The purpose of the protocol discriminator is to distinguish messages for user-network call control from other messages(to be defined). It also distinguishes messages of this Standard from those OSI network layer protocol units which are coded to other CCITT Recommendations and other standards.
Note - A protocol discriminator field is also included in the User-user information element to indicate the user protocol within the user information; however, the coding of the protocol discriminator in this case is shown in 4.5.30.
The protocol discriminator is the first part of every message. The protocol discriminator is coded according to Table 4-1.
Bits |
||||||||
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
Octet |
Q.931/I.451 user-network call control messages |
||||||||
0 |
0 |
0 |
0 |
1 |
0 |
0 |
0 |
1 |
Protocol discriminator |
||||||||
Protocol discriminator
Protocol discriminator
8 7 6 5 4 3 2 1 |
|
0 0 0 0 0 0 0 0 |
Assigned in subclause 4.5.30; not available for use in the message protocol discriminator |
0 0 0 0 1 0 0 0 |
Recommendation Q.931/I.451 user-network call control messages |
0 0 0 0 1 0 0 1 |
Recommendation Q.2931 user-network call control messages |
0 0 0 1 0 0 0 0 |
Reserved for other network layer or layer 3 protocols, including Recommendation X.25 [5] (Note) |
0 1 0 0 0 0 0 0 |
National use |
0 1 0 1 0 0 0 0 |
Reserved for other network layer or layer 3 protocols, including Recommendation X.25 (Note) |
All other values are reserved. |
|
Note - These values are reserved to discriminate these protocol discriminators from the first octet of a Recommendation X.25 packet including general format identifier. |
|