Page 1146 - 5G Basics - Core Network Aspects

2 Transport aspects

15.7.2.4 OTU OTN synchronisation message channel (OSMC)

For synchronisation purposes, one byte is defined in the OTU overhead as an OTN synchronisation message
channel to transport SSM and PTP messages within SOTU and MOTU interfaces. The OSMC bandwidth is
listed in Table 15-3.

The OTUk contains one instance of OTU OSMC overhead.
NOTE 1 – OTU OSMC is not defined for MOTUm and SOTUm interfaces.
NOTE 2 – Support of OTU OSMC in a SOTU or MOTU interface is optional.
NOTE 3 – Equipment designed prior to Edition 5.0 of this Recommendation may not be able to support OTU OSMC via
their SOTU or MOTU interfaces.
NOTE 4 – SOTU and MOTU interfaces with vendor specific application identifiers may support an OSMC function. The
encapsulation of the messages and overhead location are then vendor specific.

Table 15-3 – OSMC bandwidth

OTUk OSMC Bandwidth (kbit/s)
OTU1 163.361
OTU2 656.203
OTU3 2,635.932
OTU4 6,851.101

The SSM and PTP messages within a SOTU or MOTU interface are encapsulated into GFP-F frames as
specified into [ITU-T G.7041]. PTP event messages are timestamped and after encapsulation into GFP-F
frames inserted into the OSMC as specified in clause 15.7.2.4.1. GFP-F encapsulated SSM messages (and
PTP non-event messages) are inserted into the OSMC at the earliest opportunity. GFP Idle frames may be
inserted between successive GFP frames.

The mapping of generic framing procedure (GFP) frames is performed by aligning the byte structure of
every GFP frame with the byte of the OSMC overhead field. Since the GFP frames are of variable length and
longer than one byte, a frame crosses the OTUk (k=1,2,3,4) frame boundary.

15.7.2.4.1 Generation of event message timestamps
15.7.2.4.1.1 SOTU and MOTU interface event message timestamp point

The SOTU and MOTU interface message timestamp point [ITU-T G.8260] for a PTP event message
transported over the OSMC shall be the X-frame multiframe event preceding the beginning of the GFP
frame in which the PTP event message is carried. See Figure 15-16. Since the GFP frames may be longer
than X-4 bytes, a frame may cross the X-frame multiframe boundary. The X-frame multiframe contains
frames numbered 0, 1, .., X-1.

15.7.2.4.1.2 Event timestamp generation
All PTP event messages are timestamped on egress and ingress SOTU and MOTU interfaces. The timestamp
shall be the time at which the event message timestamp point passes the reference plane [ITU-T G.8260]
marking the boundary between the PTP node (i.e., OTN node) and the network.
OTUk
Event message timestamps are generated every X-frame multiframe period at the OTUk Access Point. X is
64 for k = 1 and 256 for k = 2,3,4.
The first byte of a GFP(PTP event message) frame is inserted into the OTUk OSMC between 4 and X-1
frames after the X-frame multiframe boundary.
NOTE 1 – The first byte of a GFP(PTP event message) frame is not inserted into the OTUk OSMC in frame 0…3 of the
X-frame multiframe.

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