Page 901 - 5G Basics - Core Network Aspects

Transport aspects 2

Upon entering the RQ mode, the message index shall initially be set to 0116 and shall be incremented by
one in every subsequent message. The index shall wrap around and set to 0116 after reaching the value of
FF16. The value 0016 has a special meaning, as described below, and shall be skipped. The index shall not be
incremented if an O/R-REPEAT_REQUEST message is received, i.e., if message is re-sent. The message index
of an O/R-ACK-SEG message shall be increased by one when a new segment is received.
The segmentation index shall be set to 1116 if the message is not segmented, and as specified in
clause 12.2.4.6 if the message is segmented. The segmentation index shall not be changed if the message
(segment) is re-sent.
The message index and segmentation index of the O/R-REPEAT_REQUEST message shall be set to 0016.
These fields shall be ignored by the receiver.

12.2.2.3 Non-repeat (NR) mode
In NR mode, SOC messages or segments are transmitted one after another, separated by four or more 7E16
flags (IDLE). Neither messages nor message segments are acknowledged or repeated.
Transmission of messages is terminated upon reception of an appropriate termination message (e.g., ACK
or another regular SOC message) or by reception of an O-P-SYNCHRO signal.
The message index and segmentation index shall be as specified for repeat request (RQ) mode.

12.2.3 SOC IDLE (O-IDLE, R-IDLE)
When the SOC is in the active state but has no message to send, it shall send IDLE (the FTU-O shall send
O-IDLE and FTU-R shall send R-IDLE).
Both O-IDLE and R-IDLE states shall consist of HDLC flags 7E16 that shall be sent repeatedly instead of HDLC
frames.

12.2.4 SOC messages

12.2.4.1 Message codes
The message payload of any SOC message shall contain an integer number of bytes. The payload shall start
with a one byte field containing a unique code to identify the type of message. For one-byte messages the
message code is the entire content of the message. The message codes for all defined messages are shown
in Table 12-7.
NOTE – Other than O/R-REPEAT_REQUEST and O/R-ACK-SEG, all messages sent by the FTU-O have the MSB of the
message code equal to ZERO, whilst messages sent by the FTU-R have the MSB of the message code equal to ONE.

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