in addition:
Traffic Multiframe Structures - The 26 traffic multiframe structure is
used to send information on the traffic channel. The 26 traffic
multiframe structure is used to combine user data (traffic), slow
control signaling (SACCH), and idle time period. The idle time period
allows a mobile device to perform other necessary operations such as
monitoring the radio signal strength level of a beacon channel from
other cells. The time interval of a 26 frame traffic multiframe is 6
blocks of speech coder data (120 msec).
(
http://www.althos.com/tutorial/GSM-tutorial-frame-structure.html)
one encoded speech block lasts 20ms.
576,92307692307692307692307692308us per slot
4615,3846153846153846153846153846 per 8 slots (1 frame)
120ms per 26 frames (6 speech blocks)