2:1 or 4:1 is a type of Statistical Multiplexing Mode.
Statistical multiplexing is a technology where n channels share one 64 kbit/s timeslot, each in a different time slice.
For example you use 1:1
For 10 TRX it is necessary on Abis interface:
1OML (64 kbit/s) + 10RSL (64 kbit/s) + 10*8TCH (16 kbit/s ) = 31 timeslots
In other case for 4:1
(1OML (64 kbit/s) + 10RSL (64 kbit/s))/4 + 10*8TCH (16 kbit/s ) =23 timeslots