640 lines
30 KiB
Plaintext
640 lines
30 KiB
Plaintext
==Phrack Magazine==
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Volume Six, Issue Forty-Seven, File 19 of 22
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A Guide To British Telecom's Caller ID Service
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By DrB0B
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(DrB0b@grex.cyberspace.org)
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Introduction:
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Whilst caller ID services are old news to American readers, to UK phone-
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phreaks they are a new and potentially exciting addition to British-
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Telecom's network services. Many people will have already read articles
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describing CNID, almost invariably these articles have been based on systems
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utilizing Bellcore's CLASS signalling requirements, it should be
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noted that while BT's Caller Display System is also based on CLASS there
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are some significant technical differences. I have tried to make the
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information in this article as comprehensible as possible, unfortunately the
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telecommunications industry is one of the most jargon-infested industries in
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the world so if you have any questions about anything in this article don't
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hesitate to contact me at the above address, I'll do my best to help.
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BT hope to have their Caller Display Service available by November 1994.
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LATE NEWS: Today, Nov 1st 1994, BT announce that caller ID services would
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be withheld for a while longer as the public are too stupid to understand
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what it means, I swear I'm not making this up. According to BT newsline
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(0800 500005) "The public failed to comprehend that caller display services
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meant that caller number would be transmitted with every call, nor did they
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understand that CDS could be blocked on a per call basis by using the 141
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prefix, or on a per line basis by arrangement with BT. Go figure !
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New date for service launch is towards the end of November.
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(1) What is Calling Line Identification Presentation.
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When BT introduce their Caller Display Service over the analogue local
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access network the first service available will be Calling Line Identification
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Presentation (CLIP), this provides for the delivery of the callers
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number when a telephone call arrives, in the near future it will also
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provide the callers name. When the callers name or number cannot be delivered
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then one of two reasons for the failure will be displayed, (1) name or number
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not available (the caller has an unlisted number), or, (2) name and number
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withheld by customer (this is done by the caller dialling 141 before dialling
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the called number, this results in the message "CLI Withheld" being displayed
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on the recipients equipment). In addition to caller identification the CLIP
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service can also deliver network messages, the time, date, and, (optionally),
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some indication of call type.
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As noted above callers can choose to withhold CLI information by using the
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prefix 141, users should be aware that this has no effect on BT's ability
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to trace a call, the 141 prefix is a service activation code whilst call
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tracing is an operator function.
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(2) Some Necessary Definitions
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From here it gets a bit more complex, your mileage may vary. It's probably
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best if I define some of the terms to be used before going any further.
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Line Reversal
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The potential difference between the two wires of the exchange line (A+B)
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will always be equal to or greater than 15 volts. An incoming Caller
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Display message will be preceded by a polarity reversal between the two wires.
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Idle State Tone Alert Signal
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Signals sent in the idle state will be preceded by a Tone Alert signal and
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a Channel Seizure signal. Terminal equipment may recognize the Idle State
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Tone Alert Signal by the detection of both frequencies together, or by
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detection of a single (the lower) frequency. In the case of single
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frequency detection the recognition time should be not less than 30ms,
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if both frequencies are detected the recognition time can be reduced to not
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less than 20ms.
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Fig 1. The Idle State Tone Alert Signal
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--------------------------------------------------------------
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|Frequencies | 2130 hz and 2750 hz +/- 1% |
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--------------------------------------------------------------
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|Received Signal Level | -2dBV to -40 dBV |
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--------------------------------------------------------------
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|AC and DC load impedance | AC load is high impedance as |
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| | required by NET4 |
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--------------------------------------------------------------
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|Unwanted Signals | Total power of extraneous signals|
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| | in the voice band (300-3400hz) is|
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| | at least 20dB below the signal |
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| | levels |
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--------------------------------------------------------------
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|Duration | 88 to 110 msec |
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--------------------------------------------------------------
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Note: NET4 is European Telecommunications Standard ETS 300 001;
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Attachments to PSTN; general technical requirements for equipment
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connected to an analogue subscriber interface in the PSTN).
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DC Load
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NET4 requires that the total of terminal equipment on a line shall not
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draw in excess of 120 microA in the idle state. The Caller Data Service
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terminal equipment may, as an option, draw DC of up to 0.5 mA par device
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at 50 V line voltage, but only during CDS idle state, otherwise the
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conditions of NET4 apply.
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DC Wetting Pulse
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In order to improve reliability of idle state data reception (by reducing
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noise), it is mandatory that the terminal equipment shall draw a short
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pulse of current from the line by applying a resistive load for a
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specified time.
