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# ISDN user-network interface layer 3 specification for basic call control (Q.931)
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## History
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XXX - add a brief description of Q.931 history
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## Protocol dependencies
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Q.931 was originally used as the call control protocol for [ISDN](/ISDN). It is also used for VoIP; when used for VoIP, it's usually transported atop [TPKT](/TPKT) atop [TCP](/TCP).
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## Example traffic
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XXX - Add example traffic here (as plain text or Wireshark screenshot).
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## Wireshark
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The Q.931 dissector is fully functional.
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## Preference Settings
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- Make sure that preferences in [TCP](/TCP) and [TPKT](/TPKT) are set to allow reassembly of packets.
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- Whether to reassemble Q.931 messages spanning multiple TCP segments or not. (Default: yes)
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- Reassemble segmented Q.931 messages or not. (Default: yes)
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## Example capture file
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XXX - Add a simple example capture file to the [SampleCaptures](/SampleCaptures) page and link from here. Keep it short, it's also a good idea to gzip it to make it even smaller, as Wireshark can open gzipped files automatically.
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## Display Filter
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A complete list of Q.931 display filter fields can be found in the [display filter reference](http://www.wireshark.org/docs/dfref/q/q931.html)
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Show only the Q.931 based traffic:
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```
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q931
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```
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## Capture Filter
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You cannot directly filter Q.931 protocols while capturing. However, if you know the [TCP](/TCP) port used (see above), you can filter on that one.
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## External links
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- Q.931 is specified by ITU in [Q.931](http://www.itu.int/rec/T-REC-Q.931-199805-I).
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## Discussion
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---
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Imported from https://wiki.wireshark.org/Q.931 on 2020-08-11 23:23:44 UTC |