GSM Channels | Teleinf Edu

Physical and Logical Channels in GSM

In GSM, communication uses radio waves. The available spectrum is divided into frequency channels by FDMA, and each frequency channel is divided into eight time slots by TDMA. A selected frequency combined with a selected time slot forms a . The physical channel identifies when and where a burst is transmitted in the radio interface. A GSM carrier has a 200 kHz raster, and the eight-slot TDMA frame repeats in about 4.615 ms. A normal burst occupies its assigned slot together with the guard period required for timing uncertainty.

The network also needs to identify the type of information assigned to that radio resource. It may carry a part of a voice call, a power-control command, or system information containing the cell identity. This purpose is defined by a . Logical channels are mapped onto physical channels according to the GSM frame and multiframe configuration. A physical channel can therefore carry different logical channels at different times. Traffic channels commonly use a 26-frame multiframe, while many control-channel arrangements use a 51-frame multiframe. The mapping determines the burst positions, training sequence, coding, and interleaving used for each logical channel.

Families of Logical Channels

A GSM network maps voice, circuit-switched data, access procedures, mobility management, and radio-resource control onto logical channels. The logical channels form two main families, classified by the information they carry. The classification describes the service or signalling role, while the physical mapping describes the carrier, time slot, burst format, and multiframe position used over the air.

Traffic Channels (TCH): These channels carry user-plane information, including coded speech and circuit-switched data for services such as fax or CSD. Their allocation is tied to a particular mobile connection. The codec, channel coding, interleaving, and selected bearer determine how application data becomes radio bursts.
Control Channels (CCH):These channels carry signalling and control information required for network operation. They support call setup, location updating, access, power control, measurements, and synchronization. Their payload consists of control information, and the network schedules it according to the mobile station's state and the channel configuration of the cell.

Traffic Channels (TCH)

Traffic Channels transport the user's speech or data. GSM defines full-rate and half-rate traffic-channel configurations, which provide different relationships between voice quality, coding rate, and capacity. The radio interface adds channel coding and interleaving so that a receiver can correct or conceal some errors caused by fading and interference.

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

TCH/F assigns one physical time slot in each TDMA frame to a connection. In the classic full-rate speech configuration, the radio-interface stream is $22.8 \text{ kbit/s}$ . It includes coded speech bits, tail and stealing-related fields, and error-protection bits. The speech codec input rate is commonly $13 \text{ kbit/s}$ for the original full-rate speech codec. A compatible circuit-switched data bearer can provide up to $9.6 \text{ kbit/s}$ in the configuration described here. The exact user rate depends on the bearer and coding option. One TCH/F allocation therefore consumes one slot position in each frame, even when the speech source has no user payload for part of the interval.

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

TCH/H lets two half-rate speech channels share the same physical time-slot position. Each connection uses the position in alternating TDMA frames, so User A can use frames 1, 3, 5 and User B can use frames 2, 4, 6 in the illustrative sequence. In the classic half-rate speech configuration, each connection has about $11.4 \text{ kbit/s}$ of radio-interface capacity and an original half-rate speech codec rate near $5.6 \text{ kbit/s}$ . The arrangement increases potential speech capacity while using a lower speech-coding rate and a different error-protection profile. The two calls still require independent signalling and coding context, so the gain is a capacity arrangement rather than a claim that every radio condition supports the same speech quality.

Control Channels (CCH)

Control channels carry the signalling needed to establish connections, distribute cell information, provide access, and maintain an active radio link. They are divided into three groups according to their direction and function. Broadcast and common channels are shared at cell level, while dedicated channels are assigned to one mobile station for a signalling task or an associated traffic connection.