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(3) Signalling
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For an understanding of the processes involve we need to have some under-
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standing of the four layers used in Basic Mode communication. Basic Mode
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communication covers transmission of data between network and terminal
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equipment, either before ringing is applied or without any ringing,
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transmission is either down-stream (network to terminal equipment), or
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up-stream (terminal equipment to network).
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Physical Layer: This defines data symbol encoding and modulation, and
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analogue line conditions.
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Datalink Layer: This defines framing of messages for transmission and a
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simple error checking procedure.
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Presentation Layer: This defines how application-related information is
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assembled into a message.
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Application Layer: This defines the application that uses the signalling.
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In this case Calling Line Identity Presentation.
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Now we'll go into a little more detail about each of these layers.
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Physical Layer:
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Signalling may occur in either the idle state or loop state. We won't
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discuss loop state signalling here, as it's not pertinent at this stage.
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An incoming CDS call is indicated by a polarity reversal on the A and B
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wires, usually followed by ringing current applied to the B wire. The
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Terminal Equipment responds to the Idle State Tone Alert by drawing a DC
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wetting pulse and applying a DC load and an AC load. The DC wetting pulse
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is applied during the idle period following the end of the Idle State
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Tone Alert signal. The AC load is applied at the same time as the DC
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wetting pulse. It is removed after the end of the V.23 signals. The DC load
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is applied and removed at the same time as the AC load impedance. On removal
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of the DC and AC loads the CPE reverts to the idle state. For some
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applications the Channel Seizure may be delayed by up to 5 seconds,
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either or both silent periods may be extended in this case.
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If a terminal equipment loop state condition is detected the CDS message
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is aborted and the call presented as a non-CDS call. All data transmitted
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by the physical layer consists of 8-bit characters transmitted asynchronously
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preceded by one start-bit and followed by one stop bit. With the exception
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of the mark signal immediately following channel seizure there should be
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no more than 10 stop bits between characters.
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Values for octets are given in the following format:
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S2 M B7 B6 B5 B4 B3 B2 L S1
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(Order of bits S1 first S2 last)
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where S1 = start bit
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S2 = stop bit
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M = most significant bit
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L = least significant bit
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B* = bit numbers 2 to 7
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Octets are transmitted with most significant octet first.
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Datalink Layer:
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The datalink layer provides framing of data into packets that can be
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distinguished from noise, and has error detection in the form of a check-
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sum.
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Fig 2. Datalink Packet Format
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-------------------------------------------------------------
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|Channel |Mark |Message |Message |Message |Check- |
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|Seizure |Signal |Type |Length | |sum | | | | | | | |
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-------------------------------------------------------------
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^^^^^^^^^^
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Presentation
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Layer
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Analysis of the fields in a Datalink Packet:
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Channel Seizure
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The channel seizure consists of a continuous sequence of alternate 0 and 1
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bits at 1200 bits/s. The purpose of channel seizure is to minimize the possibility of noise mimicking a genuine carrier. The length of channel
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seizure as seen by terminal equipment is at least 96 bits (80 msec). It
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may be longer, up to 315 bits (262 msec)
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Mark Signal
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The mark signal seen by terminal equipment is at least 55 bits (45 msec)
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of continuous mark condition (equivalent to a series of stop bits, or no
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data being transmitted).
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Message Type
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The message type is a single binary byte. The value depends on the
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application.
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Message Length
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The message length is a single binary byte indicating the number of bytes in
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the message, excluding the message type, message length, and checksum bytes.
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This allows a message of between 0 and 255 bytes.
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Message
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The message consists of between 0 and 255 bytes, according to the message
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length field. This is the presentation layer message (explained later).
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Any 8-bit value may be sent, depending on the requirements of the
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presentation layer and the application.
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Checksum
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The checksum consists of a single byte equal to the two's complement sum
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of all bytes starting from the "message type" word up to the end of the
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message block. Carry from the most significant bit is ignored. The
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receiver must compute the 8-bit sum of all bytes starting from "message
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type" and including the checksum. The result must be zero or the message
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must be assumed to be corrupt.
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Presentation Layer:
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Fig 3. Presentation Layer Message format
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-------------------------------------------------------------------
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|Parameter|Parameter|Parameter| ... |Parameter|Parameter|Parameter|
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|Type |Length |Byte(s) | |Type |Length |Byte(s) |
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-------------------------------------------------------------------
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The fields Parameter Type, Length, and Byte, together describe one
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presentation layer parameter, and may be repeated.
