SDCCH vs. SACCH: Understanding GSM Control Channels

This page describes the difference between SDCCH and SACCH channels used in GSM technology.

Both channels fall under the signaling logical channel category and are dedicated control channels. They are used in both the uplink and downlink directions and are carried by a normal burst structure.

• SACCH: Slow Associated Control Channel
• SDCCH: Stand Alone Dedicated Control Channel

SDCCH is the channel used for the exchange of signaling messages between a GSM mobile and the network (base station). It is used for pure signaling purposes or to establish a traffic connection.

SDCCH Allocation and Usage

The SDCCH is allocated to the mobile in the AGCH response from the base station, in response to the RACH sent by the MS. The SDCCH allocation is short-lived. It is used for location updates, and after this procedure is complete, the SDCCH channel is released. If the SDCCH is used to establish a traffic channel connection, it is also released once the connection is established.

SACCH Role and Functionality

When a dedicated channel is allocated to the Mobile Subscriber (MS), whether a dedicated traffic channel or a dedicated SDCCH channel, another channel is required to exchange measurement information regularly. This need is fulfilled in the GSM system by providing the SACCH channel. The data rate is slow, hence the name “Slow Associated Control Channel”.

SACCH Timing and Data Rate

One multiframe structure has only one SACCH slot, and complete information of SACCH carried by a normal burst requires four bursts. Therefore, four multiframes are required to carry an SACCH message.

This means that one SACCH message can be delivered in approximately 480 ms, explaining the “slow” designation.

For consecutive timeslots, one SACCH slot is on the 12th frame, and another is on the 25th frame.