X21 » History » Version 3
laforge, 03/04/2023 02:36 PM
1 | 1 | laforge | h1. X.21 |
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3 | X.21 is a specification for a differential, serial interface introduced in the mid-1970 by ITU-T. |
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5 | It was used as a digital signaling interface between (typically customer-owned) [[DTE]] and (typically provider-owned) [[DCE]]. |
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7 | X.21 was primarily used in the [[CSPDN__Datex-L|CSPDN]] networks such as DATEX-L. |
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9 | 2 | laforge | Key aspects of X.21: |
10 | * all signals differential |
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11 | * not just data (Rx/Tx), but also bit-clock and optionally byte clock |
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12 | * state machines and signaling for call control |
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13 | ** outbound calls |
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14 | ** progress signals |
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15 | ** inbound calls (+accept/reject) |
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17 | 1 | laforge | h2. Types of Signals |
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19 | Note: Each of those signals actually has two physical circuits as the signals are all differential. |
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21 | |_.Name|_.Direction|_.Description| |
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22 | |Transmit Data|DTE -> DCE|Data transmitted by the DTE| |
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23 | |Control|DTE -> DCE|Indicates the meaning of the data sent on _Transmit Data_| |
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24 | |Receive Data|DTE <- DCE|Data received by the DTE| |
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25 | |Indication|DTE <- DCE|Indicates the type of data received on _Receive Data_| |
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26 | |Signal Element Timing|DTE <- DCE|The bit-clock| |
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27 | |Byte Timing|DTE <- DCE|The byte-clock (optional)| |
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28 | 2 | laforge | |
29 | h2. Example sequence of events for outbound call |
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31 | !x21_figure_b1.png! |
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32 | 1 | laforge | |
33 | 3 | laforge | h2. Electical Signals / Coding |
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35 | h3. Data lines (R/T) |
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36 | |||
37 | |B > A|1| |
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38 | |B < A|0| |
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40 | h3. Control lines (I/C) |
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42 | |B > A|OFF| |
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43 | |B < A|ON| |
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45 | 1 | laforge | h2. External Links |
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47 | * https://en.wikipedia.org/wiki/X.21 |
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48 | * http://www.arcelect.com/X21_interface.htm |
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49 | * http://old.pinouts.ru/SerialPorts/x21_pinout.shtml |
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50 | * https://www.itu.int/rec/T-REC-X.21/recommendation.asp?lang=en&parent=T-REC-X.21-199209-I |