2447 lines
98 KiB
Plaintext
2447 lines
98 KiB
Plaintext
.oO Phrack 49 Oo.
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Volume Seven, Issue Forty-Nine
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File 05 of 16
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Introduction to Telephony and PBX
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by Cavalier[TNO]
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Table of Contents
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1. . . . . . . . . . . . . . . . . . . . The Central Office
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2. . . . . . . . . . . . . . .Private Branch Exchange (PBX)
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3. . . . . . . . . Properties of Analog and Digital Signals
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4. . . . . . . . . . . . . . . . .Analog-Digital Conversion
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5. . . . . . . . . . . . . . . . . . . Digital Transmission
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6. . . . . . . . . . . . . . . . . . . . . . . Multiplexing
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7. . . . . . . . . . . . . . . . . . . . Transmission Media
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8. . . . . . . . . . . . . . . . . . . . . . . . .Signaling
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.--------------------.
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1 | The Central Office |
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`--------------------'
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Telephones alone do nothing special. Their connection to the rest of
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world makes them one of mankind's greatest achievements.
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In the early days of telephone communications, users had to establish
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their own connections to other telephones. They literally had to string
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their own telephone lines.
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Although the customer inconvenience of building their own connections
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limited the availability of phone service, an even greater problem soon
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arose. As the telephone became more popular, more people wanted to be
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connected. At the time, each phone had to be directly wired to each
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other. In a very short time there was a disorganized maze of wires
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running from the homes and businesses.
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A simple mathematical formula demonstrates the growth in the number of
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connections required in a directly wired network:
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I = N(N-1)/2
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(I = number of interconnections; N = number of subscribers)
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I = 100(100-1)/2
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If just 100 subscribers attempted to connect to each other, 4950
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separate wire connections would be needed! Obviously, a better method
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was needed.
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Switching
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A Central Office (CO) switch is a device that interconnects user
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circuits in a local area, such as a town. The CO is a building where
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all subscriber phone lines are brought together and provided with a
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means of interconnection. If someone wants to call a neighbor, the call
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is routed through the CO and switched to the neighbor.
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What if someone wanted to call a friend in the next town? If their
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friend was connected to a different CO, there was no way to communicate.
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The solution was to interconnect COs. Then, CO-A routed calls to CO-B
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to complete the connection.
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Today every CO in the world is connected to every other CO in a vast
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communication highway known as the Public Switched Network (PSN). The
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PSN goes by a variety of different names:
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Dial-up network
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Switched network
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Exchange network
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The CO provides all users (subscribers) with a connection to each other.
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A critical note, however, is that no CO has the resources to switch all
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their users simultaneously. It would be too expensive and it is
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unnecessary to attempt to do so because for the vast majority of the
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time, only a small percentage of subscribers are on the phone at the
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same time.
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If, on a rare occasion, all the circuits are busy, the next call will be
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blocked. A call is blocked if there are no circuits available to switch
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it because all the circuits are in use.
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The term `probability of blocking` is a statistical logarithm which
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determines the chance that a call cannot be switched. For modern day
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commercial COs, the probability of blocking is very low.
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History of COs
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Operating switching
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In the first COs, a subscriber who wanted to place a call cranked a
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magneto-generator to request service from the local phone company. An
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operator at the CO monitored subscriber connections by observing lamps
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on a switchboard console. When a subscriber's lamp lit, indicating the
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request for service, the operator would answer: "Number please...".
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The operator connected one call to another by plugging one end of a cord
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into the jack of the caller and the other end of the cord into the jack
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of the called party, establishing a manual, physical connection.
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The switchboard had to have a jack for every incoming and outgoing line
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that needed service. The number of lines an operator could monitor was
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limited by her arm's reach. Billing was accomplished by the operators
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writing up a ticket for each call designating its starting and ending
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times.
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When telephone subscribers were few in number, this method worked fine.
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As the popularity of the phone increased, more phones placed more calls
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and it became increasingly unmanageable and expensive to manually switch
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and bill each call.
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Strowger Step-by-Step Switch
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A mechanical switch was invented in the 1890's by a Kansas City
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mortician named Almon B. Strowger. He became very suspicious because
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callers looking for a mortician were continually referred to his
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competition instead to him. When he learned that the local operator was
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the wife of his rival, his suspicions were confirmed. He set about to
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invent a switching system that would not be dependent upon human
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intervention.
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His creation, called the Strowger or Step-by-Step switch, was the first
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automated electromechanical switching system. It placed switching
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control in the hands of the subscriber instead of the operator by adding
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a dialing mechanism to the phone.
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The Strowger switch completed a call by progressing digit by digit
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through two axes of a switching matrix in the CO. A call was stepped
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vertically to one of ten levels and rotated horizontally to one of ten
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terminals.
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It was called step-by-step because calls progress one step at a time as
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the customer dialed each digit of the number. When the final digit was
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dialed, the switch seized an available circuit and connected the call.
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The result of the step-by step switch was to eliminate the need for
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manual operator connection and grant privacy and call control to the
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subscriber.
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The step-by-step switch was a wonderful invention for its day. Today
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it is obsolete. Compared to modern day switches, it is slow, noisy
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and too expensive to maintain. It is also both bulky and inefficient.
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The Crossbar Switch
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The crossbar switch was invented and developed in the late 1920s. One
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of its main technological advanced was the introduction of a hard wired
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memory to store dialed digits until the dialing was complete.
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Unlike the step-by-step method, calls are not processed under the
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direct control of incoming dial pulses. In the step-by-step method,
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each phone call controlled its own pathway through the switching matrix
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at the speed the digits were dialed by the user. The crossbar switch
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introduced a better method.
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Devices called registers stored the digits in memory as they were dialed
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by the callers. Not until all the digits were dialed would the call
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begin to be switched. Once all the digits were received and stored in
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the register, the register handed the digits to a processor to be
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examined and used to route the call.
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When a pathway had been established and the call was connected, the
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register and processor would release and become available to handle
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another call. Collectively, this process was called `common control`.
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Common control resulted in faster call completion and increased capacity
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of the switch. With the old step-by-step, the time it would take a user
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to physically dial the digits would occupy valuable switch time because
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dialing the digits was the most time consuming part of switching a call.
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This 8 to 12 seconds of dialing time prevented other users from
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accessing the switching matrix and generally slowed things down.
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The genius of the crossbar common control was to store the dialed digits
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as they came in and then after the user finished dialing, send the
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digits off for processing. The act of dialing no longer kept other
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calls waiting for switch resources.
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Common control created the separation of the control functions (setting
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up and directing the call) from the switching functions (physically
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creating the connections).
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Crossbar Switching Matrix
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Calls were connected by sharing a dedicated wire path through the
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switching matrix. Crossbar switches used the intersection of two points
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to make a connection. They selected from a horizontal and vertical
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matrix of wires, one row connected to one column. The system still
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stepped the call through the network, but only after all the digits were
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dialed. This method created a more efficient allocation of switch
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resources.
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There are four important components of a crossbar switch.
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. The marker is the brain of a crossbar switch. It identifies a
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line requesting service and allocates a register.
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. The register provides dial tone and receives and stores the dialed
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digits.
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. The matrix is a set of horizontal and vertical bars. The point at
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which the crosspoints meet establishes the connection.
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. A trunk interface unit, also called a sender, processes calls from
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a PBX.
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Although crossbar is faster and less bulky than step-by-step, it is
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still electromechanical and requires a lot of maintenance. It requires
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huge amounts of space, generates a lot of heat, and makes a great deal of
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noise.
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Electronic Switching System (ESS)
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The advent of electronic switching (also called stored program
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switching) was made possible by the transistor. Introduced in 1965, the
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Electronic Switching System (ESS) greatly sped up switch processing
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capacity and speed and has done nothing less than revolutionize the
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industry.
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Modern ESS switches perform five main functions to establish and
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maintain service in a public network.
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1. Establish a connection between two or more points
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2. Provide maintenance and testing services
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3. Record and sort customer billing charges
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4. Offer customer features, such as call waiting
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5. Allow access to operators for special services
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An ESS uses computer-based logic to control the same two primary
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operations we introduced with the crossbar -- common control and the
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switching matrix.
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(In an ESS, the terms stored program control, common control, and
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electronic switching are all synonymous.)
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ESS Common Control
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The function of the common control is similar to its function in the
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crossbar. The difference is that common control is accomplished
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electronically instead of electromechanically. Like the crossbar, one
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group of control devices controls the functions of all lines. However,
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instead of the hard wired logic of the crossbar, the control device
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consists of a computer with memory, storage, and programming capability.
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In the ESS, the computer governs the common control. It monitors all
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the lines and trunks coming into the CO, searching for changes in the
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electrical state of the circuit, such as a phone going off-hook. When a
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subscriber goes off- hook and dials a number, the common control
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equipment detects the request for service and responds by returning the
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dial tone. It then receives, stores, and interprets the dialed digits.
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Again, similar to the workings of the crossbar, once the digits have
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been processed, the computer establishes a path through the switching
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matrix to complete the call. After the connection for the call has been
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established, the common control equipment releases and becomes available
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to complete other calls.
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ESS Switching Matrix
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Recall that in the crossbar, calls were connected by sharing a dedicated
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wire path through the matrix, establishing a connection between an input
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and an output. The matrix in an ESS is logically similar to the
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crossbar grid except the pathway is electronic instead of
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electromechanical. Called a TDM bus, it is solid state circuitry and is
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printed into small computer controlled circuit boards. The computer
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controls the connections and path status map to determine which path
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should be established to connect the calling and called parties.
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Remember
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Crossbar switching matrix = maze of physical wire cross connections
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ESS switching matrix = electronic multiplexed TDM (time division
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multiplexing) bus
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ESS Advancements
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The unprecedented advancement of the ESS was the speed and processing
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power advantage it had over the crossbar because it switched calls
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digitally instead of electromechanically. The processing capacity that
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would have required a city block of crossbar technology could be
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accomplished by one floor of ESS equipment. Much less effort was
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required to maintain the ESS because it was smaller and had fewer moving
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parts.
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Telephone companies would have moved to the new technology for these
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advantages alone. But, there was much more to be offered. There was
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the power of the computer.
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There are major advantages to a computer stored program. It allows the
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system to perform functions earlier switches were incapable of. For
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example, the switch can collect statistical information to determine its
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effectiveness. It can perform self-diagnostics of circuit and system
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irregularities and report malfunctions. If trouble occurs, technicians
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can address it via a keyboard and terminal. The same terminal, often
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called a system managers terminal, allows personnel to perform system
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changes and to load new software, eliminating the need for manually
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rewiring connections.
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The computer uses two types of memory:
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. Read Only Memory (ROM) is used to store basic operating
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instructions and cannot be altered by the end user. The contents
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of this memory can only be changed by the manufacturer.
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. Random Access Memory (RAM) stores configuration and database
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information. The contents of its memory can be changed by a
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system administrator.
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Other important functions of the computer include
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. Performing telephone billing functions
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. Generating traffic analysis reports
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. Generating all tones and announcements regarding the status of
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circuits and calls
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Computer control operates under the direction of software called its
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generic program. Periodically updating or adding to the generic program
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allows the ESS to be much more flexible and manageable than previous
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switch generations because it is the software, not the hardware, that
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normally has to be upgraded.
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Electronic switching heralded the introduction of new customer features
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and services. Credit card calls, last number redial, station transfer,
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conference calling, and automatic number identification (ANI) are just
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a few examples of unprecedented customer offerings.
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The ESS is an almost fail-safe machine. Its design objective is one
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hour's outage in 20 years. In today's competitive environment for
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higher quality communication equipment, ESS machines provide a level of
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service and reliability unachievable in the past.
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.-----------------------------------.
