I-frame control field format:
8 7 6 5 4 3 2 1
+--------------------------+---+
octet | N(S) | | I - format
4 | | 0 | information transfer
+--------------------------+---+
octet | N(R) | | N(S) Transmitter send sequence number
5 | | P | N(R) Transmitter receive sequence number
+--------------------------+---+ P/F Poll bit when issued as a command, final bit when issued
as a response
The general format of the DSS1 Basic Call Control:
+------------------------------+ <- The start of the information field
octet | Protocol discriminator |
6 |0 0 0 0 1 0 0 0 | message delimiter
+------------------------------+
| Call reference | the call identifier
+--+---------------------------+
|0 | Message type | the message identification code
+--+---------------------------+
| Other Information elements |
The first three information elements are mandatory in all messages.
The number of other information elements are variable in the messages.
The other information elements are mandatory or optional.