Page 1157 - 5G Basics - Core Network Aspects

Transport aspects 2

A P-CMEP sets these bits to "000".

A TC-CMEP ingress point sets these bits to either "001" to indicate to its peer TC-CMEP egress point that
there is no incoming alignment error (IAE), or to "010" to indicate that there is an incoming alignment
error.

The TC-CMEP egress point may use this information to suppress the counting of bit errors, which may occur
as a result of a frame phase change of the ODU at the ingress of the tandem connection.

15.8.2.2.6 TCM overhead field assignment
Each TC-CMEP will be inserting/extracting its TCM overhead from one of the 6 TCMi overhead fields and
one of the 6 DMti fields. The specific TCMi/DMti overhead field is provisioned by the network operator,
network management system or switching control plane.
At a domain interface, it is possible to provision the maximum number (0 to 6) of tandem connection levels
which will be passed through the domain. The default is three. These tandem connections should use the
lower TCMi/DMti overhead fields TCM1/DMt1...TCMMAX/DMtMAX. Overhead in TCM/DMt fields beyond the
maximum (TCMmax+1/DMtmax+1 and above) may/will be overwritten in the domain.

The TCM6 overhead field is assigned to monitor an ODUk connection which is supported by two or more
concatenated ODUk link connections (supported by back-to-back OTUk trails). [ITU-T G.7714.1] specifies a
discovery application which uses the TCM6 TTI SAPI field as discovery message channel. [ITU-T G.873.2]
specifies an ODUk SRP-1 protection application which uses the TCM6 field to monitor the
status/performance of the ODUk connection between two adjacent ODUk SRP-1 nodes.

Example
For the case of an ODUk leased circuit, the user may have been assigned one level of TCM, the service
provider one level of TCM and each network operator (having a contract with the service provider) four
levels of TCM. For the case where a network operator subcontracts part of its ODUk connection to another
network operator, these four levels are to be split; e.g., two levels for the subcontracting operator.

This would result in the following TCM OH allocation:
– User: TCM1/DMt1 overhead field between the two user subnetworks, and
TCM1/DMt1..TCM6/DMt6 within its own subnetwork;
– Service provider (SP): TCM2/DMt2 overhead field between two UNIs;
– Network operators NO1, NO2, NO3 having contract with service provider: TCM3/DMt3,
TCM4/DMt4, TCM5/DMt5, TCM6/DMt6. Note that NO2 (which is subcontracting) cannot use
TCM5/DMt5 and TCM6/DMt6 in the connection through the domain of NO4;

– NO4 (having subcontract with NO2): TCM5/DMt5, TCM6/DMt6.
See Figure 15-24.

Figure 15-24 – Example of TCM overhead field assignment

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