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2 | The Private Branch Exchange (PBX) |
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`-----------------------------------'
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The two primary goals of every PBX are to
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. facilitate communication in a business
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. be cost effective
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Organizations that have more than a few phones usually have an internal
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switching mechanism that connects the internal phones to each other and
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to the outside world.
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A PBX is like a miniature Central Office switching system designed for a
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private institution. A PBX performs many of the same functions as a CO
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does. In fact, some larger institutions use genuine COs as their private
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PBX.
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Although a PBX and a CO are closely related, there are differences
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between them
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. A PBX is intended for private operation within a company. A CO is
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intended for public service.
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. A PBX usually has a console station that greets outside callers
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and connects them to internal extensions.
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. Most PBXs do not maintain the high level of service protection
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that must be maintained in a CO. Assurance features such as
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processor redundancy (in the event of processor failure) and
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battery backup power, which are standard in a CO, may not be a
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part of a PBX.
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. COs require a seven digit local telephone number, while PBXs can
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be more flexible and create dialing plans to best serve their
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users (3, 4 5, or 6 digit extensions).
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. A PBX can restrict individual stations or groups of stations from
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certain features and services, such as access to outside lines. A
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CO usually has no interest in restricting because these features
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and services are billed to the customer. COs normally provide
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unlimited access to every member on the network.
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A PBX is composed of three major elements.
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1. Common equipment (a processor and a switching matrix)
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2. CO trunks
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3. Station lines
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Common Equipment
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The operation of a PBX parallels the operation of a Central Office ESS.
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Its common control is
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. A computer operated Central Processing Unit (CPU) running software
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that intelligently determines what must be done and how best to do
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it.
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. A digital multiplexed switching matrix printed on circuit boards
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that establishes an interconnection between the calling and called
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parties.
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The CPU stores operating instructions and a database of information from
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which it can make decisions. It constantly monitors all lines for
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supervisory and control signals. A switching matrix sets up the
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connections between stations or between stations and outgoing trunks.
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Housed in equipment cabinets, PBX common equipment is often compact
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enough to occupy just a closet or small room. Given the extremely high
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rental rates many companies have, a major benefit of a PBX is its small
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size.
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CO Trunks and Station Lines
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A trunk is a communication pathway between switches. A trunk may
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provide a pathway between a PBX and the CO or between two PBXs and two
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COs. A trunk may be privately owned or be a leased set of lines that
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run through the Public Switched Network.
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A line is a communication pathway between a switch and terminal
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equipment, such as between a PBX and an internal telephone or between a
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CO and a home telephone.
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The function of the PBX is to interconnect or switch outgoing trunks
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with internal lines.
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Two Varieties of Lines
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Station lines are either analog or digital, depending on the station
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equipment it is connecting. If the phone on one desk is digital, it
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should be connected to a digital line. If the phone on the desk is
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analog, it should be connected to an analog line.
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Varieties of Trunks
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There exists a wide variety of trunks that can be connected to a PBX for
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off-premises communication. Each variety has different functions and
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capabilities. It is important to be able to distinguish them.
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Tie Trunks
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Organizations supporting a network of geographically dispersed PBXs
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often use tie trunks to interconnect them. A tie trunk is a permanent
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circuit between two PBXs in a private network. Tie trunks are usually
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leased from the common carrier; however, a private microwave arrangement
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can be established. Usually, leased tie trunks are not charged on a per
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call basis but rather on the length of the trunk. If a tie trunk is
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used more than one or two hours a day, distance sensitive pricing is
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more economical.
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A T1 trunk is a digital CO leased trunk that is capable of being
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multiplexed into 24 voice or data channels at a total rate of 1.544
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Mbps. T1 trunks are used as PBX-to-PBX tie trunks, PBX-to-CO trunks as
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well as PBX trunks to bypass the local CO and connect directly to a long
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distance carrier. It is a standard for digital transmission in North
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America and Japan.
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T1 uses two pairs of normal, twisted wire--the same as would be found in
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a subscriber's residence. Pulse Code Modulation is the preferred method
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of analog to digital conversion.
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A T2 trunk is capable of 96 multiplexed channels at a total rate of
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6.312 Mbps.
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A T3 trunk is capable of 672 multiplexed channels at a total rate of
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44.736 Mbps.
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A T4 trunk is capable of 4,032 multiplexed channels at a total of
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274.176 Mbps.
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Direct Inward Dialing (DID) Trunks
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Incoming calls to a PBX often first flow through an attendant position.
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DID trunks allow users to receive calls directly from the outside
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without intervention from the attendant. DID offers three main
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advantages.
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1. It allows direct access to stations from outside the PBX.
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2. It allows users to receive calls even when the attendant
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switchboard is closed.
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3. It takes a portion of the load off the attendants.
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Trunk Pools
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Trunks do not terminate at a user's telephone station. Instead trunks
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are bundled into groups of similarly configured trunks called trunk
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pools. When a user wants to access a trunk, he can dial a trunk access
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code--for example, he can dial 9 to obtain a trunk in the pool. Trunk
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pools make system administration less complicated because it is easier
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to administer a small number of groups than a large number of individual
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trunks.
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Ports
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Ports are the physical and electrical interface between the PBX and a
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trunk or station line.
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PBX Telephones
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Telephone stations in a PBX are not directly connected to the CO but to
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the PBX instead. When a station goes off-hook, the PBX recognizes it
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and sends to the station its own dial tone. The PBX requires some
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access digit, usually "9" to obtain an idle CO trunk from a pool to
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connect the station with the public network. This connection between
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the telephone and the PBX allows stations to take advantage of a myriad
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of PBX features.
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The attendant console is a special PBX telephone designed to serve
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several functions. Traditionally, most PBXs have used attendants as the
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central answering point for incoming calls. Calls placed to the PBX
|
|
first connected to the attendant, who answered the company name. The
|
|
attendant then established a connection to the desired party. The
|
|
attendant also provided assistance to PBX users, including directory
|
|
assistance and reports of problems.
|
|
|
|
In recent years a number of cost-saving improvements have been made to
|
|
the attendant console. A feature commonly called automated attendant
|
|
can establish connections without a human interface, substantially
|
|
decreasing PBX operating costs.
|
|
|
|
Blocking versus Non-blocking
|
|
|
|
Blocking is a critical aspect of the functioning of a PBX. A
|
|
non-blocking switch is one that provides as many input/output interface
|
|
ports as there are lines in the network. In other words, the switching
|
|
matrix provides enough paths for all line and trunk ports to be
|
|
connected simultaneously.
|
|
|
|
PBX systems are usually blocking. It requires an exponential increase
|
|
in resources and expense to ensure non-blocking. Based on call traffic
|
|
studies and the nature of calls, it is generally acceptable to engineer
|
|
a low level of blocking in exchange for a major savings of common
|
|
equipment resources.
|
|
|
|
Grades of service are quantitative measurements of blocking. They are
|
|
written in the form:
|
|
|
|
P.xx
|
|
|
|
where xx is a two digit number that indicates how many calls out of a
|
|
hundred will be blocked. The smaller the number, the better the grade
|
|
of service.
|
|
|
|
P.01 means one call out of a hundred will be blocked. It is a better
|
|
grade of service than P.05 that block five calls out of a hundred.
|
|
Naturally the P.05 service costs less than the better grade of service
|
|
provided by P.01.
|
|
|
|
Even if a PBX's switching matrix is non-blocking, an internal caller may
|
|
still not be able to reach an outside trunk if all the trunks are busy.
|
|
CO trunks cost money, and very few PBXs dedicate one trunk to every
|
|
internal line. Instead, traffic studies are performed to determine the
|
|
percentage of time a station will be connected to an outside trunk
|
|
during peak hours.
|
|
|
|
If, for example, it is determined that the average station uses a trunk
|
|
only 20% of the time during peak hours, then the switch may be
|
|
configured to have a 5:1 line-to-trunk ratio, meaning for every five
|
|
lines (or extensions) there is one trunk. Most PBXs are configured on
|
|
this principle as a major cost saving method.
|
|
|
|
|
|
PBX Features
|
|
|
|
COs and PBXs share many of the same attributes and functionality.
|
|
However, COs are built to perform different tasks than a PBX, resulting
|
|
in feature differences between them. The following is an overview of
|
|
common PBX features not found in a CO.
|
|
|
|
Automatic Route Selection (ARS)
|
|
|
|
A primary concern of any telecommunications manager is to keep costs
|
|
down. One of these costs is long distance service. ARS is a feature
|
|
that controls long distance costs.
|
|
|
|
Most PBXs have more than just public CO trunks connected to them. They
|
|
may have a combination of tie trunks to other PBXs (T1/E1 trunks and
|
|
many others). Each type of trunk has a separate billing scheme,
|
|
relatively more or less expensive for a given number of variables.
|
|
|
|
It is extremely difficult to attempt to educate company employees on
|
|
which trunks to select for which calls at what time of day. It defeats
|
|
the productivity-raising, user-transparency goal of any PBX if employees
|
|
must pour over tariffing charts every time they want to use the phone.
|
|
|
|
Instead, ARS programs the PBX central processor to select the least
|
|
expensive trunk on a call by call basis. When a user places a call, the
|
|
computer determines the most cost effective route, dials the digits and
|
|
completes the call.
|
|
|
|
|
|
Feature Access
|
|
|
|
PBXs support a wide variety of user features. For example, call
|
|
forward, hold, and call pickup are all user features. There are two
|
|
methods of activating a feature. A code, such as "*62" can be assigned
|
|
to the call forward feature. To activate call forward the user presses
|
|
"*62" and continues dialing.
|
|
|
|
Dial codes are not the preferred method of feature access. The problem
|
|
is that users tend to forget the codes and either waste time looking
|
|
them up or do not take advantage of time saving features, thereby
|
|
defeating the purpose of buying them.
|
|
|
|
Dedicated button feature access is a better solution. Programmable
|
|
feature buttons, located on most PBX telephones, are pressed to activate
|
|
the desired feature. If a user wants to activate call forward, he
|
|
presses a button labeled "call forward" and continues dialing.
|
|
|
|
The only drawback of telephones with programmable feature buttons is
|
|
that they are more expensive than standard phones.
|
|
|
|
|
|
Voice Mail
|
|
|
|
For a voice conversation to occur, there is one prerequisite so obvious
|
|
it is usually overlooked. The called party must be available to answer
|
|
the call. In today's busy world, people are often not accessible which
|
|
can create a major problem resulting in messages not being received and
|
|
business not being conducted.
|
|
|
|
Statistics confirm the need for an alternate method.
|
|
|
|
75% of call attempts fail to make contact with the desired party.
|
|
|
|
50% of business calls involve one-way information--one party
|
|
wishing to deliver information to another party without any
|
|
response necessary.
|
|
|
|
50% of incoming calls are less important than the activity they
|
|
interrupt.
|
|
|
|
Voice mail (also known as store and forward technology) is a valuable
|
|
feature that is designed around today's busy, mobile office. It is like
|
|
a centralized answering machine for all telephone stations in a PBX.
|
|
When a telephone is busy or unattended, the systems routes the caller to
|
|
a voice announcement that explains that the called party is unavailable
|
|
and invites the caller to leave a message. The message is stored until
|
|
the station user enters a security dial access code and retrieves the
|
|
message.
|
|
|
|
|
|
Automated Attendant
|
|
|
|
Automated attendant is a feature sometimes included with voice mail. It
|
|
allows outside callers to bypass a human attendant by routing their own
|
|
calls through the PBX. Callers are greeted with a recorded announcement
|
|
that prompts them to dial the extension number of the desired position,
|
|
or stay on the line to be connected to an attendant.
|
|
|
|
Reducing cost is the primary goal of automated attendant. The decreased
|
|
attendant work load more d) an pays for the cost of the software and
|
|
equipment.
|
|
|
|
When automated attendant was first introduced, it met with substantial
|
|
resistance from the general public. People did not want to talk to a
|
|
machine. But, as its cost effectiveness drove many companies to employ
|
|
it, the public has slowly adjusted to the new technology.
