GSM Logical and Physical Channels Explained: Types, Functions and Classification

Introduction to GSM Channels

In GSM communication, channels are used to transfer voice and data between the mobile station (MS) and the base station subsystem (BSS). These channels are classified into logical channels and physical channels, each serving a specific purpose in communication.

Common and Dedicated Channels

GSM channels can also be classified as common channels and dedicated channels.

Common Channels

Common channels are used for general communication between the network and mobile devices. They are mainly used during call setup and network access.

• Paging the mobile for incoming calls
• Broadcasting system information
• Responding to channel requests

The return common channel is known as the Random Access Channel (RACH), which is used by mobile stations to request access to the network.

Dedicated Channels

Dedicated channels are assigned to a specific user for communication. These are further divided into:

• Signaling Channels: Used for call setup, maintenance, handover, and termination.
• Traffic Channels: Used to carry the actual voice or data payload.

Types of GSM Logical Channels

GSM defines several logical channels, each with a specific function:

• TCH/F (Full Rate Traffic Channel): Used for full-rate voice communication.
• TCH/H (Half Rate Traffic Channel): Used for half-rate voice communication to increase system capacity.
• BCCH (Broadcast Control Channel): Broadcasts network information such as control channel structure.
• SCH (Synchronization Channel): Synchronizes mobile stations with the network.
• FCCH (Frequency Correction Channel): Provides frequency correction information to mobile stations.
• AGCH (Access Grant Channel): Acknowledges channel requests and assigns a Standalone Dedicated Control Channel (SDCCH).
• PCH (Paging Channel): Notifies the mobile station of incoming calls.
• RACH (Random Access Channel): Used by mobiles to request network access.
• FACCH (Fast Associated Control Channel): Used for urgent signaling such as handover. It temporarily replaces traffic data.
• SACCH (Slow Associated Control Channel): Used for continuous signaling like power control and link monitoring.
• SDCCH (Standalone Dedicated Control Channel): Used for signaling during call setup, registration, and location updates.

Signaling Channels in Detail

• FACCH: Handles time-critical signaling. It “steals” traffic bursts when required.
• SACCH: Provides ongoing communication between the mobile and base station, mainly for monitoring and control.
• SDCCH: Supports signaling tasks such as authentication and call setup.

Traffic Channels

Traffic channels (TCH) are used to carry the actual voice or user data. They can operate in:

• Full Rate (TCH/F)
• Half Rate (TCH/H)

Half-rate channels allow more users to be supported but may slightly reduce voice quality.

Voice Codecs in GSM

GSM systems use different audio codecs (vocoders) to compress voice data efficiently. Some commonly used codecs include:

• RPE-LPC: Used in early GSM systems
• Half Rate Codec: Reduces bandwidth usage
• AMR (Adaptive Multi-Rate): Most widely used modern codec
• AMR-WB (Wideband): Provides enhanced voice quality

With advancements in processing power, voice codec technology has significantly improved, resulting in better voice quality and efficient bandwidth utilization.

Conclusion

GSM logical and physical channels form the backbone of mobile communication. While logical channels define the type of information being transmitted, physical channels represent the actual transmission medium. Understanding these channels is essential for analyzing GSM communication, optimizing network performance, and ensuring efficient use of available resources.