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Parameter Type will be discussed more fully in the next section.
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Parameter Length is a single binary byte of a value between 0 and 255. In
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Basic Mode a complete message must be contained within a single datalink
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packet, this means that the total length of presentation layer parameters
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must not exceed 255 bytes.
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Parameter Byte(s) contains zero or more bytes of application related
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information. The information contained in this parameter should be en-
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coded in BT ISDN Character Set IA5 format.
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Parameter Type:
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There are eight parameter types associated with CLIP
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Fig 4. Parameter Type values
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| Parameter Type Value | Parameter Name |
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| 00010001 | Call Type |
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-------------------------------------------------------------
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| 00000001 | Time & Date |
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-------------------------------------------------------------
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| 00000010 | Calling line directory number (DN)|
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-------------------------------------------------------------
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| 00000011 | Called directory number |
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-------------------------------------------------------------
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| 00000100 | Reason for absence of DN |
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-------------------------------------------------------------
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| 00000111 | Caller name/text |
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-------------------------------------------------------------
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| 00001000 | Reason for absence of name |
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-------------------------------------------------------------
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| 00010011 | Network message system status |
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-------------------------------------------------------------
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The calling line directory number is the number of the line from which the
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call was made, or a substitute presentation number. The called directory
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number is the number that was called. This is of significance when the call
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has been diverted.
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There may be parameters of other types present. the call type parameter, if
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present will always be sent first, other parameters may be sent in any
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order. at least seven of these eight parameters must be recognized for the
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CLIP service (Called directory number is not necessary). Parameters may be
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sent with zero length. In such cases parameter length will be zero and the
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checksum will be correct. Parameters are usually encoded in IA5. The
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version used is a 7-bit code and is sent in 8-bit bytes with the most
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significant bit set to zero. Non-displayable characters (codes 0-32
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decimal) are not used. In the tables following byte number 1 is sent first
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followed by byte number 2 and so on.
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Call Type Parameter
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------------------------------------------------------
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| Byte Number| Contents |
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------------------------------------------------------
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| 1 | Call Type Parameter Type Code |
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| | (00010001) |
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| 2 | Parameter Length |
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| 3 | Call Type |
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------------------------------------------------------
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------------------------------------------------------
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| Call Type Encoding | Call Type |
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------------------------------------------------------
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| 00000001 | Voice Call |
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| 00000010 | ring-back-when-free-call |
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| 10000001 | message waiting call |
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------------------------------------------------------
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If the call type parameter is omitted then the call type is "voice call".
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Additional Call Types may be defined later. Other call types, ie FAX, will
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be used when they are available. The "message waiting" call type is used
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to give an indication of a new message from a specific caller.
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Time and Date Parameter
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The Time parameter indicates the date and time (+/- 1 minute) of the event
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associated with the supplementary information message. Where the call type
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has a value 127 (01111111) or less, then the time is the current time and
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can be used to set internal terminal equipment clocks and calendars. For
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a call of type "message waiting" the time and date refer to the time
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message was left or recovered. For other call types with value 128
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(10000000) or greater, the time and date may refer to some unspecified event
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and not necessarily current time.
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--------------------------------------------------------
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| Byte Number | Contents |
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--------------------------------------------------------
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| 1 | Time & Date parameter type code |
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| | (00000001) |
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| 2 | Parameter length (8) |
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| 3 | Month |
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| 4 | Month |
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| 5 | Day |
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| 6 | Day |
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| 7 | Hours |
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| 8 | Hours |
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| 9 | Minutes |
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| 10 | Minutes |
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--------------------------------------------------------
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Calling Line Directory Number Parameter
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The maximum length of number sent is 18 characters. The first digit sent is
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in byte 3. The Calling Line Directory Number is a number that may be used
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to call back the caller, or the same service. It may not be the directory
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number of the originating call, for example, an 0800 may be associated
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with the caller. Where an alternative to the directory number of the caller
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is sent this is known as a Presentation Number. There is no indication of
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which type of number is sent, this may change.
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If only a partial number is known then that partial number may be sent. This
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will be followed by a "-". For instance, where a call comes from outside the
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digital network the area code may still be sent and shown as:
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0171-250-
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or, (under the new national code) for an international call from France;
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00 33-
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assuming the new international access code of 00.