|
|
|
|
Restriction
|
|
|
|
Nearly every PBX enforces some combination of inside and outside calling
|
|
restrictions on certain phones. Depending upon the sophistication of
|
|
the PBX, a system administrator can have nearly unlimited flexibility in
|
|
assigning restrictions. For example, a tire manufacturing plant could
|
|
restrict all lobby phones at corporate headquarters to internal and
|
|
local calls only. The phones at the storage warehouse could be
|
|
restricted for only internal calling. But, all executive phones could
|
|
be left unrestricted.
|
|
|
|
Long distance toll charges can be a crippling expense. Toll fraud is a
|
|
major corporate problem. Restriction combats unauthorized use of
|
|
company telephone resources and is a prime function of any PBX.
|
|
|
|
|
|
Tandems
|
|
|
|
As stated earlier, it is necessary to have a switching mechanism to
|
|
interconnect calls. If a number of phones all wish to be able to talk
|
|
to each other, an enormous amount of cabling would be wasted tying each
|
|
of them together. Thus, the switch was born.
|
|
|
|
The same principle applies for interconnecting PBXs. Large firms that
|
|
have PBXs scattered all over the country want each PBX to have the
|
|
ability to access every other one. But the expense of directly
|
|
connecting each could drive a company out of business. The solution is
|
|
to create a centrally located tandem switching station to interconnect
|
|
the phones from one PBX with the phones from any other. This solution
|
|
creates a Private Switched Network.
|
|
|
|
Directing digits are often used to inform the tandem switch where to
|
|
route the call. Each PBX is assigned a unique number. Let's say a PBX
|
|
in Paris is numbered "4." To call the Paris PBX from a PBX in Chicago,
|
|
a user would dial "4- XXXX."
|
|
|
|
|
|
Uniform Dialing Plan
|
|
|
|
A network of PBXs can be configured poorly so that calling an extension
|
|
at another PBX could involve dialing a long, confusing series of numbers
|
|
and create a lot of user frustration. A Uniform Dialing Plan enables a
|
|
caller to dial another internal extension at any PBX on the network with
|
|
a minimum of digits, perhaps four or five. The system determines where
|
|
to route the call, translates the digits and chooses the best facility,
|
|
all without the knowledge of the user. As far as the user knows, the
|
|
call could have been placed to a station at the next desk.
|
|
|
|
|
|
Call Accounting System (CAS) and Station Message Detail Recording (SMDR)
|
|
|
|
CAS works in conjunction with SMDR to identify and monitor telephone
|
|
usage in the system. SMDR records call information such as the calling
|
|
number, the time of the call, and its duration. The raw data is usually
|
|
listed chronologically and can be printed on reports.
|
|
|
|
SMDR by itself is not particularly useful because the sheer volume and
|
|
lack of sorting capability of the reports make them difficult to work
|
|
with. A Call Accounting Systems is a database program that addresses
|
|
these shortcomings by producing clear, concise management reports
|
|
detailing phone usage.
|
|
|
|
The primary function of CAS reports is to help control and discourage
|
|
unnecessary or unauthorized use and to bill back calling charges to
|
|
users. Many law firms use a call accounting system to bill individual
|
|
clients for every call they make on behalf of each client.
|
|
|
|
|
|
Attendant Features
|
|
|
|
A number of features are available to improve the efficiency of
|
|
attendant consoles.
|
|
|
|
Here are a few of them.
|
|
|
|
Direct Station Selection (DSS) allows attendants to call any
|
|
station telephone by pressing a button labeled with its extension.
|
|
|
|
Automatic Timed Reminder alerts the attendant that a station has
|
|
not picked up its call. The attendant may choose to reconnect to
|
|
the call and attempt to reroute it.
|
|
|
|
Centralized Attendant Service groups all network attendants into
|
|
the same physical location to avoid redundancies of service and
|
|
locations.
|
|
|
|
|
|
Power Failure Schemes
|
|
|
|
If a city or a town experiences a commercial power failure, telephones
|
|
connected directly to the CO will not be affected because the CO gets
|
|
power from its own internal battery source. A PBX, however, is
|
|
susceptible to general power failures because it usually gets its power
|
|
from the municipal electric company.
|
|
|
|
There are several different ways a PBX can be configured to overcome a
|
|
power failure.
|
|
|
|
A PBX can be directly connected to a DC battery which serves as
|
|
its source of power. The battery is continually recharged by an
|
|
AC line to the electric company. In the event of a power failure,
|
|
the PBX will continue functioning until the battery runs out.
|
|
|
|
A PBX can have an Uninterruptable Power Supply (UPS) to protect
|
|
against temporary surges or losses of power.
|
|
|
|
A PBX can use a Power Failure Transfer (PFT) which, in the event
|
|
of a power failure, immediately connects preassigned analog phones
|
|
to CO trunks, thereby using power from the CO instead of from the
|
|
PBX.
|
|
|
|
|
|
Outgoing Trunk Queuing
|
|
|
|
In the event all outgoing trunks are busy, this feature allows a user to
|
|
dial a Trunk Queuing code and hang up. As soon as a trunk becomes free,
|
|
the system reserves it for the user, rings the station and connects the
|
|
outside call automatically.
|
|
|
|
|
|
System Management
|
|
|
|
PBXs can be so large and complex that without a carefully designed
|
|
method of system management chaos can result. The best, most advanced
|
|
systems mimic CO management features--computer access terminals which
|
|
clearly and logically program and control most system features. The
|
|
system manager has a wide variety of responsibilities which may include,
|
|
but is not limited to
|
|
|
|
Programming telephone moves, additions, and changes on the system
|
|
|
|
Performing traffic analysis to maximize system configuration
|
|
resources and optimize network performance
|
|
|
|
Responding to system-generated alarms
|
|
|
|
Programming telephone, system, attendant, and network features.
|
|
|
|
|
|
ISDN
|
|
|
|
|
|
ISDN is not a product. Rather, it is a series of standards created by
|
|
the international body, ITU (previously known as CCITT), to support the
|
|
implementation of digital transmission of voice, data, and image through
|
|
standard interfaces. Its goal is to combine all communications services
|
|
offered over separate networks into a single, standard network. Any
|
|
subscriber could gain access to this vast network by simply plugging
|
|
into the wall. (At this time not all PBXs are compatible with the ISDN
|
|
standard.)
|
|
|
|
|
|
Alternatives to a PBX
|
|
|
|
There are two main alternatives to purchasing a PBX. They are
|
|
purchasing a Key system or renting Centrex service from the local
|
|
telephone company.
|
|
|
|
|
|
Key System
|
|
|
|
Key systems are designed for very small customers, who typically use
|
|
under 15 lines. There is no switching mechanism as in a PBX. Instead
|
|
every line terminates on every phone. Hence, everyone with a phone can
|
|
pick up every incoming call.
|
|
|
|
Key systems are characterized by a fat cable at the back of each phone.
|
|
The cables are fat because each phone is directly connected to each
|
|
incoming line and each line has to be wired separately to each phone.
|
|
|
|
Fat cables have become a drawback to Key systems as building wire
|
|
conduits have begun to fill with wire. It has become increasingly
|
|
difficult to add and move stations because technicians must physically
|
|
rewire the bulky cables instead of simply programming a change in the
|
|
software.
|
|
|
|
Key telephones are equipped with line assignment buttons that light on
|
|
incoming calls and flash on held calls. These buttons enable a user to
|
|
access each line associated with each button. Unlike a PBX, there is no
|
|
need to interface with an attendant console to obtain an outside line.
|
|
|
|
|
|
Differences between Key and PBX Systems
|
|
|
|
Key systems have no switching matrix. In a Key system, incoming
|
|
calls terminate directly on a station user's phone. In a PBX,
|
|
incoming calls usually first go to the attendant who switches the
|
|
call to the appropriate station.
|
|
|
|
PBX accesses CO trunk pools by dialing an access code such as "9."
|
|
Key systems CO trunks are not pooled. They are accessed directly.
|
|
|
|
Key systems make use of a limited number of features, many of them
|
|
common to the PBX. These include
|
|
|
|
Last number redial
|
|
Speed dialing
|
|
Message waiting lamp
|
|
Paging
|
|
Toll restriction
|
|
|
|
Today's PBXs can simulate Key system operation. For example, telephones
|
|
can have a line directly terminating on a button for direct access.
|
|
|
|
|
|
Centrex
|
|
|
|
The other alternative to purchasing a PBX is leasing a Centrex service.
|
|
|
|
Centrex is a group of PBX-like service offerings furnished by the local
|
|
telephone company. It offers many of the same features and functions
|
|
associated with a PBX, but without the expense of owning and maintaining
|
|
equipment and supporting in-house administrative personnel.
|
|
|
|
Because network control remains the responsibility of the CO, companies
|
|
that choose Centrex service over purchasing and maintaining a private
|
|
PBX can ignore the sophisticated world of high tech telecommunications
|
|
and leave it up to the telephone company representatives.
|
|
|
|
To provide Centrex service, a pair of wires is extended from the CO to
|
|
each user's phone. Centrex provides an "extension" at each station
|
|
complete with its own telephone number. No switching equipment is
|
|
located at the customer premises. Instead, Centrex equipment is
|
|
physically located at the CO.
|
|
|
|
There are a number of reasons a company would choose a Centrex system
|
|
over owning their own PBX. Currently Centrex has six million customers
|
|
in the United States market.
|
|
|
|
Advantages of a Centrex System over a PBX:
|
|
|
|
Nearly uninterruptable service due to large redundancies in the CO
|
|
|
|
Easily upgraded to advanced features.
|
|
|
|
No floor space requirement for equipment.
|
|
|
|
No capital investment
|
|
|
|
24-hour maintenance coverage by CO technicians
|
|
|
|
Inherent Direct Inward Dialing (DID). All lines terminate at
|
|
extensions, instead of first flowing through a switchboard.
|
|
|
|
Call accounting and user billing as inherent part of the service.
|
|
|
|
Reduced administrative payroll.
|
|
|
|
|
|
Disadvantages of a Centrex System:
|
|
|
|
Cost. Centrex is tariffed by the local telephone company and can
|
|
be very expensive. Companies are charged for each line connected
|
|
to the Centrex, as well for the particular service plan chosen.
|
|
Additionally, Centrex service may be subject to monthly increases.
|
|
|
|
Feature availability. Centrex feature options are generally not
|
|
state of the art, lagging behind PBX technology. Not all COs are
|
|
of the same generation and level of sophistication--a company
|
|
associated with an older CO may be subject to inferior service and
|
|
limited or outdated feature options.
|
|
|
|
Control of the network is the responsibility of the CO. While
|
|
this release from responsibility is often cited as a positive
|
|
feature of Centrex, there are drawback to relinquishing control.
|
|
CO bureaucracy can be such that a station move, addition or change
|
|
can sometimes take days to achieve. Furthermore, each request is
|
|
charged a fee. Also, some companies are more particular about
|
|
certain features of their network (security for example) and
|
|
require direct control for themselves.
|
|
|
|
|
|
|
|
.------------------------------------------.
|
|
3 | Properties of Analog and Digital Signals |
|
|
`------------------------------------------'
|
|
|
|
A man in Canada picks up a telephone and dials a number. Within
|
|
seconds, he begins talking to his business partner in Madrid. How can
|
|
this be?
|
|
|
|
Telephony is a constantly evolving technology with scientific rules and
|
|
standards. You will learn to make sense of what would otherwise seem
|
|
impossible.
|
|
|
|
Voice travels at 250 meters per second and has a range limited to the
|
|
strength of the speaker's lungs. In contrast, electricity travels at
|
|
speeds approaching the speed of light (310,000 Km per second) and can be
|
|
recharged to travel lengths spanning the globe. Obviously, electricity
|
|
is a more effective method of transmission.
