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Tech Stuff - Q.931 Packet Format

Generic Format

This is the generic format of a Q.931 packet carried within a LAPD (Q.921) frame.

Humble Pie: Until pointed out by Andreas Hagerer we had incorrectly stated that the call originator set the call reference flag = 1, the call originator sets the flag = 0 the other side sets the flag = 1. Thanks to Andreas for taking the time.

Name Octets Notes
Protocol Discriminator 1 Always 0x08 for ISDN
Call Reference 1,2 or 3 The call reference field.
Message Type 1 or 2 The message type value.
Information Elements various Information Elements (IEs) may or may not be present.

Call Reference

The call reference value may take any of the following formats.

8 7 6 5 4 3 2 1
0 0 0 0 CR length
Flag Optional call reference
Optional extended call reference

Notes

  1. Call Reference length = 0 (with no further octets) is called the NULL (or Dummy) Call Reference and is used to carry information of global significance for the line i.e. it is not relevant to a specific call.

  2. Custom LUCENT also uses an alternate format for a NULL call reference with a length of 1 and a call reference value = 0 (a.k.a. the Global call reference).

  3. Call Reference values are unique for any given user-network interface. They are not significant on an end-to-end basis.

  4. The call originator (either user or network side) sets flag = 0, the other end will always set flag = 1.

  5. Single octet call reference values lie in the range 1 to 127.

  6. Dual Octet call reference values lie in the range 1 to 32768 (and are typically only used in Primary Rate Interfaces).



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