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SDCCH Vs. SACCH

This page on GSM SDCCH Vs. SACCH describes difference between SDCCH and SACCH channels used in GSM technology. Both the channels fall under signaling logical channel category. Both are dedicated control channel. They both are used in uplink as well as downlink direction. Both are carried by normal burst structure.



SACCH stands for Slow Associated Control Channel and SDCCH stands for Stand Alone Dedicated Control Channel.

SDCCH is the channel used for exchange of signaling messages between GSM mobile and network(base station). This is used for pure signaling purpose or establishing traffic connection. SDCCH is allocated to the mobile in AGCH response from base station for the RACH sent by MS.

The SDCCH allocation is short-lived. It is used for location update and after this procedure is said to be complete and the SDCCH is channel is released.

If this SDCCH is used to establish the traffic channel connection then also once the connection is established the SDCCH channel is released.

When the dedicated channel is allocated to Mobile Subscriber(MS) i.e. dedicated traffic channel or a dedicated SDCCH channel. Now another channel is very much required to exchange measurement information regularly. This is fulfilled in the GSM system by providing SACCH channel. The data rate is very slow and hence the name.

VISIT following links for allocation of SDCCH and SACCH in GSM multiframe structure of combined and non-combined type.
• GSM noncombined channel configuration
• GSM combined channel configuration

One multiframe structure has only one SACCH slot and complete information of SACCH carried by normal burst needs four bursts. Hence four multiframes are required to carry an SACCH message.This indicates that one SACCH message can be delivered on about 480 ms.This is the reason of name as slow channel. For consecutive timeslots one SACCH slot is on 12th and another is on 25th frame.

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