|
|
|
|
To capitalize on the transmission properties of electricity, voice is
|
|
first converted into electrical impulses and then transmitted. These
|
|
electrical impulses represent the varying characteristics that
|
|
distinguish all of our voices. The impulses are transmitted at high
|
|
speeds and then decoded at the receiving end into a recognizable
|
|
duplication of the original voice.
|
|
|
|
For a hundred years, scientists have been challenged by how best to
|
|
represent voice by electrical impulses. An enormous amount of effort
|
|
has been devoted to solving this puzzle. The two forms of electrical
|
|
signals used to represent voice are analog and digital.
|
|
|
|
Both analog and digital signals are composed of waveforms. However,
|
|
their waveforms have very distinctive properties which distinguish them.
|
|
To understand the science of telephony, it is necessary to understand
|
|
how analog and digital signals function, and what the differences
|
|
between them are.
|
|
|
|
If you do not possess a fundamental understanding of basic waveforms,
|
|
you will not understand many of the more advanced concepts of
|
|
telecommunications.
|
|
|
|
|
|
Analog Signal Properties
|
|
|
|
Air is the medium that carries sound. When we speak to one another, our
|
|
vocal chords create a disturbance of the air. This disturbance causes
|
|
air molecules to become expanded and compress thus creating waves. This
|
|
type of wave is called analog, because it creates a waveform similar to
|
|
the sound it represents.
|
|
|
|
Analog waves are found in nature. They are continually flowing and have
|
|
a limitless number of values. The sine wave is a good example of an
|
|
analog signal.
|
|
|
|
|
|
Three properties of analog signals are particularly important in
|
|
transmission:
|
|
|
|
amplitude frequency phase
|
|
|
|
Amplitude
|
|
|
|
Amplitude refers to the maximum height of an analog signal. Amplitude
|
|
is measured in decibels when the signal is measured in the form of
|
|
audible sound. Amplitude is measured in volts when the signal is in the
|
|
form of electrical energy.
|
|
|
|
|
|
Amplitude of an Analog Wave
|
|
|
|
|
|
Volts represent the instantaneous amount of power an analog signal
|
|
contains.
|
|
|
|
Amplitude, wave height, and loudness of an analog signal represent the
|
|
same property of the signal. Decibels and volts are simply two
|
|
different units of measurement which are used to quantify this property.
|
|
|
|
Frequency
|
|
|
|
Frequency is the number of sound waves or cycles that occur in a given
|
|
length of time. A cycle is represented by a 360 degree sine wave.
|
|
Frequency is measured in cycles per second, commonly called hertz (Hz).
|
|
|
|
Frequency corresponds to the pitch (highness or lowness) of a sound. The
|
|
higher the frequency, the higher the pitch. The high pitch tone of a
|
|
flute will have a higher frequency than the low pitch tone of a bass.
|
|
|
|
Phase refers to the relative position of a wave at a point in time. It
|
|
is useful to compare the phase of two waves that have the same frequency
|
|
by determining whether the waves have the same shape or position at the
|
|
same time. Waves that are in-step are said to be in phase, and waves
|
|
that are not synchronized are called out-of-phase.
|
|
|
|
Modulation
|
|
|
|
|
|
The reason these three properties are significant is that each can be
|
|
changed (modulated) to facilitate transmission.
|
|
|
|
The term modulation means imposing information on an electrical signal.
|
|
|
|
The process of modulation begins with a wave of constant amplitude,
|
|
frequency, and phase called carrier wave. Information signals
|
|
representing voice, data, or video modulate a property (amplitude,
|
|
frequency, or phase) of the carrier wave to create a representation of
|
|
itself on the wave.
|
|
|
|
Amplitude Modulation is a method of adding information to an analog
|
|
signal by varying its amplitude while keeping its frequency constant. AM
|
|
radio is achieved by amplitude modulation.
|
|
|
|
Frequency Modulation adds information to an analog signal by varying its
|
|
frequency while keeping its amplitude constant. FM radio is achieved by
|
|
frequency modulation.
|
|
|
|
Phase Modulation adds information to an analog signal by varying its
|
|
phase.
|
|
|
|
The modulated wave carrying the information is then transmitted to a
|
|
distant station where it is decoded and the information is extracted
|
|
from the signal.
|
|
|
|
|
|
Properties of Digital Signals
|
|
|
|
|
|
Unlike analog signals, digital signals do not occur in nature. Digital
|
|
signals are an invention of mankind. They were created as a method of
|
|
coding information. An early example of digital signals is the Morse
|
|
Code.
|
|
|
|
Digital signals have discrete, non-continuous values. Digital signals
|
|
have only two states:
|
|
|
|
|
|
Type of Signal State
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
Light switch On Off
|
|
|
|
|
|
Voltage Voltage Level 1 Voltage Level 2
|
|
(-2 volts) (+2 volts)
|
|
|
|
Morse Short beat Long beat
|
|
|
|
|
|
|
|
Computers and humans cannot communicate directly with each other. We do
|
|
not understand what tiny bits and voltage changes mean. Computers do
|
|
not understand the letters of the alphabet or words.
|
|
|
|
For computers and humans to communicate with each other, a variety of
|
|
binary (digital) languages, called character codes, have been created.
|
|
Each character of a character code represents a unique letter of the
|
|
alphabet: a digit, punctuation mark, or printing character.
|
|
|
|
The most popular character code is call ASCII (America Standard Code for
|
|
Information Interchange). It uses a seven bit coding scheme-- each
|
|
character consists of a unique combination of seven 1s and 0s. For
|
|
example, the capital letter T is represented by the ASCII 1010100; the
|
|
number 3 by the ACSII 0110011. The maximum number of different
|
|
characters which can be coded in ASCII is 128).
|
|
|
|
|
|
English ASCII
|
|
|
|
T 1010100
|
|
|
|
3 0110011
|
|
|
|
|
|
Another character code is called Extended ASCII. Extended ASCII builds
|
|
upon the existing ASCII character code. Extended ASCII codes characters
|
|
into eight bits providing 256 character representations). The extra 127
|
|
characters represent foreign language letters and other useful symbols.
|
|
|
|
|
|
Signal Loss - Attenuation
|
|
|
|
Analog and digital signals are transmitted to provide communication over
|
|
long distances. Unfortunately, the strength of any transmitted signal
|
|
weakens over distance. This phenomenon is called attenuation. Both
|
|
analog and digital signals are subject to attenuation, but the
|
|
attenuation is overcome in very different ways.
|
|
|
|
|
|
Analog Attenuation
|
|
|
|
Every kilometer or so, an analog signal must be amplified to overcome
|
|
natural attenuation. Devices called amplifiers boost all the signals
|
|
they receive, strengthening the signals to their original power. The
|
|
problem is that over distance, noise is created and it is boosted along
|
|
with the desired signal.
|
|
|
|
The result of using amplifiers is that both the noise (unwanted
|
|
electrical energy) and the signal carrying the information are
|
|
amplified. Because the noise is amplified every kilometer, it can build
|
|
up enough energy to make a conversation incomprehensible. If the noise
|
|
becomes too great, communication may become impossible.
|
|
|
|
Two different types of noise affect signal quality.
|
|
|
|
White noise is the result of unwanted electrical signals over
|
|
lines. When it becomes loud enough, it sounds like the roar of
|
|
the ocean at a distance.
|
|
|
|
Impulse noise is caused by intermittent disturbances such as
|
|
telephone company switch activity or lightning. It sounds like
|
|
pops and crack over the line.
|
|
|
|
As analog signals pass through successive amplifiers, the noise is
|
|
amplified along with the signal and therefore causes the signal to
|
|
degenerate.
|
|
|
|
|
|
Digital Attenuation
|
|
|
|
Although digital signals are also affected by attenuation, they are
|
|
capable of a much more effective method to overcome signal loss. A
|
|
device called a regenerative repeater determines whether the incoming
|
|
digital signal is a 1 or a 0. The regenerative repeater then recreates
|
|
the signal and transmits it at a higher signal strength. This method is
|
|
more effective than repeating an analog signal because digital signals
|
|
can only be one of two possible states. Remember that an analog signal
|
|
is comprised of an infinite number of states.)
|
|
|
|
The advantage of a digital regenerator is that noise is not reproduced.
|
|
At each regenerative repeater, all noise is filtered out-- a major
|
|
advantage over analog amplification.
|
|
|
|
|
|
Advantages of Digital over Analog Signals
|
|
|
|
|
|
1. Digital regenerative repeaters are superior to analog amplifiers.
|
|
|
|
A buildup of noise causes a distortion of the waveform. If the
|
|
distortion is large enough, a signal will not arrive in the same
|
|
form as it was transmitted. The result is errors in transmission.
|
|
|
|
In digital transmission, noise is filtered out leaving a clean,
|
|
clear signal. A comparison of average error rates shows
|
|
|
|
Analog: 1 error every 100,000 signals
|
|
|
|
Digital: 1 error every 10,000,000 signals
|
|
|
|
2. The explosion of modern digital electronic equipment on the market
|
|
has greatly reduced its price, making digital communications
|
|
increasingly more cost effective. The price of computer chips,
|
|
the brains of electronic equipment, has dropped dramatically in
|
|
recent years further reducing the price of digital equipment.
|
|
|
|
This trend will almost certainly continue adding more pressure to
|
|
use digital methods.
|
|
|
|
|
|
3. An ever increasing bulk of communication is between digital
|
|
equipment (computer-to-computer)
|
|
|
|
For most of telephony history, long distance communication meant
|
|
voice telephone conversations. Because voice is analog in nature,
|
|
it was logical to use analog facilities for transmission. Now the
|
|
picture is changing. More and more communication is between
|
|
computers, digital faxes, and other digital transmission devices.
|
|
|
|
Naturally, it is preferable to send digital data over digital
|
|
transmission equipment when both sending and receiving devices are
|
|
digital since there is no need to convert the digital signals to
|
|
analog to prepare them for analog transmission.
|
|
|
|
Historically, telephone networks were intended to carry analog voice
|
|
traffic. Therefore, equipment was designed to create, transmit, and
|
|
process analog signals. As technology in computers (microprocessors)
|
|
and digital transmission has advanced, nearly all equipment installed in
|
|
new facilities are digital.
|
|
|
|
|
|
.---------------------------.
|
|
4 | Analog-Digital Conversion |
|
|
`---------------------------'
|
|
|
|
|
|
Because it offers better transmission quality, almost every long
|
|
distance telephone communication now uses digital transmission on the
|
|
majority of their lines. But since voice in its natural form is analog,
|
|
it is necessary to convert these. In order to transmit analog waves
|
|
over digital facilities to capitalize on its numerous advantages, analog
|
|
waves are converted to digital waves.
|
|
|
|
|
|
Pulse Code Modulation (PCM)
|
|
|
|
The conversion process is called Pulse Code Modulation (PCM) and is
|
|
performed by a device called a codec (coder/decoder). PCM is a method
|
|
of converting analog signals into digital 1s and 0s, suitable for
|
|
digital transmission. At the receiving end of the transmission, the
|
|
coded 1s and 0s are reconverted into analog signals which can be
|
|
understood by the listener.
|
|
|
|
|
|
Three Step Process of PCM
|
|
|
|
|
|
Step 1 - Sampling
|
|
|
|
|
|
Sampling allows for the recording of the voltage levels at discrete
|
|
points in prescribed time intervals along an analog wave. Each voltage
|
|
level is called a sample. Nyquist's Theorem states:
|
|
|
|
If an analog signal is sampled at twice the rate of the highest
|
|
frequency it attains, the reproduced signal will be a highly
|
|
accurate reproduction of the original.
|
|
|
|
The highest frequency used in voice communications is 4000 Hz (4000
|
|
cycles per second). Therefore, if a signal is sampled 8000 times per
|
|
second, the listener will never know they have been connected and
|
|
disconnected 8000 times every second! They will simply recognize the
|
|
signal as the voice of the speaker.
