Result: standard mapping
How many STM-256s are in an OTU4?
An OTU4 carries 2 STM-256s, per ITU-T G.709. This is a client mapping of STM-256 into ODU3.
2
STM-256s in an OTU4
Working
OTU4 → 1 × ODU4 → 2 × ODU3 → 1 × STM-256
- 1 × ODU4 in each OTU4 ITU-T G.709The OTU4 frame carries exactly one ODU4.
- 2 × ODU3 in each ODU4 ITU-T G.709Each ODU3 takes 31 of the 80 ODU4 tributary slots, so 2 fit and 18 slots stay unused.
- 1 × STM-256 in each ODU3 ITU-T G.709Client mappingCBR STM-256 mapped into OPU3.
- 1 ODU4
- 1 × 2 = 2 ODU3s
- 1 × 2 × 1 = 2 STM-256s
Leftover capacity
32.183733568 Gbit/s of each OTU4 line rate (111.809973568 Gbit/s) sits outside 2 × 39.813120 Gbit/s of STM-256 line rate (79.626240 Gbit/s). That remainder is overhead, fixed stuff or justification. It is not another whole tributary.