mkt, it’s not because many people say something that it is correct 🙂
why 8 TS.
the reason is certainly related to the fact that the speech is put in frame of 20ms. And when the GSM was invented, 20ms could be encoded/compressed/etc. into 4×0.577 ms.
20ms => 4×0.577ms = 2.3ms
each 20ms, one user needs 2.3ms.
So during 20ms on the air, only 8 calls can be multiplexed.
20/2.3 = 8.7
The remaining is used for in band signalling (sacch).