|
|
|
|
To visualize this procedure better, consider how a movie works. Single
|
|
still frames are sped past a light and reproduced on a screen. Between
|
|
each of the frames is a dark space. Since the frames move so quickly,
|
|
the eye does not detect this dark space. Instead the eye perceives
|
|
continuous motion from the still frames.
|
|
|
|
PCM samples can be compared to the still frames of a movie. Since the
|
|
voice signal is sampled at such frequent intervals, the listener does
|
|
not realize that there are breaks in the voice and good quality
|
|
reproduction of voice can be achieved. Naturally, the higher the
|
|
sampling rate, the more accurate the reproduction of the signal. Dr.
|
|
Nyquist was the one who discovered that only 8000 samples per second are
|
|
needed for excellent voice reproduction.
|
|
|
|
The 8000 samples per second are recorded as a string of voltage levels.
|
|
This string is called a Pulse Amplitude Modulation (PAM) signal.
|
|
|
|
|
|
Step 2 - Quantizing
|
|
|
|
|
|
Since analog waves are continuous and have an infinite number of values,
|
|
an infinite number of PAM voltage levels are needed to perfectly
|
|
describe any analog wave. In practice, it would be impossible to
|
|
represent each exact PAM voltage level. Instead, each level is rounded
|
|
to the nearest of 256 predetermined voltage levels by a method called
|
|
Quantizing.
|
|
|
|
Quantizing assigns each PAM voltage level to one of 256 amplitude
|
|
levels. The amplitude levels do not exactly match the amplitude of the
|
|
PAM signal but are close enough so only a little distortion results.
|
|
|
|
This distortion is called quantizing error. Quantizing error is the
|
|
difference between the actual PAM voltage level and the amplitude level
|
|
it was rounded to. Quantizing error produces quantizing noise.
|
|
Quantizing noise creates an audible noise over the transmission line.
|
|
|
|
Low amplitude signals are affected more than high amplitude signals by
|
|
quantizing noise. To overcome this effect, a process call companding is
|
|
employed. Low amplitude signals are sampled more frequently than high
|
|
amplitude signals. Therefore, changes in voltage along the waveform
|
|
curve can be more accurately distinguished.
|
|
|
|
Companding reduces the effect of quantizing error on low amplitude
|
|
signals where the effect is greatest by increasing the error on high
|
|
amplitude signals where the effect is minimal. Throughout this process,
|
|
the total number of samples remains the same at 8000 per second.
|
|
|
|
Two common companding formulas are used in different parts of the world.
|
|
The United States and Japan follow a companding formula called Mu-Law.
|
|
In Europe and other areas of the world, the formula is slight different
|
|
and is called A-Law. Although the two laws differ only slightly, they
|
|
are incompatible. Mu-Law hardware cannot be used in conjunction with
|
|
A-Law hardware.
|
|
|
|
|
|
Step 3 - Encoding
|
|
|
|
Encoding converts the 256 possible numeric amplitude voltage levels into
|
|
binary 8-bit digital codes. The number 256 was not arrived at
|
|
accidentally. The reason there are 256 available amplitude levels is
|
|
that an 8-bit code contains 256 (28) possible combinations of 1s and 0s.
|
|
These codes are the final product of Pulse Codes Modulation (PCM) and
|
|
are ready for digital transmission.
|
|
|
|
PCM only provides 256 unique pitches and volumes. Every sound that is
|
|
heard over a phone is one of these 256 possible sounds.
|
|
|
|
Digital-Analog Conversion
|
|
|
|
After the digital bit stream is transmitted, it must be convert back to
|
|
an analog waveform to be audible to the human ear. This process is
|
|
called Digital-Analog conversion and is essentially the reverse of PCM.
|
|
|
|
This conversion occurs in three steps.
|
|
|
|
Step 1 - Decoding
|
|
|
|
Decoding converts the 8-bit PCM code into PAM voltage levels.
|
|
|
|
Step 2 - Reconstruction
|
|
|
|
Reconstruction reads the converted voltage level and reproduces
|
|
the original analog wave
|
|
|
|
Step 3 - Filtering
|
|
|
|
The decoding process creates unwanted high frequency noise in the
|
|
4000 Hz - 8000 Hz range which is audible to the human ear. A
|
|
low-pass filter blocks all frequencies above one-half the sampling
|
|
rate, eliminating any frequencies above 4000 Hz.
|
|
|
|
|
|
.----------------------.
|
|
5 | Digital Transmission |
|
|
`----------------------'
|
|
|
|
Importance of Digital Transmission
|
|
|
|
Digital transmission is the movement of computer-encoded binary
|
|
information from one machine to another. Digital information can
|
|
represent voice, text, graphics, and video.
|
|
|
|
Digital communication is important because we use it everyday. You have
|
|
used digital communications if
|
|
|
|
- your credit card is scanned at the checkout line of a department
|
|
store.
|
|
|
|
- you withdraw money from an automated teller machine.
|
|
|
|
- you make an international call around the world.
|
|
|
|
There are a million ways digital communication affects us every day.
|
|
|
|
As computer technology advances, more and more of our lives are affected
|
|
by digital communication. A vast amount of digital information is
|
|
transmitted every second of every day. Our bank records, our tax
|
|
records, our purchasing records, and so much more is stored as digital
|
|
information and transferred whenever and wherever it is needed. It is
|
|
no exaggeration to say that digital communications will continue to
|
|
change our lives from now on.
|
|
|
|
|
|
Digital Voice Versus Digital Data
|
|
|
|
|
|
The difference between voice and non-voice data is this:
|
|
|
|
Voice transmission represents voice while data transmission
|
|
represents any non-voice information, such as text, graphics, or
|
|
video. Both can be transmitted in identical format--as digitized
|
|
binary digits
|
|
|
|
In order to distinguish digital voice binary code from digital data,
|
|
since they both look like strings of 1s and 0s, you must know what the
|
|
binary codes represent.
|
|
|
|
This leads us to another important distinction-- that between digital
|
|
transmission and data transmission. Although these two terms are often
|
|
confused, they are not the same thing.
|
|
|
|
Digital transmission describes the format of the electrical
|
|
signal--1s and 0s as opposed to analog waves.
|
|
|
|
Data transmission describes the type of information transmitted-
|
|
-text, graphics, or video as opposed to voice.
|
|
|
|
Basic Digital Terminology
|
|
|
|
A bit is the smallest unit of binary information--a "1" or a "0"
|
|
|
|
A byte is a "word" of 7 or 8 bits and can represent a unit of
|
|
information such as a letter, a digit, a punctuation mark, or a printing
|
|
character (such as a line space).
|
|
|
|
BPS (bits per second) or bit rate refers to the information transfer
|
|
rate-- the number of bits transmitted in one second. BPS commonly refers
|
|
to a transmission speed.
|
|
|
|
Example:
|
|
|
|
A device rated at 19,200 bps can process more information than one
|
|
rated at 2,400 bps. As a matter of fact, eight times more. Bps
|
|
provides a simple quantifiable means of measuring the amount of
|
|
information transferred in one second.
|
|
|
|
Bits per second is related to throughput. Throughput is the amount of
|
|
digital data a machine or system can process. One might say a machine
|
|
has a "high throughput," meaning that it can process a lot of information.
|
|
|
|
|
|
Digital Data Transmission
|
|
|
|
|
|
Data communications is made up of three separate parts:
|
|
|
|
1. Data Terminal Equipment (DTE) is any digital (binary code) device,
|
|
such as a computer, a printer, or a digital fax.
|
|
|
|
2. Data Communications Equipment (DCE) are devices that establish,
|
|
maintain, and terminate a connection between a DTE and a facility.
|
|
They are used to manipulate the signal to prepare it for
|
|
transmission. An example of DCE is a modem.
|
|
|
|
3. The transmission path is the communication facility linking DCEs
|
|
and DTEs.
|
|
|
|
|
|
The Importance of Modems
|
|
|
|
|
|
A pair of modems is required for most DTE-to-DTE transmissions made over
|
|
the public network.
|
|
|
|
The function of a modem is similar to the function of a codec, but in
|
|
reverse. Codecs convert information that was originally in analog form
|
|
(such as voice) into digital form to transmit it over digital
|
|
facilities. Modems do the opposite. They convert digital signals to
|
|
analog to transmit them over analog facilities.
|
|
|
|
It continues to be necessary to convert analog signals to digital and
|
|
then back again because the transmission that travels between telephone
|
|
company COs is usually over digital facilities. The digital signals
|
|
travel from one telephone company Central Office to another over high
|
|
capacity digital circuits. Digital transmission is so superior to
|
|
analog transmission that it is worth the time and expense of converting
|
|
the analog signals to digital signals.
|
|
|
|
Since computers communicate digitally, and most CO-to-CO facilities are
|
|
digital, why then is it necessary to convert computer-generated digital
|
|
data signals to analog before transmitting them?
|
|
|
|
The answer is simple. Most lines from a local Central Office to a
|
|
customer's residence or business (called the local loop) are still
|
|
analog because for many years, the phone company has been installing
|
|
analog lines into homes and businesses. Only very recently have digital
|
|
lines begun to terminate at the end user's premises.
|
|
|
|
It is one thing to convert a telephone company switch from analog to
|
|
digital. It is quite another to rewire millions of individual customer
|
|
sites, each one requiring on-site technician service. This would
|
|
require a massive effort that no institution or even industry could
|
|
afford to do all at one time.
|
|
|
|
In most cases, therefore, we are left with a public network that is part
|
|
analog and part digital. We must, therefore, be prepared to convert
|
|
analog to digital and digital to analog.
|
|
|
|
|
|
Modulation/Demodulation
|
|
|
|
|
|
To transmit data from one DCE to another, a modem is required when any
|
|
portion of the transmitting facility is analog. The modem (modulater/
|
|
demodulater) modulates and demodulates digital signals for
|
|
transmission over analog lines. Modulation means "changing the
|
|
signals." The digital signals are changed to analog, transmitted, and
|
|
then changed back to digital at the receiving end.
|
|
|
|
Modems always come in pairs-- one at the sending end and one at the
|
|
receiving end. Their transmission rates vary from 50 bps to 56 Kbps
|
|
(Kilobits per second).
|
|
|
|
|
|
Synchronous Versus Asynchronous
|
|
|
|
|
|
There are two ways digital data can be transmitted:
|
|
|
|
Asynchronous transmission sends data one 8-bit character at a time. For
|
|
example, typing on a computer sends data from the keyboard to the
|
|
processor of the computer one character at a time. Start and stop bits
|
|
attach to the beginning and end of each character to alert the receiving
|
|
device of incoming information. In asynchronous transmission, there is
|
|
no need for synchronization. The keyboard will send the data to the
|
|
processor at the rate the characters are typed. Most modems transmit
|
|
asynchronously.
|
|
|
|
Synchronous transmission is a method of sending large blocks of data at
|
|
fixed intervals of time. The two endpoints synchronize their clocking
|
|
mechanisms to prepare for transmission. The success of the transmission
|
|
depends on precise timing.
|
|
|
|
Synchronous transmission is preferable when a large amount of data must
|
|
be transmitted frequently. It is better suited for batch transmission
|
|
because it groups data into large blocks and sends them all at once.
|
|
|
|
The equipment need for synchronous transmission is more expensive than
|
|
for asynchronous transmission so a data traffic study must be made to
|
|
determine if the extra cost is justified. Asynchronous transmission is
|
|
more cost effective when data communication is light and infrequent.
|
|
|
|
|
|
Error Control
|
|
|
|
|
|
The purpose of error control is to detect and correct errors resulting
|
|
from data transmission.
