Understanding GSM Channels in 2G Networks

In GSM (Global System for Mobile Communications), channels manage and control the flow of data between the network and mobile devices. These GSM channels fall into physical and logical categories, further classified into combined and non-combined configurations. Let’s explore the breakdown of different channel types, their functions, and their roles in a 2G GSM network system.

1. Physical Channels

A physical channel in GSM refers to the actual radio frequency (RF) transmission path used to carry information. It’s defined by a time slot on a carrier frequency within the network’s frequency spectrum. GSM divides each frequency into 8 time slots, forming a frame. Each time slot is assigned to a user and repeats periodically. Each physical channel represents one time slot on a given frequency carrier.

Example: Time slots in a carrier frequency (e.g., 935.2 MHz).

2. Logical Channels

Logical channels are higher-level channels that define the type of information transmitted over the physical channels. They’re mapped onto the physical channels and divided into traffic and control channels.

2.a Traffic Channels (TCH)

Traffic channels carry user data like voice or data.

• TCH/F (Full rate traffic channel): Supports full-rate voice or data transmission.
• TCH/H (Half rate traffic channel): Supports half-rate voice or data transmission, allowing two calls to share a single physical channel.

2.b Control Channels

Control channels carry signaling information necessary for establishing and managing connections between mobile stations and the network. They’re further divided into subtypes.

• Broadcast Control Channels (BCCH): Provides network information to all mobile devices, including network identification, frequency allocations, and available features.
• Frequency Correction Channel (FCCH): Helps mobile stations synchronize with the network’s frequency.
• Synchronization Channel (SCH): Provides timing information for mobile stations to synchronize with the network’s time frame structure.
• Common Control Channels (CCCH): Manages communication between the mobile station and the base station during connection setup.
  • Random Access Channel (RACH): Used by mobile stations to request a connection.
  • Paging Channel (PCH): Used to inform the mobile station of incoming calls.
  • Access Grant Channel (AGCH): Used to assign a dedicated channel to a mobile station that has requested access.
• Dedicated Control Channels (DCCH): Provides signaling between a mobile station and the network once a connection is established.
  • Standalone Dedicated Control Channel (SDCCH): Carries signaling information for call setup, registration, SMS, etc.
  • Slow Associated Control Channel (SACCH): Provides continuous signaling for maintaining the call (e.g., power control).
  • Fast Associated Control Channel (FACCH): Used for urgent signaling, such as handover, by stealing traffic channel resources.

3. Combined and Non-Combined Channels

GSM can operate in combined or non-combined configurations depending on a cell’s capacity and requirements.

A. Combined Configuration

In the combined configuration, the Broadcast Control Channel (BCCH) and the Standalone Dedicated Control Channel (SDCCH) share the same physical channel. This configuration is typically used in small cells with low traffic density to save bandwidth.

B. Non-Combined Configuration

In the non-combined configuration, the BCCH and SDCCH are assigned separate physical channels. This is generally used in large cells or areas with higher traffic to ensure better performance and channel availability.

There are certain rules governing how different channel types are used in different time slots. These rules map logical channels to physical channels. The most important slot is time slot TS0, where BCCH is mapped and is very useful because SI messages (system information messages) are transmitted over this channel.

The following are possible channel combinations in a GSM system which a network (BTS) will adopt based on the need for traffic channels versus signaling (control) GSM channels. They are called combined and non-combined types.

• Noncombined 51-frame multiframe configuration

  In the non-combined configuration, dedicated signaling channels are not combined with BCCH/CCCH and thus require a separate time slot (TS1). FCCH, SCH, BCCH, and CCCH channels are mapped on TS0.

• Combined 51 frame multiframe configuration

  In the combined configuration, FCCH, SCH, BCCH, and CCCH channels are present along with SDCCH on time slot TS0. Hence, dedicated signaling channels SDCCH are combined with BCCH/CCCH on the same time slot TS0. SDCCH also can be mapped on TS1 in addition to TS0; even SDCCH can be mapped onto any other time slots also.

Conclusion

These 2G GSM channels work together to ensure that communication between the mobile device and the 2G network is efficient, reliable, and adaptable to different traffic conditions.