Definition of OTN Frame Payload Type(PT) 20 & 21?

As defined in G.709 an ODUk container consist of an OPUk (Optical Payload Unit) plus a specific ODUk Overhead (OH).    OPUk OH information is added to the OPUk information payload to create anOPUk.  It includes information to support the adaptation of client signals.Within the OPUk overhead there is the payload structure identifier (PSI) that includes the     payload type (PT).  The payload type (PT) is used to indicate the composition of the OPUk signal.

When an ODUj signal is multiplexed into an ODUk, the ODUj signal is first extended with frame alignment overhead and   then mapped into an Optical channel Data Tributary Unit (ODTU). Two different types of ODTU are   defined in G.709:

– ODTUjk ((j,k) = {(0,1), (1,2), (1,3), (2,3)}; ODTU01,ODTU12,ODTU13 and ODTU23) in which an ODUj signal is mapped via the asynchronous mapping procedure (AMP), defined in clause 19.5 of G.709.

– ODTUk.ts ((k,ts) = (2,1..8), (3,1..32), (4,1..80)) in which a lower order ODU (ODU0, ODU1, ODU2, ODU2e, ODU3, ODUflex) signal is mapped via the generic mapping procedure (GMP), defined in clause 19.6 of  G.709.

When PT is assuming value 20 or 21,together with OPUk type (K=1,2,3,4), it is used to discriminate two different  ODU multiplex structure ODTUGx :

– Value 20: supporting ODTUjk only,

– Value 21: supporting ODTUk.ts or ODTUk.ts and ODTUjk.

The discrimination is needed for OPUk with K =2 or 3, since OPU2 and OPU3 are able to support both the different ODU multiplex structures.For OPU4 and OPU1, only one type  of ODTUG is supported: ODTUG4 with PT=21 and ODTUG1 with PT=20.The relationship between PT and TS granularity, is in the fact that the twodifferent ODTUGk discriminated by PT and OPUk  are characterized by two different TS granularities of the relatedOPUk, the former at 2.5   Gbps, the latter at 1.25Gbps.