|
|
|
|
There are several methods of performing error control. What most
|
|
methods have in common is the ability to add an error checking series of
|
|
bits at the end of a block of data that determines whether the data
|
|
arrived correctly. If the data arrived with errors, it will contact the
|
|
sending DTE and request the information be re-transmitted. Today's
|
|
sophisticated error checking methods are so reliable that, with the
|
|
appropriate equipment, it is possible to virtually guarantee that data
|
|
transmission will arrive error-free. There are almost no reported cases
|
|
of a character error in received faxes.
|
|
|
|
Error control is much more critical in data communication than in voice
|
|
communication because in voice communication, if one or two of the 8000
|
|
PCM signals per second arrive with an error, it will make almost no
|
|
difference to the quality of the voice representation received. But,
|
|
imagine the consequences of a bank making a funds transfer and
|
|
misplacing a decimal point on a large account.
|
|
|
|
|
|
|
|
.--------------.
|
|
6 | Multiplexing |
|
|
`--------------'
|
|
|
|
|
|
Function of Multiplexers
|
|
|
|
Analog and digital signals are carried between a sender and receiver
|
|
over transmission facilities. It costs money to transmit information
|
|
signals from Point A to Point B. It is, therefore, of prime importance
|
|
to budget conscious users to minimize transmission costs.
|
|
|
|
The primary function of multiplexers is to decrease network facility
|
|
line costs.
|
|
|
|
Multiplexing is a technique that combines many individual signals to
|
|
form a single composite signal. This allows the transmission of
|
|
multiple simultaneous calls over a single line. It would cost a lot
|
|
more money to have individual lines for each telephone than to multiplex
|
|
the signals and send them over a single line.
|
|
|
|
Typical transmission facilities in use today can transmit 24 to 30 calls
|
|
over one line. This represents a significant savings for the end user
|
|
as well as for commercial long distance and local distance carriers.
|
|
|
|
|
|
Bandwidth
|
|
|
|
The bandwidth of a transmission medium is a critical factor in
|
|
multiplexing. Bandwidth is the difference between the highest and lowest
|
|
frequencies in a given range. For example, the frequency range of the
|
|
human voice is between 300 Hz and 3300 Hz. Therefore, the voice
|
|
bandwidth is
|
|
|
|
3300 Hz - 300 Hz = 3000 Hz
|
|
|
|
We also refer to the bandwidth of a transmission medium. A transmission
|
|
medium can have a bandwidth of 9600 Hz. This means that it is capable
|
|
of transmitting a frequency range up to 9600 Hz. A medium with a large
|
|
bandwidth can transmit more information and be divided into more
|
|
channels than a medium with a small bandwidth.
|
|
|
|
We will investigate three different methods of multiplexing:
|
|
|
|
Frequency Division Multiplexing (FDM)
|
|
Time Division Multiplexing (TDM)
|
|
Statistical Time Division Multiplexing (STDM)
|
|
|
|
|
|
Frequency Division Multiplexing (FDM)
|
|
|
|
FDM is the oldest of the three methods of multiplexing. It splits up
|
|
the entire bandwidth of the transmission facility into multiple smaller
|
|
slices of bandwidth. For example, a facility with a bandwidth of 9600
|
|
Hz can be divided into four communications channels of 2400 Hz each.
|
|
Four simultaneous telephone conversations can therefore be active on the
|
|
same line.
|
|
|
|
Logically, the sum of the separate transmission rates cannot be more
|
|
than the total transmission rate of the transmission facility: the 9600
|
|
Hz facility could not be divided into five 2400 Hz channels because 5 x
|
|
2400 is greater than 9600.
|
|
|
|
Guard bands are narrow bandwidths (about 1000 Hz wide) between adjacent
|
|
information channels (called frequency banks) which reduce interference
|
|
between the channels.
|
|
|
|
The use of FDM has diminished in recent years, primarily because FDM is
|
|
limited to analog transmission, and a growing percentage of transmission
|
|
is digital.
|
|
|
|
|
|
Time Division Multiplexing (TDM)
|
|
|
|
|
|
Time division multiplexing has two main advantages over frequency
|
|
division multiplexing:
|
|
|
|
- It is more efficient
|
|
- It is capable of transmitting digital signals
|
|
|
|
Instead of the bandwidth of the facility being divided into frequency
|
|
segments, TDM divides the capacity of a transmission facility into short
|
|
time intervals called time slots.
|
|
|
|
TDM is slightly more difficult to conceptualize than FDM. An analogy
|
|
helps.
|
|
|
|
The problem is
|
|
|
|
We must transport the freight of five companies from New York to
|
|
San Francisco. Each company wants their freight to arrive on the
|
|
same day. We must be as fair as we can to prevent one company's
|
|
freight from arriving before another company's. The freight from
|
|
each company will fit into 10 boxcars so a total of 50 boxcars
|
|
must be sent. Essentially, there are three different ways we can
|
|
accomplish this.
|
|
|
|
1. We can rent five separate locomotives and rent five
|
|
separate railway tracks and send each company's freight on
|
|
its own line.
|
|
|
|
2. We can rent five separate locomotives, but only one track and
|
|
send five separate trains along one line.
|
|
|
|
3. We can join all the boxcars together and connect them to one
|
|
engine and send them over a single track.
|
|
|
|
Obviously the most cost effective solution is Number 3. It saves us
|
|
from renting four extra rail lines and four extra locomotives.
|
|
|
|
To distribute the freight evenly so that each company's freight arrives
|
|
at the same time, the could be placed in a pattern as illustrated below:
|
|
|
|
Company A + Company B + Company C + Company A + Company B + Company C . . .
|
|
|
|
At San Francisco, the boxcars would be reassembled into the original
|
|
groups of 10 for each company and delivered to their final destination.
|
|
|
|
This is exactly the principle behind TDM. Use one track (communication
|
|
channel), and alternate boxcars (pieces of information) from each
|
|
sending company (telephone or computer).
|
|
|
|
In other words, each individual sample of a voice or data conversation
|
|
is alternated with samples from different conversations and transmitted
|
|
over the same line.
|
|
|
|
Let's say we have four callers in Boston (1, 2, 3, and 4) who want to
|
|
speak with four callers in Seattle (A, B, C, and D). The task is to
|
|
transmit four separate voice conversations (the boxcars) over the same
|
|
line (the track).
|
|
|
|
The voice conversations are sampled by PCM. This breaks each
|
|
conversation into tiny 8-bit packets. For a brief moment, caller 1
|
|
sends a packet to receiver A. Then, caller 2 sends a packet to receiver
|
|
B-- and so on. The result is a steady stream of interleaved
|
|
packets-- just like our train example except the boxcars stretch all
|
|
across the country. Notice that every fourth packet is from the same
|
|
conversation. At the receiving end, the packets are reassembled and
|
|
sent to the appropriate receiver at the rate of 8000 samples per
|
|
seconds.
|
|
|
|
Remember that if the receiver hears the samples at the rate of 8000
|
|
times per second, it will result in good quality voice reproduction.
|
|
Therefore, the packets are transmitted fast enough so that every 1/8000
|
|
of a second, a packet from each send arrives at the appropriate
|
|
receiver. In other words, each conversation is connected 8000 times per
|
|
second-- enough to satisfy Nyquist's Theorem.
|
|
|
|
In FDM the circuit was divided into individual frequency channels for
|
|
use by each sender. In contrast, TDM divides the circuit into
|
|
individual time channels. For a brief moment, each sender is allocated
|
|
the entire bandwidth-- just enough time to send eight bits of
|
|
information.
|
|
|
|
|
|
TDM Time Slots
|
|
|
|
|
|
Because a version of the TDM process (called STDM) is the primary
|
|
switching technique in use today, it is important that this challenging
|
|
concept be presented as clearly and understandably as possible. Here is
|
|
a closer look at TDM, emphasizing the "T"--which stands for time.
|
|
|
|
Each transmitting device is allocated a time slot during which it is
|
|
permitted to transmit. If there are three transmitting devices, for
|
|
example, there will be three time slots. If there are four devices
|
|
there will be four time slots.
|
|
|
|
Two devices, one transmitting and one receiving, are interconnected by
|
|
assigning them to the same time slot of a circuit. This means that
|
|
during their momentary shared time slot, the transmitting device is able
|
|
to send a short burst of information (usually eight bits) to the
|
|
receiving device. During their time slot, they use the entire bandwidth
|
|
of the transmission facility but only for a short period of time. Then,
|
|
in sequence, the following transmitting devices are allocated time slots
|
|
during which they too use the whole bandwidth.
|
|
|
|
Clock A and Clock B at either end of the transmission must move
|
|
synchronously. They rotate in unison, each momentarily making contact
|
|
with the two synchronized devices (one sender and one receiver). For
|
|
precisely the same moment, Clock A will be in contact with Sender 1 and
|
|
Clock B will be in contact with Receiver 1, allowing one sample (8 bits)
|
|
of information to pass through. The they will both rotate so that clock
|
|
A comes into contact with Sender 2 and Clock B with Receiver 2. Again,
|
|
one sample of information will pass. This process is repeated for as
|
|
long as needed.
|
|
|
|
How fast must the clocking mechanism rotate? Again, the answer is
|
|
Nyquist's theorem. If a signal is sampled 8000 times per second, an
|
|
accurate representation of voice will result at the receiving end. The
|
|
same theory applies with TDM. If the clocking mechanism rotates 8000
|
|
times per second, the rate of transfer from each sender and receiver
|
|
must also be 8000 times per second. This is so because every revolution
|
|
of the two clocking mechanisms result in each input and output device
|
|
making contact once. TDM will not work if the clocking mechanism
|
|
synchronization is off.
|
|
|
|
Each group of bits from one rotation of the clocking mechanism is called
|
|
a frame. One method for maintaining synchronization is inserting a frame
|
|
bit at the end of each frame. The frame bit alerts the demultiplexer of
|
|
the end of a frame.
|
|
|
|
|
|
Statistical Time Division Multiplexing (STDM)
|
|
|
|
|
|
STDM is an advanced form of TDM and is the primary switching technique
|
|
is use now. The drawback of the TDM process is that if a device is not
|
|
currently transmitting, its time slot is left unused and is therefore
|
|
wasted.
|
|
|
|
In contrast, is STDM, carrying capacity is assigned dynamically. If a
|
|
device is not transmitting, its time slot can be used by the other
|
|
devices, speeding up their transmission. In other words, a time slot is
|
|
assigned to a device only if it has information to send. STDM
|
|
eliminates wasted carrying capacity.
|
|
|
|
|
|
|
|
.--------------------.
|
|
7 | Transmission Media |
|
|
`--------------------'
|
|
|
|
|
|
Voice and data information is represented by waveforms and transmitted
|
|
to a distant receiver. However, information does not just magically
|
|
route itself from Point A to Point B. It must follow some predetermined
|
|
path. This path is called a transmission medium, or sometimes a
|
|
transmission facility.
|
|
|
|
The type of transmission medium selected to join a sender and receiver
|
|
can have a huge effect on the quality, price, and success of a
|
|
transmission. Choosing the wrong medium can make the difference between
|
|
an efficient transmission and an inefficient transmission.
|
|
|
|
Efficient means choosing the most appropriate medium for a given
|
|
transmission. For example, the most efficient medium for transmitting a
|
|
normal call from your home to your neighbor is probably a simple pair of
|
|
copper wires. It is inexpensive and it gets the job done. But if we
|
|
were to transmit 2-way video teleconferencing from Bombay to Burbank,
|
|
one pair of wires might be the least efficient medium and get us into a
|
|
lot of trouble.
|
|
|
|
A company may buy all the right equipment and understand all the
|
|
fundamentals, but if they transmit over an inappropriate medium, they
|
|
would probably be better off delivering handwritten messages than trying
|
|
to use the phone.
|
|
|
|
There are a number of characteristics that determine the appropriateness
|
|
of each medium for particular applications:
|
|
|
|
- cost
|
|
- ease of installation
|
|
- capacity
|
|
- rate of error
|
|
|
|
In choosing a transmission medium, these and many other factors must be
|
|
taken into consideration.
