Page 899 - 5G Basics - Core Network Aspects

Transport aspects 2

12.2 Special operations channel (SOC)

A special operations channel (SOC) is established for message exchange between the FTU-O and FTU-R
during initialization.
The SOC provides a bidirectional communication of messages between the FTU-O and the FTU-R to support
initialization.
The SOC has two states; active and inactive. In the active state, the FTU transmits SOC messages separated
by one or more high-level data link control (HDLC) flags. In the inactive state, no SOC messages and no
HDLC flags are transmitted.

The state of SOC is determined by the initialization procedure and is indicated in the timing diagrams in
Figures 12-8, 12-9 and 12-12. The list of messages used for each phase of initialization is shown in
Table 12-7.

12.2.1 Message format
The SOC shall use a HDLC-like format with byte stuffing, as specified in clause 12.2.1.1, and a frame check
sequence (FCS) to monitor errors as specified in clause 11.2.1.1.
The structure of a HDLC frame carrying an SOC message shall be as shown in Figure 12-6.

Size in bytes Meaning Value

1 Flag 7E 16
1 Address field Message index
1 Control field Segmentation index
Up to 1 024 Information payload Message payload

1 Frame check sequence FCS
1 Frame check sequence FCS
1 Flag 7E 16

Figure 12-6 – Structure of SOC message transmitted using HDLC-like frame

The message index is determined by the acknowledgement mode (i.e., automatic repeat (AR), repeat
request (RQ) or non-repeat (NR)) and whether a message is being repeated, as defined in clause 12.2.2.
The segmentation index facilitates the message segmentation as described in clause 12.2.4.6. If no
segmentation is used, the segmentation index shall be set to 1116.
The number of SOC bytes (before byte stuffing) transmitted in a single HDLC frame shall not exceed 1 024.

12.2.1.1 Byte stuffing

With byte stuffing, any data byte that is equal to 7E16 (flag) shall be replaced, as described below. This is
necessary to avoid detection of false flags.

After the FCS computation, the transmitter examines the entire SOC message between the opening and
closing flags. Any data byte that is equal to 7E16 or to 7D16 shall be replaced by a two-byte sequence as
following:
• a data byte 7E16 shall be replaced by 7D16 5E16;
• a data byte 7D16 shall be replaced by 7D16 5D16.

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