In GSM (Global System for Mobile Communications), BCCH and TCH are two types of logical channels used for communication between the mobile device and the base station. These channels play distinct roles in the GSM network, serving different purposes in the exchange of control and traffic information.
BCCH (Broadcast Control Channel):
- Purpose:
- BCCH is a broadcast channel that serves the purpose of broadcasting essential control information to mobile devices within the cell. It is a downlink channel, meaning information flows from the base station to the mobile device.
- Broadcasted Information:
- BCCH carries information such as system information messages, cell identity, frequency lists, and other parameters that help the mobile device to camp onto the network, obtain synchronization, and access important network-related details.
- Continuous Transmission:
- BCCH operates continuously and is broadcasted on a specific frequency within the cell. Mobile devices use BCCH for initial cell selection, location area updating, and acquiring necessary network information.
- Fixed Time Slots:
- BCCH uses fixed time slots in the time division multiple access (TDMA) frame, allowing mobile devices to reliably receive system information at regular intervals.
TCH (Traffic Channel):
- Purpose:
- TCH is a traffic channel used for the actual exchange of voice or data traffic between the mobile device and the network. Unlike BCCH, TCH can be either a downlink (from the base station to the mobile device) or an uplink (from the mobile device to the base station).
- Voice and Data Transmission:
- TCH is dedicated to carrying voice or data during a conversation or data session. It is used for the transmission of actual user information between the mobile device and the network.
- Dynamic Allocation:
- TCH is dynamically allocated based on the demand for traffic channels in the cell. When a mobile device initiates a call or data session, a TCH is assigned for the duration of that communication.
- Time Slot Allocation:
- TCH uses specific time slots within the TDMA frame for communication. The assignment of time slots depends on the network’s capacity and the ongoing traffic in the cell.
Relationship between BCCH and TCH:
- BCCH provides essential control information for mobile devices to access the network, while TCH is responsible for the actual transmission of voice or data during a call or session.
- A mobile device first uses BCCH to acquire information about the network, and once it establishes a connection, it is assigned a TCH for the duration of the communication.
In summary, BCCH (Broadcast Control Channel) in GSM is a broadcast channel used for essential control information, while TCH (Traffic Channel) is a dedicated channel for the exchange of voice or data traffic between the mobile device and the network.