|
|
|
|
|
|
Terminology
|
|
|
|
|
|
The transmission media used in telecommunications can be divided into
|
|
two major categories: conducted and radiated. Examples of conducted
|
|
media include copper wire, coaxial cable, and fiber optics. Radiated
|
|
media include microwave and satellite.
|
|
|
|
A circuit is a path over which information travels. All of the five
|
|
media serve as circuits to connect two or more devices.
|
|
|
|
A channel is a communication path within a circuit. A circuit can
|
|
contain one or more channels. Multiplexing divides one physical link
|
|
(circuit) into several communications paths (channels).
|
|
|
|
The bandwidth of a circuit is the range of frequencies it can carry.
|
|
The greater the range of frequencies, the more information can be
|
|
transmitted. Some transmission media have a greater bandwidth than
|
|
others and are therefore able to carry more traffic.
|
|
|
|
The bandwidth of a circuit is directly related to its capacity to carry
|
|
information.
|
|
|
|
Capacity is the amount of information that may pass through a circuit in
|
|
a given amount of time. A high capacity circuit has a large amount of
|
|
bandwidth-- a high range of frequencies-- and can therefore transmit a
|
|
lot of information.
|
|
|
|
Copper Cable
|
|
|
|
Copper cable has historically been the most common medium. It has been
|
|
around for many years and today is most prevalent in the local loop--the
|
|
connection between a residence or business and the local telephone
|
|
company.
|
|
|
|
Copper cables are typically insulated and twisted in pairs to minimize
|
|
interference and signal distortion between adjacent pairs. Twisting the
|
|
wires into pairs results in better quality sound which is able to travel
|
|
a greater distance.
|
|
|
|
Shielded twisted pair is copper cable specially insulated to reduce the
|
|
high error rate associated with copper transmission by significantly
|
|
reducing attenuation and noise.
|
|
|
|
Copper cable transmission requires signal amplification approximately
|
|
every 1800 meters due to attenuation.
|
|
|
|
Advantages of Copper Cable
|
|
|
|
There is plenty of it and its price is relatively low.
|
|
|
|
Installation of copper cable is relatively easy and inexpensive.
|
|
|
|
|
|
Disadvantages of Copper Cable
|
|
|
|
Copper has a high error rate.
|
|
|
|
Copper cable is more susceptible to electromagnetic interference (EMI) and
|
|
radio frequency interference (RFI) than other media. These effects can
|
|
produce noise and interfere with transmission.
|
|
|
|
Copper cable has limited bandwidth and limited transmission capacity.
|
|
|
|
The frequency spectrum range (bandwidth) of copper cable is relatively low
|
|
-- approximately one megahertz (one million Hz). Copper circuits can be
|
|
divided into fewer channels and carry less information than the other media.
|
|
|
|
|
|
Typical Applications of Copper Cable
|
|
|
|
Residential lines from homes to the local CO (called the local loop).
|
|
|
|
Lines from business telephone stations to an internal PBX.
|
|
|
|
Coaxial Cable
|
|
|
|
Coaxial cable was developed to provide a more effective way to isolate
|
|
wires from outside influence, as well as offering greater capacity and
|
|
bandwidth than copper cable.
|
|
|
|
Coaxial cable is composed of a central conductor wire surrounded by
|
|
insulation, a shielding layer and an outer jacket.
|
|
|
|
Coaxial cable requires signal amplification approximately every 2000
|
|
meters.
|
|
|
|
|
|
Advantages of Coaxial Cable
|
|
|
|
Coaxial cable has higher bandwidth and greater channel capacity than
|
|
copper wire. It can transmit more information over more channels than
|
|
copper can.
|
|
|
|
Coaxial cable has lower error rates. Because of its greater insulation,
|
|
coaxial is less affected by distortion, noise, crosstalk (conversations
|
|
from adjacent lines), and other signal impairments.
|
|
|
|
Coaxial cable has larger spacing between amplifiers.
|
|
|
|
Disadvantages of Coaxial Cable
|
|
|
|
Coaxial cable has high installation costs. It is thicker and
|
|
less flexible and is more difficult to work with than copper wire.
|
|
|
|
Coaxial cable is more expensive per foot than copper cable.
|
|
|
|
|
|
Typical Applications
|
|
|
|
- Data networks
|
|
|
|
- Long distance networks
|
|
|
|
- CO-to-CO connections
|
|
|
|
Microwave
|
|
|
|
For transmission by microwave, electrical or light signals must be
|
|
transformed into high-frequency radio waves. Microwave radio transmits
|
|
at the high end of the frequency spectrum --between one gigahertz (one
|
|
billion Hz) and 30 GHz.
|
|
|
|
Signals are transmitted through the atmosphere by directly aiming one
|
|
dish at another. A clear line-of-sight must exist between the
|
|
transmitting and receiving dishes because microwave travels in a
|
|
straight line. Due to the curvature of the earth, microwave stations
|
|
are spaced between 30 and 60 kilometers apart.
|
|
|
|
To compensate for attenuation, each tower is equipped with amplifiers
|
|
(for analog transmission) or repeaters (for digital transmission) to
|
|
boost the signal.
|
|
|
|
Before the introduction of fiber optic cable in 1984, microwave served
|
|
as the primary alternative to coaxial cable for the public telephone
|
|
companies.
|
|
|
|
|
|
Advantages of Microwave
|
|
|
|
|
|
Microwave has high capacity. Microwave transmission offers greater
|
|
bandwidth than copper or coaxial cable resulting in higher transmission
|
|
rates and more voice channels.
|
|
|
|
Microwave has low error rates.
|
|
|
|
Microwave systems can be installed and taken down quickly and inexpensively.
|
|
They can be efficiently allocated to the point of greatest need in a
|
|
network. Microwave is often used in rural areas because the microwave
|
|
dishes can be loaded on trucks, moved to the desired location, and
|
|
installed quickly.
|
|
|
|
Microwave requires very little power to send signals from dish to dish
|
|
because transmission does not spread out into the atmosphere. Instead
|
|
it travels along a straight path toward the next tower.
|
|
|
|
Microwave has a low Mean Time Between Failures (MTBF) of 100,000
|
|
hours-- or only six minutes of down time per year.
|
|
|
|
Microwave is good for bypassing inconvenient terrain such as mountains
|
|
and bodies of water.
|
|
|
|
Disadvantages of Microwave
|
|
|
|
|
|
Microwave is susceptible to environmental distortions. Factors such as
|
|
rain, snow, and heat can cause the microwave beam to bend and vary.
|
|
This affects signal quality.
|
|
|
|
Microwave dishes must be focused in a straight line-of-sight. This can
|
|
present a problem over certain terrain or in congested cities.
|
|
Temporary physical line-of-sight interruptions, such as a bird or plane
|
|
flying through the signal pathway, can result in a disruption of
|
|
signals.
|
|
|
|
Microwave usage must be registered with appropriate regulatory agencies.
|
|
These agencies monitor and allocate frequency assignments to prevent
|
|
systems from interfering with each other.
|
|
|
|
Extensive use of microwave in many busy metropolitan areas has filled up
|
|
the airwaves, limiting the availability of frequencies.
|
|
|
|
|
|
Typical Applications
|
|
|
|
- Private networks
|
|
|
|
- Long distance networks
|
|
|
|
|
|
Satellite
|
|
|
|
|
|
Satellite communication is a fast growing segment of the
|
|
telecommunications market because it provides reliable, high capacity
|
|
circuits.
|
|
|
|
In most respects, satellite communication is similar to microwave
|
|
communication. Both use the same very high frequency (VHF) radio waves
|
|
and both require line-of-sight transmission. A satellite performs
|
|
essentially the same function as a microwave tower.
|
|
|
|
However, satellites are positioned 36,000 kilometers above the earth in
|
|
a geosynchronous orbit, This means they remain stationary relative to a
|
|
given position on the surface of earth.
|
|
|
|
Another difference between microwave and satellite communications is
|
|
their transmission signal methods. Microwave uses only one frequency to
|
|
send and receive messages. Satellites use two different
|
|
frequencies--one for the uplink and one for the downlink.
|
|
|
|
A device called a transponder is carried onboard the satellite. It
|
|
receives an uplink signal beam from a terrestrial microwave dish,
|
|
amplifies (analog) or regenerates (digital) the signal, then retransmits
|
|
a downlink signal beam to the destination microwave dish on the earth.
|
|
Today's satellites have up to 48 transponders, each with a capacity
|
|
greater than 100 Mbps.
|
|
|
|
Because of the long distance traveled, there is a propagation delay of
|
|
1/2 second inherent in satellite communication. Propagation delay is
|
|
noticeable in phone conversations and can be disastrous to data
|
|
communication.
|
|
|
|
A unique advantage of satellite communication is that transmission cost
|
|
is not distance sensitive. It costs the same to send a message across
|
|
the street as around the world.
|
|
|
|
Another unique characteristic is the ability to provide
|
|
point-to-multipoint transmission. The area of the surface of the earth
|
|
where the downlinked satellite signals can be received is called its
|
|
footprint. Information uplinked from the earth can be broadcast and
|
|
retransmitted to any number of receiving dishes within the satellite's
|
|
footprint. Television broadcast is a common application of
|
|
point-to-multipoint transmission.
|
|
|
|
|
|
Advantages of Satellite Transmission
|
|
|
|
|
|
Satellite transmission provides access to wide geographical areas (up to the
|
|
size of the satellite's footprint), point-to-multipoint broadcasting, a large
|
|
bandwidth, and is very reliable.
|
|
|
|
|
|
Disadvantages of Satellite Transmission
|
|
|
|
|
|
Problems associated with satellite transmission include: propagation delay,
|
|
licensing requirement by regulatory agencies security issue concerning the
|
|
broadcast nature of satellite transmission. Undesired parties within a
|
|
satellites footprint may illicitly receive downlink transmission.
|
|
|
|
Installation requires a satellite in orbit.
|
|
|
|
|
|
Fiber Optics
|
|
|
|
|
|
Fiber optics is the most recently developed transmission medium. It
|
|
represents an enormous step forward in transmission capacity. A recent
|
|
test reported transmission rates of 350 Gbps (350 billion bits), enough
|
|
bandwidth to support millions of voice calls. Furthermore, a recently
|
|
performed record- setting experiment transmitted signals 10,000 Km
|
|
without the use of repeaters, although in practice 80 to 300 Km is the
|
|
norm. Recall the need for repeaters every kilometer or so with copper
|
|
wire and coaxial.
|
|
|
|
Fiber optics communication uses the frequencies of light to send
|
|
signals. A device called a modulator converts electrical analog or
|
|
digital signals into light pulses. A light source pulses light on and
|
|
off billions and even trillions of times per second (similar to a
|
|
flashlight turned on and off-- only faster). These pulses of light are
|
|
translated into binary code. The positive light pulse represents 1; a
|
|
negative light pulse (no light) represents 0. Fiber optics is digital
|
|
in nature.
|
|
|
|
The light is then transmitted along a glass or plastic fiber about the
|
|
size of a human hair. At the receiving end, the light pulses are
|
|
detected and converted back to electrical signals by photoelectric
|
|
diodes.
|
|
|
|
Advantages of Fiber Optics
|
|
|
|
Fiber optics has an extremely high bandwidth. In fact, fiber optic
|
|
bandwidth is almost infinite, limited only by the ability of engineers
|
|
to increase the frequency of the pulses of light. Current technology
|
|
achieves a frequency of 100 terahertz (one million billion).
|
|
|
|
Fiber optics is not subject to interference or electromagnetic
|
|
impairments as are the other media.
|
|
|
|
Fiber optics has an extremely low error rate-- approximately one error
|
|
per 1,000,000,000,000.
|
|
|
|
Fiber optics has a low energy loss translating into fewer
|
|
repeaters/regenerators per long distance transmission.
|
|
|
|
Fiber is a glass and glass is made of sand. There will never by a
|
|
shortage of raw material for fiber.
|
|
|
|
|
|
Disadvantages of Fiber Optics
|
|
|
|
|
|
Installation costs are high for a fiber optic system. Currently it
|
|
costs approximately $41,000 per km to install a fiber optic system. The
|
|
expense of laying fiber is primarily due to the high cost of splicing
|
|
and joining fiber. The cost will almost certainly decrease dramatically
|
|
as less expensive methods of splicing and joining fiber are introduced.
|
|
|
|
A potential disadvantage of fiber optics results from its enormous
|
|
carrying capacity. Occasionally a farmer or construction worker will
|
|
dig into the earth and unintentionally split a fiber optic cable.
