AGCH stands for Access grant channel in GSM.
The AACH is used by the base station to provide forward link communication to the mobile. It carries data that instructs the mobile to operate in a specific channel with a particular dedicated control channel.
It is the message sent by the base station before a subscriber is moved off the control channel.
The AGCH is used by the base station to respond to RACH sent by the mobile station in the earlier CCCH frame.