Result: standard mapping
How many DS2s are in a STM-1?
A STM-1 carries 21 DS2s, per ITU-T G.707 and ANSI T1.107.
21
DS2s in a STM-1
Working
STM-1 → 3 × DS3 → 7 × DS2
- 3 × DS3 in each STM-1 ITU-T G.707Three VC-3s, each able to carry a DS3.
- 7 × DS2 in each DS3 ANSI T1.107M13 multiplex: seven DS2s.
- 3 DS3s
- 3 × 7 = 21 DS2s
Leftover capacity
22.968 Mbit/s of each STM-1 line rate (155.520 Mbit/s) sits outside 21 × 6.312 Mbit/s of DS2 line rate (132.552 Mbit/s). That remainder is overhead, fixed stuff or justification. It is not another whole tributary.
By bit rate only
Dividing the line rates gives 24.63, which floors to 24 DS2s by bit rate only. The standard mapping is 21. Use the mapping.