AGCH (Access Grant Channel)

Last updated on Feb 24, 2023

The Access Grant Channel (AGCH) is a control channel used in mobile telecommunication systems such as GSM (Global System for Mobile Communications) and its successors. The AGCH plays a vital role in establishing the initial communication between a mobile station (MS) and the base transceiver station (BTS) or the base station (BS). In this article, we will explore the AGCH in detail, including its function, structure, and how it is used in mobile networks.

Function of AGCH:

The primary function of AGCH is to provide the MS with an access grant message that contains a dedicated channel for establishing the call. In other words, when a MS initiates a call, it sends a request to the BTS or BS. The BTS or BS acknowledges the request by sending an access grant message on the AGCH. The access grant message includes the frequency and timeslot on which the MS should communicate with the BTS or BS to establish the call. Therefore, the AGCH provides a dedicated channel for establishing the call between MS and BTS or BS.

AGCH Structure:

The AGCH is a downlink channel that is transmitted by the BTS or BS. It consists of 184 bits and is transmitted at a data rate of 270.833 kbps. The AGCH is transmitted using GMSK (Gaussian Minimum Shift Keying) modulation, which is a form of phase shift keying modulation. The AGCH is also transmitted using a specific timeslot and frequency, which is specified by the BTS or BS.

The AGCH consists of several fields that are used to carry different types of information. The following are the fields that make up the AGCH:

Synchronization bits (8 bits): The synchronization bits are used to synchronize the receiver with the transmitted signal.
Training sequence (26 bits): The training sequence is used to calibrate the receiver and to estimate the channel characteristics.
Access grant message (116 bits): The access grant message contains the information that the MS needs to establish the call, including the frequency and timeslot.
Guard period (8 bits): The guard period is used to prevent interference between adjacent channels.
Tail bits (26 bits): The tail bits are used to fill the remaining bits in the AGCH.

How AGCH is used in Mobile Networks:

The AGCH is used in the following manner in mobile networks:

Step 1: The MS sends a request to the BTS or BS to initiate a call. This request is transmitted on the Random Access Channel (RACH).

Step 2: The BTS or BS acknowledges the request by sending an access grant message on the AGCH. The access grant message contains the frequency and timeslot on which the MS should communicate with the BTS or BS to establish the call.

Step 3: The MS receives the access grant message on the AGCH and tunes its receiver to the frequency and timeslot specified in the access grant message.

Step 4: The MS establishes the call by transmitting a call setup message on the allocated dedicated channel.

Step 5: The BTS or BS acknowledges the call setup message, and the call is established.

In summary, the AGCH plays a vital role in establishing the initial communication between the MS and BTS or BS in mobile networks. The AGCH provides a dedicated channel for establishing the call, and the access grant message on the AGCH contains the frequency and timeslot on which the MS should communicate with the BTS or BS to establish the call. The AGCH is a downlink channel that consists of several fields, including the synchronization bits, training sequence, access grant message, guard period, and tail bits. The AGCH is used in conjunction with the RACH to establish a call in mobile networks.

The AGCH is a critical component of the GSM and its successors, as it is responsible for assigning dedicated channels for establishing the call between the MS and BTS or BS. The AGCH operates on the same frequency as the other channels in the GSM network, but it uses a different timeslot. This enables the MS to receive the access grant message without interrupting ongoing calls on the network.

The AGCH also has some additional features that make it an essential component of the GSM network. One of these features is the use of power control. The power control feature ensures that the MS and BTS or BS transmit at the optimal power level to avoid interference and conserve battery life. The AGCH also uses a scrambling code, which adds another layer of security to the transmission and reception of the access grant message.

Another feature of the AGCH is that it can be used to transmit other types of messages besides the access grant message. For example, it can be used to transmit messages for updating the location of the MS or for transmitting short messages to the MS. However, the primary use of the AGCH is to assign a dedicated channel for establishing the call between the MS and BTS or BS.

In conclusion, the Access Grant Channel (AGCH) is a critical component of mobile telecommunications systems such as GSM and its successors. The AGCH provides a dedicated channel for establishing the call between the MS and BTS or BS, and the access grant message on the AGCH contains the frequency and timeslot on which the MS should communicate with the BTS or BS to establish the call. The AGCH is a downlink channel that consists of several fields, and it uses power control and scrambling to optimize the transmission and reception of the access grant message. The AGCH is a vital component of the GSM network, and its use ensures efficient and secure communication between the MS and BTS or BS.