Tech Info - 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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