GSM Access Grant Channel (AGCH) Explained

This page explains the GSM Access Grant Channel (AGCH). The AGCH is a downlink channel in the GSM system, transmitting from the network to the Mobile Subscriber (MS).

It carries information that allows the mobile to determine whether its access to the network has been granted or denied. Critically, it provides information about the Traffic Channel (TCH) or Standalone Dedicated Control Channel (SDCCH) that the mobile needs to use for subsequent communication with the network. The AGCH information is mapped using a normal burst, as illustrated below.

In the 51-frame multiframe structure, the first two time slots (TS0) in consecutive TDMA frames are used for the Frequency Correction Channel (FCCH) and Synchronization Channel (SCH), repeating every 10 frames. The next four slots are reserved for the Broadcast Control Channel (BCCH), and the remaining slots are used by the Common Control Channels (CCCH), specifically the AGCH and Paging Channel (PCH). The mapping of CCCH (AGCH/PCH) and SDCCH/SACCH varies depending on whether a non-combined or combined configuration is used (typically differentiating between low-capacity and high-capacity cells).