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---------------------------------------------------------
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| Byte Number | Contents |
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---------------------------------------------------------
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| 1 | Calling Line Directory Number |
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| | Parameter type code (00000010) |
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| 2 | Parameter length (n) |
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| 3 | First digit |
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| 4 | Second digit |
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| . | . |
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| . | . |
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|n+2 | nth digit |
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---------------------------------------------------------
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Reason for Absence of Directory Number Parameter
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------------------------------------------------------------
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| Byte Number | Contents |
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------------------------------------------------------------
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| 1 | Reason for Absence of DN parameter type |
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| | code (00000100) |
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| 2 | Parameter length (1) |
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| 3 | Reason |
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------------------------------------------------------------
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The reason will be one of the following BT IA5-encoded values
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"P" = "Number Withheld"
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"O" = "Number Unavailable"
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Called Directory Number Parameter
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The Called Directory Number is the telephone number used by the caller when
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making the call. The maximum length of characters sent is 18, the first digit
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of the number is sent in byte 3, the second in byte 4 and so on.
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---------------------------------------------------------
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| Byte Number | Contents |
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---------------------------------------------------------
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| 1 | Called Directory Number Parameter |
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| | type code (00000011) |
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| 2 | Parameter length (n) |
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| 3 | First digit |
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| 4 | Second digit |
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| . | . |
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| . | . |
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| n+2 | nth digit |
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---------------------------------------------------------
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Caller Name/Text parameter
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At the launch of the service the Caller Name will not be available, the
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parameter will contain text only.
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The Name/Text consists of between 1 and 20 BT-IA5 characters. The parameter
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may be used for other information when no name is available.
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---------------------------------------------------------
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| Byte Number | Contents |
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---------------------------------------------------------
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| 1 | Caller Name/Text Parameter type code |
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| | (00000111) |
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| 2 | Parameter length (n) |
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| 3 | First digit |
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| 4 | Second digit |
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| . | . |
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| . | . |
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| n+2 | nth digit |
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---------------------------------------------------------
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Reason for Absence of Name Parameter
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The reason will be one of the following;
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P "Name Withheld"; Caller has withheld delivery of name
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O "Name Unavailable"; The name is not available
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---------------------------------------------------------
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| Byte Number | Contents |
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---------------------------------------------------------
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| 1 | Reason for Absence of Name type |
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| | parameter (00001000) |
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| 2 | Parameter length (1) |
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| 3 | Reason |
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---------------------------------------------------------
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Network Message System Status Parameter
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The value of the Network Message System Status parameter is a binary
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encoded value indicating the number of messages waiting in the message
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system. 0 means no messages, 1 means one or an unspecified number, other
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values, up to 255, indicate that number of messages waiting.
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This parameter is not necessarily associated with a normal phone call, and
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will probably be sent as a no ring call.
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---------------------------------------------------------
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| Byte Number | Contents |
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---------------------------------------------------------
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| 1 | Network System Message Status |
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| | Parameter (00010011) |
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| 2 | Parameter length (1) |
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| 3 | Network System Message Status |
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---------------------------------------------------------
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Unless a Call Type parameter is also set, then any time parameter sent with
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the Network System Status parameter will indicate current clock time. This
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is to enable the terminal equipment to assume the time is current time and
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to set it's internal clock where no Call Type parameter is sent.
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(4) Message Length
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The longest CLIP message, excluding datalink layer information is currently
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64 bytes. This length is expected for call types "Voice", "Ring-back-when-
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free", "Message Waiting". In future there may be additional parameters that
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could extend message length, these will be sent after the parameters Call
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Type, caller number, name/text, reason for absence of name or number, and
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Network Message System Status.
