Link Access Procedure, Channel D (LAPD)

Link Access Procedures on the D channel (LAPD), specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.

It is heavily based on HDLC.


History of HDLC and LAP

Protocol dependencies

  • ISDN: Typically, LAPD uses ISDN as its transport mechanism.

  • RTP: Wireshark also supports decoding LAPD frames transported via RTP payload

Example traffic



The LAPD dissector is fully functional.

Preference Settings

  • Use GSM SAPI values. Default OFF.
  • RTP payload type for embedded LAPD. Default 0, that means LAPD dissection in RTP packets is disabled.

Example capture file

Display Filter

A complete list of LAPD display filter fields can be found in the display filter reference

Show only the LAPD based traffic:


Capture Filter

You cannot directly filter LAPD protocols while capturing. XXX - is this true?

External links


Imported from on 2020-08-11 23:15:55 UTC