In GSM (Global System for Mobile Communications), the RACH (Random Access CHannel) is a channel used for the establishment of a connection between a mobile device and the network. The RACH channel plays a vital role in the initial access procedure and is primarily employed for call setup and location updating. Here’s a detailed overview:
Definition:
The RACH channel is a shared channel used by mobile devices to request access to the GSM network. It serves as the medium through which a mobile device initiates communication with the network, especially when there is a need to establish a connection, make a call, or update its location.
Key Characteristics and Usage:
- Access Requests:
- Mobile devices use the RACH channel to send access requests to the network. This is particularly important during scenarios such as initiating a call, sending an SMS, or when a device enters a new location area.
- Random Access Procedure:
- The RACH channel is part of the random access procedure, where mobile devices contend for access to the network.
- When a device needs to establish a connection, it randomly selects a timing advance value and transmits a burst on the RACH channel.
- Burst Transmission:
- The mobile device transmits a burst on the RACH channel to inform the network of its presence and the need for access.
- The burst includes information like the Mobile Identity (e.g., IMSI – International Mobile Subscriber Identity) and the timing advance value.
- Access Burst:
- The burst transmitted on the RACH channel is referred to as the access burst. It contains specific information to facilitate the network’s identification and coordination of the connection setup process.
- Contention Resolution:
- In cases where multiple devices attempt to access the network simultaneously, a contention resolution mechanism is employed.
- The network responds to the access bursts, and if contention arises, it resolves the access requests to avoid collisions.
- Call Setup and Location Update:
- The RACH channel is crucial for call setup procedures, allowing mobile devices to request access when initiating a call.
- It is also used during location updating, where the device informs the network about its current location area.
Connection Establishment:
- Burst Transmission:
- When a mobile device needs to establish a connection, it selects a random timing advance value and transmits an access burst on the RACH channel.
- Network Response:
- The network monitors the RACH channel and responds to the access burst by assigning a dedicated channel (e.g., SDCCH – Standalone Dedicated Control Channel) for further communication.
- Call Setup or Location Update:
- The assigned channel is then used for call setup or location update procedures, depending on the nature of the access request.
Significance:
The RACH channel is fundamental for the establishment of communication between a mobile device and the GSM network. It ensures an organized and efficient approach to handling access requests from multiple devices, facilitating call setup and location updating procedures.
In summary, the RACH channel in GSM is a shared channel used by mobile devices to send access requests to the network, playing a crucial role in call setup, location updating, and the initial access procedure.