I’m trying to understand a basic thing about GSM RF and timeslots. I’m pretty sure at least one of these statements must be false, because they don’t add up properly.

a) One pulse of GSM RF can carry 116 bits of data (2 x 58, from ETSI TS 145.002)

b) A single timeslot of a single frame corresponds to a single pulse of RF.

c) A control channel in LAPDm carries up to 23 bytes, or 184 bits of data.

So how can those 184 bits of data fit onto 116 bits in the pulse? What have I got wrong?

OK, I found a bit of information that clarifies the control channels: ‘The 184 bits of raw information are processed to yeild 456 bits. These 456 bits are then transmitted through four time slots’.

I hadn’t picked up that one LAPDm message is spread over four frames of a timeslot.

3 T + 57 CD + 1 S + 26 TS + 1 S + 57 CD + 3 T + 8.25 GP

T – Tail bit (3 + 3) = 6
CD – Coded data (57 + 57) = 114
S – Stealing Flag (1 + 1) = 2
TS – Training Sequence (26) = 26
GP – Guard period (8.25) = 8.25