EXFO FTB-800 Series for FTB-1 - Manual (Page 568)

Glossary
558 FTB-700G/800 Series
G.709 Optical Transport Network (OTN)
ODU Multiplexing
The ODU multiplexer is a function that allows the multiplexing of ODU
tributary signals into higher OTN signal rates. The G.709 standard supports
2 types of ODU multiplexer which can be classified as follows:

Legacy architecture is based on multi-stage architecture to bring an
ODUk client to a higher OTN interface rate. This multiplexer is
identified by Payload Type 20 (PT 20).

New architecture uses a single stage architecture to bring an ODUk
client to any higher OTN interface rate. This method supports the
ODUflex client signal. The multiplexer is identified by Payload Type 21
(PT 21).
Note: Refer to the OTN BERT on page 40 for the ODU multiplexing capabilities.
The multiplexing strategy is based on the concept of tributary slots, which
is similar in concept to the SONET timeslot. The multiplexing of 4 ODU1 in
one ODU2 is made by distributing the ODU1 structure in a repetitive
sequence of 4 ODU2 Tributary slots, a similar strategy is used for ODU3
multiplexing where the repetitive sequence is made of 16 ODU3 tributary
slots, refer to G.709 standard for detailed information.