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.
|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.|
The call reference value may take any of the following formats.
|Flag||Optional call reference|
|Optional extended call reference|
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.
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).
Call Reference values are unique for any given user-network interface. They are not significant on an end-to-end basis.
The call originator (either user or network side) sets flag = 0, the other end will always set flag = 1.
Single octet call reference values lie in the range 1 to 127.
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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