I. Broadcast Channels (BCH): Downlink System Information

Broadcast channels are downlink channels transmitted by the Base Transceiver Station (BTS) to Mobile Stations (MS) in the cell. A mobile station in idle mode monitors them to obtain network and cell information. The BCH family consists of three key channels:

Broadcast Control Channel (BCCH): BCCH carries system information required for cell selection, access, and idle-mode operation. The mobile station reads the relevant system-information messages before selecting a cell and uses their parameters when it evaluates reselection or access. This information includes:
- The network and cell identity codes (PLMN, LAI, CI).
- The list of frequencies used by neighboring cells, which is essential for preparing for handovers.
- The specific rules for accessing the network in this cell, such as which control channels are available and the parameters for the Random Access Channel (RACH).
Frequency Correction Channel (FCCH): FCCH carries the defined Frequency Correction Burst with a fixed bit pattern. After GSM modulation, the burst produces a known frequency-correction tone. The mobile station uses it to estimate the carrier frequency error and tune its frequency reference before decoding further channels. FCCH supports acquisition of the serving carrier; it does not carry user speech or an arbitrary data payload.
Synchronization Channel (SCH):After frequency correction, the mobile station decodes the SCH synchronization burst and its training sequence. SCH provides the Base Station Identity Code (BSIC) and the TDMA frame number, allowing the receiver to align with the serving cell's frame timing and identify the decoded synchronization information. FCCH and SCH are therefore normally processed as an acquisition sequence before ordinary broadcast information is decoded.

II. Common Control Channels (CCCH): Shared Access

The CCCH group supports initial access, paging, and assignment procedures. These channels are shared by mobile stations in the cell and provide the signalling path before a dedicated channel is assigned.

Paging Channel (PCH) (Downlink):When the network has an incoming call or message for a mobile station in idle mode, it sends a paging message on PCH. The message uses the temporary identity associated with the mobile station's paging procedure, allowing the device to wake and respond. Paging groups and discontinuous reception let an idle device monitor scheduled opportunities instead of decoding every downlink block.
Random Access Channel (RACH) (Uplink):A mobile station sends an Access Burst on RACH to request an initial connection or to answer paging. Several stations can transmit in the shared channel at overlapping times, so the access procedure uses the cell's broadcast parameters, a request reference, timing rules, and retransmission behaviour to resolve unsuccessful attempts. The network returns an Immediate Assignment or a failure-related response, and a repeated attempt is governed by the broadcast access parameters.
Access Grant Channel (AGCH) (Downlink): After processing an access request, the network sends an Immediate Assignment on AGCH. The assignment gives the mobile station a temporary dedicated resource, commonly an SDCCH or a TCH, together with the frequency and time-slot information needed to use it. The mobile station then leaves the shared access procedure and follows the assigned channel description.

III. Dedicated Control Channels (DCCH): Point-to-Point Signalling

After the initial CCCH procedure, the network assigns a DCCH to one mobile station for a defined signalling task. The channel carries point-to-point control information for that assignment.

Standalone Dedicated Control Channel (SDCCH): SDCCH carries signalling after the Immediate Assignment, including call setup, location updating, authentication, and ciphering control. In circuit-switched GSM it can also transport SMS procedures and other signalling that does not require a TCH. The exact SMS transport path depends on the radio and core-network procedure. SDCCH is normally released or followed by a traffic-channel assignment when the dedicated signalling phase is complete.
Slow Associated Control Channel (SACCH): SACCH is associated with a TCH or SDCCH and carries periodic link-control information. Examples include power-control commands, timing-advance information, and measurement reports about serving and neighbouring cells used by radio-resource procedures. Its signalling blocks occur less frequently than the associated traffic blocks. The regular reports give the network inputs for maintaining the link and evaluating a handover.
Fast Associated Control Channel (FACCH): FACCH carries time-critical signalling during an active TCH connection, for example handover commands. The network temporarily replaces a traffic block with a FACCH block and sets the stealing flags in the normal burst. The receiver uses those flags to decode the block as FACCH signalling rather than speech or user data. The replaced traffic block is handled by the associated speech or data error-concealment procedure.