SIGNALING CHANNEL - GSM RADIO INTERFACE

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Tue, 08/04/2009 - 18:30

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GSM RADIO CONCEPTS

Radio Signaling Channels

Below are the main types of signaling Channels used for radio communication

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 signaling channels can be further divided into the following logical channels:

Broadcast Channels (BCH)
     Broadcast Control Channel (BCCH)
     Frequency Correction Channel (FCCH)
     Synchronization Channel (SCH)
     Cell Broadcast Channel (CBCH)

Common Control Channels (CCCH)
     Paging Channel (PCH)
     Random Access Channel (RACH)
     Access Grant Channel (AGCH)

Standalone Dedicated Control Channel (SDCCH)
     Associated Control Channel (ACCH)
     Fast Associated Control Channel (FACCH)
     Slow Associated Control Channel (SACCH)

Let's discuss each type of logical channel individually.

Broadcast Channels (BCH)

Broadcast Control Channel (BCCH) – BCCH is a downlink channel. This channel contains system parameters needed to identify the network and gain access. 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) - FCCH is a downlink channel. This channel is used by the MS as a frequency reference. This channel contains frequency correction bursts.
Synchronization Channel (SCH
) SCH is a downlink channel. 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 hyperframe.

Cell Broadcast Channel (CBCH) CBCH is a downlink channel. 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) – PCH is a downlink channel. 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) –
RACH is a Uplink channel. 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) – AGCH is a downlink channel. 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)

SDCCH is used as both uplink and 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) – Used as both 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) – Used as both UPLINK/DOWNLINK - This channel is a continuous stream channel that is used for control and supervisory signals associated with the traffic channels.

Signaling Channel Mapping

Normally the first two timeslots are allocated to signaling channels.

Need to remember that Control Channel composed of 51 TDMA frames.

On a time slot within the multiframe, the 51 TDMA frames are divided up and allocated to the various logical channels.

There are several channel combinations allowed in GSM. Some of the more common ones are:

FCCH + SCH + BCCH + CCCH
BCCH + CCCH
FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)
SDCCH/8(0 .7) + SACCH/C8(0 . 7)

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