Discover types of GSM Channels in 2G networks : Logical, Physical, Combined & Non-combined configuration

In GSM (Global System for Mobile Communications), the channels are used to manage and control the flow of data between the network and mobile devices. These GSM channels can be categorized into physical and logical channels, and further classified into combined and non-combined configurations. Let us explore breakdown of the different types of channels including their functions and roles in 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 is defined by a time slot on a carrier frequency within the frequency spectrum used by the network.

GSM divides each frequency into 8 time slots, forming a frame. Each time slot is assigned to a user and is repeated 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 being transmitted over the physical channels. These are mapped onto the physical channels and are divided into traffic channels and control channels.

2.a Traffic Channels (TCH)

Traffic channels are used to carry user data such as 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 are used to carry signaling information necessary for establishing and managing connections between mobile stations and the network. Control channels are further divided into several subtypes.
• Broadcast Control Channels (BCCH): Provides information about the network 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 the 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 the capacity and requirements of a cell.
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 to 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 by which different channel types are used in different time slots. These rules are used to map logical channels to physical channels. The most important slot is the time slot TS0 where in BCCH is mapped and is very useful as SI messages(system information messages) are transmitted over this channel.

Following are possible channel combinations in GSM system which network(BTS) will adopt based on need of traffic channels versus signalling(control) gsm channels. They are called as combined and non-combined type, refer following links for more.

Noncombined 51-frame multiframe configuration

Follow link below for the complete chart of this configurations for TS0 and TS1. In noncombined configuration, dedicated signaling channels are not combined with BCCH/CCCH and thus require separate time slot(TS1). FCCH,SCH,BCCH and CCCH channels are mapped on TS0.
➨GSM Noncombined channel Configuration➤➤

Combined 51 frame multiframe configuration

Follow link below for the complete chart of this configurations for TS0. In combined configuration, FCCH,SCH,BCCH,CCCH channels are present along with SDCCH on ime 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 on to anyother time slots also.
➨GSM Combined channel configuration➤➤

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.