RACH stands for Random access channel in GSM.
It is a reverse link channel. With the help of RACH, the mobile station originates a call, send signaling message when not on call, acknowledges message.
It uses a slotted ALOHA access method. All mobile must access or respond to a PCH alert within TS0 of a GSM frame.
At the BTS each frame will accept the RACH transmissions from the mobiles during TS0. The base station responds to the RACH transmission by allocating a channel and a stand-alone dedicated control channel for signaling in the presence of a call. The connection is confirmed by the BS full form ( base station ) over the access grant channel.
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| Random access burst |