GSM RACH (Random Access Channel) Explained

This page explains the GSM RACH (Random Access Channel). This RACH channel is used by a mobile device to access the GSM network, primarily during call setup. An access burst is used to carry the RACH information.

Whenever a mobile phone wants to initiate a Mobile Originated (MO) call, it schedules the RACH. There’s a possibility that multiple mobile devices might transmit at the same time. If this happens, their transmissions collide in the air, preventing them from gaining access to the network.

This collision occurs because the GSM standard doesn’t specify a limit on the number of mobiles transmitting in a single RACH time slot. If a collision does happen, the mobile device waits for a random period before attempting to transmit the RACH again. The duration of this waiting period is also randomized.

RACH utilizes the GSM 51-frame multiframe structure in the uplink to transmit information. Typically, Time Slot 1 (TS1) is used, but in high-capacity GSM cell areas, TS2, TS4, and TS6 can also be used for RACH, leaving TS0 (which is mainly used for broadcast frequencies).

The RACH channel allows a mobile device to gain access to the GSM network. The figure above illustrates how the RACH channel is processed through the physical layer.