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Because the cable can carry so much information, an entire city could
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lose its telephone communication from just one minor mishap.
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.-----------.
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8 | Signaling |
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`-----------'
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Types of Signals
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When a subscriber picks up the phone to place a call, he dials digits to
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signal the network. The dialed digits request a circuit and tell the
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network where to route the call--a simple enough procedure for the
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caller. But in fact, it involves a highly sophisticated maze of
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signaling to and from switches and phones to route and monitor the call.
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Signaling functions can be divided into three main categories.
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Supervisory
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Supervisory signals indicate to the party being called and the CO
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the status of lines and trunks--whether they are idle, busy, or
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requesting service. The signals detect and initiate service on
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requesting lines and trunks. Signals are activated by changes in
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electrical state and are caused by events such as a telephone
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going on-hook or off-hook. Their second function is to process
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requests for telephone features such as call waiting.
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Addressing
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Addressing signals determine the destination of a call. They
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transmit routing information throughout the network. Two of the
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most important are
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Dial Pulse: These address signals are generated by alternately
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opening and closing a contact in a rotary phone
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through which direct current flows. The number of
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pulses corresponds to the number of the dialed
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digit.
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Tone: These address signals send a unique tone or
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combination of tones which correspond to the
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dialed digit.
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Alerting
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Alerting signals inform the subscriber of call processing
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conditions.. These signals include:
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Dial tone
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The phone ringing
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Flashing lights that substitute for phone ringing
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Busy signal
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Let's take a look at how signaling is used to set up a typical call over
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the public network.
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Step 1 - Caller A goes off-hook
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Step 2 - The CO detects a change in state in the subscriber's line.
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The CO responds by sending an alerting signal (dial tone) to
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caller A to announce that dialing may begin. The CO marks
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the calling line busy so that other subscribers can not call
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into it. If another subscriber attempts to phone caller A,
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he will get the alerting busy signal. Caller A dials the
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digits using tones from the keypad or dial pulses from a
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rotary phone.
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Step 3 - The dialed digits are sent as addressing signals from caller
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A to CO A
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Step 4 - CO A routes the addressing signals to CO B.
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Step 5 - Supervisory signals in CO B test caller B to determine if the
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line is free. The line is determined to be free.
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Step 6 - CO B sends alerting signals to caller B, which causes caller
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B's telephone to ring.
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This is an example of a local call which was not billed to the customer.
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If the call had been a billable, long distance call, it would have used
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a supervisory signal known as answer supervision. When the receiving
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end of a long distance call picks up, it sends a signal to its local CO.
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The CO then sends an answer supervision signal to the caller's CO
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telling it that the phone was picked up and it is time to begin billing.
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Where on the Circuit Does Signaling Occur?
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There are only three places where signaling can occur:
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In-band means on the same circuit as voice, within the voice
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frequency range (between 300 and 3400 Hz).
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Out-of-band means on the same circuit as voice, outside of the
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voice frequency range (3400 - 3700 Hz).
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Common Channel Signaling (CCS) means signaling occurs on a
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completely separate circuit.
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The frequency range of human voice is approximately 0 - 4000 Hz.
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However, most voice signals fall in the area between 300 and 3400 Hz.
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Therefore, to save bandwidth, telephones only recognize signals between
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300 and 3400 Hz. It is conceivable that someone with an extremely high
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voice would have difficulty communicating over the telephone.
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In-band and Out-of-band
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In-band signaling (300 to 3400 Hz) can take the form of either a single
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frequency tone (SF signaling) of a combination of tones (Dual Tone
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Multifrequency - DTMF). DTMF is the familiar touch tone.
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Out-of-band signaling (3400 to 3700 Hz) is always single frequency
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(SF).
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In other words, using the frequency range from 300 to 3700 Hz, there are
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three methods of signaling.
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Method A: In-band (300 to 3400 Hz) by a single frequency
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(SF)
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Method B: In-band (300 to 3400 Hz) by multifrequencies
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(DTMF)
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Method C: Out-of-band (3400 to 3700 Hz) by a single
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frequency (SF)
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Single Frequency (SF) Signaling
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Methods A and C are examples of Single Frequency (SF) signaling. SF
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signaling is used to determine if the phone line is busy (supervision)
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and to convey dial pulses (addressing).
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Method A: In-band SF signaling uses a 2600 Hz tone which is carried
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over the frequency bandwidth of voice (remember the frequency
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bandwidth of voice is between 300 and 3300 Hz), within the
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speech path. So as not to interfere with speech, it is
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present before the call but is removed once the circuit is
|
|
seized and speech begins. After the conversation is over, it
|
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may resume signaling. It does not, however, signal during
|
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the call because it would interfere with voice which also may
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transmit at 2600 Hz. Special equipment prevents occasional
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2600 Hz speech frequencies from accidentally setting off
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signals.
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Method C: To improve signaling performance, SF out-of-band signaling
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was developed. It uses frequencies above the voice frequency
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range (within the 3400 to 3700 Hz bandwidth) to transmit
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signals.
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The problem with Methods A and C is that they are easily susceptible to
|
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fraud. In the late 1960s, one of the most popular breakfast cereals in
|
|
America had a promotion in which they packaged millions of children's
|
|
whistles, one in each specially marked box. Never did General Mills,
|
|
the producer of the cereal, anticipate the fraud they would be party to.
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|
It turned out that the whistles emitted a pure 2600 Hz tone, exactly the
|
|
tone used in Method A. It did not take long for hackers to discover
|
|
that if they blew the whistles into the phones while making a long
|
|
distance phone call, it tricked the telephone company billing equipment
|
|
and no charge was made.
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This trick grew into its own little cottage industry, culminating in the
|
|
infamous mass produced Blue Boxes which played tones that fooled
|
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telephone billing equipment out of millions of dollars.
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Method B: DTMF was introduced to overcome this fraud, as well as to
|
|
provide better signaling service to the customer. Instead of
|
|
producing just one signaling frequency, DTMF transmits
|
|
numerical address information from a phone by sending a
|
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combination of two frequencies, one high and one low, to
|
|
represent each number/letter and * and # on the dial pad.
|
|
The usable tones are located in the center of the voice
|
|
communication frequencies to minimize the effects of
|
|
distortion.
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Drawbacks to SF and DTMF Signaling
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There are drawbacks to both SF and DTMF signaling that are promoting
|
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their replacement in long distance toll circuits. The most important is
|
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that these signals consume time on the circuit while producing no
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revenues. Every electrical impulse, be it a voice conversation or
|
|
signaling information, consumes circuit time. Voice conversations are
|
|
billable. Signaling is not. Therefore, it is in the best interest of
|
|
the phone carriers to minimize signaling.
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Unfortunately, almost half of all toll calls are not completed because
|
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the called party is busy, not available or because of CO blockage.
|
|
Nevertheless, signals must be generated to attempt to set up, then take
|
|
down the call. Signals are generated but no revenue is produced. For
|
|
incompleted calls, these signals compete with revenue producing signals
|
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(whose calls were completed) for scarce circuit resources.
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CCS introduced several benefits to the public network:
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. Signaling information was removed from the voice channel, so
|
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control information could travel at the same time as voice
|
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without taking up valuable bandwidth from the voice channel.
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. CCS sets up calls faster, reducing signaling time and freeing
|
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up scarce resources.
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. It cost less than conventional signaling.
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. It improves network performance.
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. It reduces fraud.
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Signaling System 7 (SS7)
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|
|
Today the major long distance carriers use a version of CCS called
|
|
Signaling System 7 (SS7). It is a standard protocol developed by the
|
|
CCITT, a body which establishes international standards.
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|
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Common Channel Signaling (CCS)
|
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|
|
Common Channel Signaling (CCS) is a radical departure from traditional
|
|
signaling methods. It transmits signals over a completely different
|
|
circuit than the voice information. The signals from hundreds or
|
|
thousands of voice conversations are carried over a single common
|
|
channel.
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|
|
Introduced in the mid-1970s CCS uses a separate signaling network to
|
|
transmit call setup, billing, and supervisory information. Instead of
|
|
sending signals over the same communication paths as voice or data, CCS
|
|
employs a full network dedicated to signaling alone.
|
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|
|
Loop Start Versus Ground Start Signaling
|
|
|
|
Establishing an electrical current connection with a CO can be done in
|
|
several different ways. Here are a few of the possibilities
|
|
|
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|
|
Loop Start
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|
|
Inside of the CO, there is a powerful, central battery that provides
|
|
current to all subscribers. Loop start is a method of establishing the
|
|
flow of current from the CO to a subscriber's phone.
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|
|
The two main components of a loop start configuration are
|
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|
|
The tip (also called the A line) is the portion of the line loop
|
|
between the CO and the subscriber's phone that is connected to the
|
|
positive, grounded side of the battery.
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|
|
The ring (also called the B line) is the portion of the line loop
|
|
between the CO and the subscriber's phone that is connected to the
|
|
negative, ungrounded side of the battery.
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|
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|
|
To establish a loop start connection with the CO, a subscriber goes
|
|
off-hook. This closes a direct current (DC) path between the tip and
|
|
ring and allows the current to flow in a loop from the CO battery to the
|
|
subscriber and back to the battery. Once the current is flowing, the CO
|
|
is capable of sending alerting signals (dial tone) to the subscriber to
|
|
begin a connection.
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|
|
|
The problem with loop start signaling is a phenomenon called glare that
|
|
occurs in trunks between a CO and a PBX. When a call comes into a PBX
|
|
from CO trunk, the only way the PBX knows that the trunk circuit is busy
|
|
is the ringing signal sent from the CO.
|
|
|
|
Unfortunately the ringing signal is transmitted at six second intervals.
|
|
For up to six seconds at a time, the PBX does not know there is a call
|
|
on that circuit. If an internal PBX caller wishes to make an outgoing
|
|
call, the PBX may seize the busy trunk call at the same time. The
|
|
result is confused users on either end of the line, and the abandonment
|
|
of both calls.
|
|
|
|
Ground Start
|
|
|
|
Ground start signaling overcomes glare by immediately engaging a circuit
|
|
seize signal on the busy trunk. The signal alerts the PBX that the
|
|
circuit is occupied with an incoming call and cannot be used for an
|
|
outgoing call.
|
|
|
|
Ground start is achieved by the CO by grounding the tip side of the line
|
|
immediately upon seizure by an incoming call. The PBX detects the
|
|
grounded tip and is alerted not to seize this circuit for an outgoing
|
|
call, even before ringing begins.
|
|
|
|
Because ground start is so effective at overcoming glare, it is commonly
|
|
used in trunks between the CO and a PBX.
|
|
|
|
|
|
E & M
|
|
|
|
E & M signaling is used in tie lines which connect two private telephone
|
|
switches. In E & M signaling, information is transmitted from one
|
|
switch to another over two pairs of wires. Voice information is sent
|
|
over the first pair, just as it would be in a Loop Start or Ground Start
|
|
trunk. However, instead of sending the signaling information over the
|
|
same pair of wires, it is sent over the second pair of wires.
|
|
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