Logical Channels of GSM Radio Interface

The association of a radio frequency channel and a time slot-the pair ARFCN and TN-uniquely defines a physical channel on both the uplink and the downlink. On top of the physical channels, logical channels ar mapped to convey the information of voice, data, and signaling. This signaling information is used for setting up a call, or to adapt the link to rapidly changing radio conditions, or to manage handovers, to give a few examples.

Logical channels can be seen as pipes, each one used for a different purpose by the higher layers of the system. Two types of logical channels exist, traffic channels and control channels. Among the control channels, according to their functions, four classes are defined: broadcast, dedicated, common, and associated. A broadcast channel is used by the network (in downlink only) to send general information to the MSs.

A channel is said to be dedicated if only one MS can transmit or receive in the ARFCN-TN defining this channel, and common if it carries information for several mobiles. An associated control channel is allocated to one mobile, in addition to a dedicated channel, and carries signaling for the operation of this channel. The purposes of the beacon are:

  • To allow a synchronization in time and frequency of the MSs to the BTS. This synchronization is needed by the MS to access the services of a cell.
  • To help the mobile in estimating the quality of the link during a communication, by measurements on the received signal from the BTS it is transmitting to, and from the other BTSs of the geographical area. These measurements are used by the network to determine when a handover is necessary, and to which BTS this handover should apply.
  • To help the mobile in the selection of a cell when it is in idle mode (that is, not in communication, but still synchronized to the system and able to receive an incoming call or to initiate a call). This selection is performed on the basis of the received power measurements made on the adjacent cells' beacon channels.
  • To access the general parameters of the cell needed for the procedures applied by the MS, or general information concerning the cell, such as its identification, the beacon frequencies of the surrounding cells, or the option supported by the cell (services).

To allow these various operations, the logical channels transmitted on the beacon are:

  • The broadcast control channel (BCCH), which continually broadcasts, on the downlink, general information on the cell, including base station identity, frequency allocations, and frequency-hopping sequences. The information is transmitted within system information (SI) blocks, which can be of different types according to the information that is carried out. The frequency with which an SI is retransmitted on the BCCH varies with the type of information.
  • The frequency control channel (FCCH), used by the MS to adjust its local oscillator (LO) to the BTS oscillator, in order to have a frequency synchronization between the MS and the BTS.
  • The synchronization channel (SCH), used by the MS to synchronize in time with the BTS, and to identify the cell.

As listed below, four channels comprise the common control channels (CCCH). Among these, the first three are used for the MS-initiated call or for call paging (notification of an incoming call toward the MS):

  • The random access channel (RACH) is used for the MS access requests to the network, for the establishment of a call, based on a slotted aloha method.
  • The paging channel (PCH) is defined to inform the MS of an incoming call.
  • The access grant channel (AGCH) is used to allocate some physical resource to a mobile for signaling, following a request on the RACH.
  • The cell broadcast channel (CBCH) may be used to broadcast specific news to the mobiles of a cell.

The dedicated control channels are:

  • The stand-alone dedicated control channel (SDCCH), utilized for registration, authentication, call setup, and location updating.
  • The slow associated control channel (SACCH), which carries signaling for the TCH or SDCCH with which it corresponds. The information that is transmitted on this channel concerns the radio link control (RLC), such as the power control on the corresponding TCH or SDCCH, or the time synchronization between the MS and the BTS.
  • The fast associated control channel (FACCH), carries the signaling that must be sent by the network to the MS to notify that a handover is occurring.

The TCHs can be of several types, according to the service that is accessed by the subscribers: voice or data, with various possible data rates.