GSM Logical Channels Explained: 2G Network Functions

GSM (Global System for Mobile Communications) utilizes logical channels to define the type of information transmitted between the mobile device and the network. These 2G GSM logical channels are categorized based on their function into traffic channels (TCH) and control channels (CCH). Each logical channel is mapped onto a physical channel (i.e., a time slot in a carrier frequency), and they are essential for both carrying user data and maintaining network control and signaling.

1. Traffic Channels (TCH)

Traffic channels are used to transmit actual user data, such as voice or data sessions. They are classified into two types based on the data rate.

A. Full-rate Traffic Channel (TCH/F)

• Function: Carries full-rate voice (13 kbps) or data (9.6 kbps) between the mobile device and the network.
• Usage: This channel is used during an ongoing call to transmit voice or data over a physical channel.

B. Half-rate Traffic Channel (TCH/H)

• Function: Carries half-rate voice (6.5 kbps) or data (4.8 kbps), allowing two users to share a single physical channel.
• Usage: Useful for improving bandwidth efficiency in high-traffic scenarios, enabling more calls within the same frequency band.

2. Control Channels (CCH)

Control channels carry signaling and management information between the mobile device and the network. Control channels are critical for functions like call setup, handover, registration, and mobility management. Control channels are further subdivided into Broadcast Control Channels (BCCH), Common Control Channels (CCCH), and Dedicated Control Channels (DCCH).

A. Broadcast Control Channels (BCCH)

Broadcast control channels provide network information to all mobile devices within the cell. These are essential for mobile stations to receive basic network parameters.

• BCCH (Broadcast Control Channel):

  • Function: Transmits general information about the network, such as network identity, control channel structure, and available services (e.g., emergency call capabilities, frequency hopping parameters).
  • Usage: Continuously broadcast to all mobile devices within the cell, helping them understand the network configuration and available features.

• FCCH (Frequency Correction Channel):

  • Function: Assists the mobile device in tuning to the correct frequency by providing frequency correction information.
  • Usage: Helps mobile stations synchronize their local oscillators to the frequency used by the base station.

• SCH (Synchronization Channel):

  • Function: Provides frame synchronization and Base Station Identity Code (BSIC) for mobile devices to sync with the base station’s timing.
  • Usage: Used for timing synchronization, enabling mobile devices to align with the network’s time slots.

B. Common Control Channels (CCCH)

Common control channels manage the initial communication between mobile devices and the network, such as call setup, paging, and resource allocation.

• RACH (Random Access Channel):

  • Function: Used by mobile devices to request access to the network.
  • Usage: When a mobile station wants to initiate a call or register, it sends a request via the RACH. It’s a contention-based channel, so multiple devices can send access requests at the same time.

• PCH (Paging Channel):

  • Function: Used by the network to alert a mobile device of an incoming call or message.
  • Usage: The network pages the mobile device by sending a notification on the PCH when there is an incoming call or SMS.

• AGCH (Access Grant Channel):

  • Function: Provides the mobile station with information about the allocation of a dedicated channel (e.g., TCH or SDCCH) after a successful RACH request.
  • Usage: Used to grant access to a mobile device after it requests network resources.

C. Dedicated Control Channels (DCCH)

Once a connection between a mobile station and the network has been established, the network uses dedicated control channels to handle ongoing signaling and manage the communication.

• SDCCH (Standalone Dedicated Control Channel):

  • Function: Carries signaling during call setup, registration, location updates, and SMS transmission. It is used before a dedicated traffic channel (TCH) is assigned.
  • Usage: When a mobile station is making a call or registering with the network, it communicates with the base station using the SDCCH before switching to a traffic channel.

• SACCH (Slow Associated Control Channel):

  • Function: Provides continuous signaling during an ongoing call. It carries information for power control, timing advance, and reporting of signal strength.
  • Usage: It accompanies every traffic channel (TCH) or SDCCH to manage and maintain the call by adjusting power levels and handling handovers.

• FACCH (Fast Associated Control Channel):

  • Function: Used to carry urgent signaling, such as handover requests and immediate network commands, by stealing time slots from the traffic channel.
  • Usage: Temporarily replaces the traffic data on a TCH to handle urgent signaling, typically during a handover between cells.

Conclusion

These GSM logical channels work together to enable efficient communication, manage call setup, maintain ongoing calls, and handle mobility across the 2G network.