GSM RACH(Random Access Channel)
This page describes GSM RACH(Random Access Channel).
This channel RACH is used by Mobile to access the GSM network during call set-up time. Access burst is used for mapping RACH information.
Whenever mobile wants to make a MO call it schedules the RACH. There is a probability that two or more mobiles transmit at the same time and their transmissions collide in the medium(air) and they will not be granted access to the network. This happens because limit for no. of mobiles transmitting in one RACH time slot is not defined in the GSM standard.
If collision happens then mobile waits for random period of time and transmits the RACH again. This waiting time is also random.
RACH uses GSM 51 frame multiframe structure in the uplink to transmit information. Mostly TS1 is used but in high capacity GSM cell areas, TS2,TS4 and TS6 is also used for RACH leaving TS0(mainly used for broadcast frequency).
RACH channel is used by mobile to obtain gsm network access. The figure depicts RACH channel processing through the physical layer.