GSM Channel Types

GSM Channel Types There are two types of GSM logical channels, called traffic channel (TCH) and control cahnnel (CCH). Traffic channels carry digitally encoded user speech or user data and control channels carry signalling and synchronizing commands between the base station and the mobile station. Control channels are also called signalling channels.

GSM divides up each ARFCN into 8 time slots. These 8 timeslots are further broken up into logical channels. Logical channels can be thought of as just different types of data that is transmitted only on certain frames in a certain timeslot. Different time slots will carry different logical channels, depending on the structure the BSS uses. There are two main categories of logical channels in GSM: 1. Signaling/Control Channels 2. Traffic Channels (TCH)

Signaling/Control Channels These are the main types of signaling Channels:

Broadcast Channels (BCH) - Transmitted by the BTS to the MS. This channel carries system parameters needed to identify the network, synchronize time and frequency with the network, and gain access to the network. Common Control Channels (CCH) - Used for signaling between the BTS and the MS and to request and grant access to the network. Standalone Dedicated Control Channels (SDCCH) - Used for call setup Associated Control Channels (ACCH) - Used for signaling associated with calls and call-setup. An ACCH is always allocated in conjunction with a TCH or a SDCCH.

The above categories can be divided into the following logical channels:

Broadcast Channels (BCH) 1. Broadcast Control Channel (BCCH) 2. Frequency Correction Channel (FCCH) 3. Synchronization Channel (SCH) 4. Cell Broadcast Channel (CBCH)

Common Control Channels (CCCH) 1. Paging Channel (PCH) 2. Random Access Channel (RACH) 3. Access Grant Channel (AGCH)

Standalone Dedicated Control Channel (SDCCH) 1. Associated Control Channel (ACCH) 2. Fast Associated Control Channel (FACCH) 3. Slow Associated Control Channel (SACCH)

Broadcast Channels (BCH) Broadcast Control Channel (BCCH) - DOWNLINK   - This channel contains system parameters needed to identify the network and gain access to the network. These parameters include the Location Area Code (LAC), the Mobile Network Code (MNC), the frequencies of neighboring cells, and access parameters. Frequency Correction Channel (FCCH) - DOWNLINK   - This channel is used by the MS as a frequency reference. This channel contains frequency correction bursts. Synchronization Channel (SCH) - DOWNLINK   - This channel is used by the MS to learn the Base Station Information Code (BSIC) as well as the TDMA frame number (FN). This lets the MS know what TDMA frame they are on within the hyper-frame. Cell Broadcast Channel (CBCH) - DOWNLINK   - This channel is not truly its own type of logical channel. The CBCH is for point-to-omnipoint messages. It is used to broadcast specific information to network subscribers; such as weather, traffic, sports, stocks, etc. Messages can be of any nature depending on what service is provided. Messages are normally public service type messages or announcements. The CBCH isn’t allocated a slot for itself, it is assigned to an SDCCH. It only occurs on the downlink. The CBCH usually occupies the second sub slot of the SDCCH. The mobile will not acknowledge any of the messages. Common Control Channels (CCCH) Paging Channel (PCH) - DOWNLINK   - This channel is used to inform the MS that it has incoming traffic. The traffic could be a voice call, SMS, or some other form of traffic. Random Access Channel (RACH) - UPLINK   This channel is used by a MS to request an initial dedicated channel from the BTS. This would be the first transmission made by a MS to access the

network and request radio resources. The MS sends an  Access Burst on this channel in order to request access.

Access Grant Channel (AGCH) - DOWNLINK  - This channel is used by a BTS to notify the MS of the assignment of an initial SDCCH for initial signaling. Standalone Dedicated Control Channel (SDCCH) - UPLINK/DOWNLINK  - This channel is used for signaling and call setup between the MS and the BTS. Associated Control Channels (ACCH) Fast Associated Control Channel (FACCH) - UPLINK/DOWNLINK  - This channel is used for control requirements such as handoffs. There is no TS and frame allocation dedicated to a FAACH. The FAACH is a burst-stealing channel, it steals a Timeslot from a Traffic Channel (TCH). Slow Associated Control Channel (SACCH) - UPLINK/DOWNLINK   - This channel is a continuous stream channel that is used for control and supervisory signals associated with the traffic channels.