Introduction to GSM: Physical Channels, Logical Channels, Network, and Operation by Lawrence Harte

Author:Lawrence Harte [Harte, Lawrence]
Language: eng
Format: azw3
Publisher: UNKNOWN
Published: 2017-03-22T04:00:00+00:00

Figure 1.22., Logical Channels Used in GSM Systems

Dedicated Control Channel Signaling Dedicated control channels are a signaling channel that is used solely for control of a specific device. The GSM system uses dedicated control channels to assist with radio channel assignment and to control the mobile telephone while it is on a traffic channel (voice call or data session).

Stand Alone Dedicated Control Channel (SDCCH) The stand alone dedicated control channel is a signaling channel that can be used to coordinate the radio channel assignment of a mobile device after it has successfully competed for access. The SDCCH channel is used for off air call setup (OACSU) to allow the mobile device to authenticate and complete other control processes without being assigned to a dedicated traffic channel.

Traffic Channel Signaling Signaling on the traffic channel is divided into two channels; the Fast Associated Control Channel (FACCH) and the Slow Associated Control Channel (SACCH). The FACCH replaces speech with signal data. The SACCH uses dedicated (scheduled) frames within each burst.

Slow Associated Control Channel (SACCH) Slow associated control channel (SACCH) is used to continuously transmit certain call processing and control signals at a low bit rate. The SACCH is normally sent along with user data so it does not subtract or use bits from the user data portion. It is therefore sometimes called “out of band” transmission. In full-rate GSM systems, the SACCH data is transmitted in the same time slot that would otherwise be used for digital subscriber traffic. During a scheduled sequence of 26 transmission frames, 24 of these carry digital subscriber traffic, one carries SACCH data, and one is not used.

SACCH is primarily used to transfer radio channel signal quality information from the mobile device to the base station to assist with the handover process. Because SACCH messages do not replace user data (voice signals), the sending of SACCH messages does not affect the quality of speech. However, the data transmission rate of the SACCH is very low and the transmission delay is approximately ½ second.

Figure 1.23 illustrates the SACCH signaling process. This example shows that SACCH messages do not replace voice data, it is sent on a dedicated SACCH time slot on 26 traffic multiframes. Because the SACCH message is distributed over multiple time slots, each SACCH message experiences a delay of approximately 480 msec.

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