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(5)Fig 5. Received Characteristics of V.23 Signals
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------------------------------------------------------------
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| Modulation | FSK |
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------------------------------------------------------------
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| Mark (Logic 1) | 1300 Hz +/- 1.5% |
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------------------------------------------------------------
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| Space (Logic 0) | 2100 Hz +/- 1.5% |
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------------------------------------------------------------
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| Received signal level | -8dBV to -40dBV |
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| for mark | |
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------------------------------------------------------------
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| Received signal level | -8dBV to -40dBV |
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| for space | |
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------------------------------------------------------------
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| Signal level | The received signal levels may |
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| differential | differ by up to 6 dB |
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------------------------------------------------------------
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| Unwanted signals | Total power of extraneous |
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| | signals in the voice band is at|
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| | least 20dB below the signal |
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| | levels |
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------------------------------------------------------------
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| AC & DC load impedance | AC load impedance is Zss (see |
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| | below) |
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| | DC load impedance has been de- |
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| | scribed above. |
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------------------------------------------------------------
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| Transmission rate | 1200 baud +/- 1% |
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------------------------------------------------------------
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| Data format | Serial binary asynchronous (1 |
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| | start bit first, then 8 data |
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| | bits with least significant |
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| | bit first, followed by 1 stop |
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| | bit minimum, up to 10 stop bits|
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| | maximum. Star bit 0, stop bit 0|
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------------------------------------------------------------
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(6)Fig 6. Zss
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Zss: a complex impedance nominally represented by the following network;
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139 nF
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----------------
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| |
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------ -------
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| | | | ------------
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| ---------------- | | |
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O----- -------- ----------O
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| | | |
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| --------------- | ------------
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| | | | 827 Ohms
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------ -------
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| |
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----------------
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1386 Ohms
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(7)Fig 7. BT IA5 alpha-numeric character set
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-----------------------------------------------------
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| B | b7 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 |
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-----------------------------------------------------
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| I | | | | | | | | | |
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-----------------------------------------------------
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| T | b6 | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 1 |
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-----------------------------------------------------
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| S | | | | | | | | | |
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-----------------------------------------------------
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| | b5 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 |
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---------------------------------------------------------
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| BITS | | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
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|b b b b | | | | | | | | | |
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|4 3 2 1 | | | | | | | | | |
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---------------------------------------------------------
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|0 0 0 0 | 0 |NUL |TC7 |SP | 0 | @ | P | ` | p |
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---------------------------------------------------------
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|0 0 0 1 | 1 |TC1 |DC1 | ! | 1 | A | Q | a | q |
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---------------------------------------------------------
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|0 0 1 0 | 2 |TC2 |DC2 | " | 2 | B | R | b | r |
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---------------------------------------------------------
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|0 0 1 1 | 3 |TC3 |DC3 | # | 3 | C | S | c | s |
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---------------------------------------------------------
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|0 1 0 0 | 4 |TC4 |DC4 | | 4 | D | T | d | t |
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---------------------------------------------------------
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|0 1 0 1 | 5 |TC5 |TC8 | % | 5 | E | U | e | u |
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---------------------------------------------------------
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|0 1 1 0 | 6 |TC6 |TC9 | & | 6 | F | V | f | v |
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---------------------------------------------------------
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|0 1 1 1 | 7 |BEL |TC10| ' | 7 | G | W | g | w |
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---------------------------------------------------------
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|1 0 0 0 | 8 |FE0 |CAN | ( | 8 | H | X | h | x |
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---------------------------------------------------------
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|1 0 0 1 | 9 |FE1 |EM | ) | 9 | I | Y | i | y |
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---------------------------------------------------------
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|1 0 1 0 | 10 |FE2 |SUB | * | : | J | Z | j | z |
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---------------------------------------------------------
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|1 0 1 1 | 11 |FE3 |ESC | + | ; | K | [ | k | { |
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---------------------------------------------------------
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|1 1 0 0 | 12 |FE4 |IS4 | , | < | L | \ | l | | |
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---------------------------------------------------------
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|1 1 0 1 | 13 |FE5 |IS3 | - | = | M | ] | m | } |
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---------------------------------------------------------
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|1 1 1 0 | 14 |SO |IS2 | . | > | N | ^ | n | ~ |
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---------------------------------------------------------
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|1 1 1 1 | 15 |SI |IS1 | / | ? | O | _ | o |DEL |
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---------------------------------------------------------
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Where;
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BEL = Bell
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CAN = Cancel
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DC = Device Control
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EM = End of Medium
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ESC = Escape
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FE = Format Effectors
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IS = Information Separator
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NUL = Null
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SI = Shift In
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SO = Shift Out
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SP = Space
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SUB = Substitute Character
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TC = Transmission Control
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Conclusion:
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My head hurts, I've been in front of this screen for eight hours, I started
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this because I was chucked out of the cinema for being drunk and disorderly
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and I'd nothing else to do, I've got through 2 packs of Marlboros, 1 bottle
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mad dog and a stack of telco manuals. Most of this has been lifted whole-
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sale from those manuals (in the great tradition of all p/h g-philes). I'm
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currently working on a round up of ISDN2 and ISDN30, a glossary for European
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phone phreaks (almost ready), and a technical description of British and
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Irish cellular communication systems. If anybody has any info to share on any
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of these things, or any questions they'd like answered (stick to the subject
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though, I don't know who killed Kennedy, #8^)), then get in touch with me at
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the above address or at any of a variety of boards.
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