functional equivalent of theIDSM D-channel and sometimes called Dm (along with the SDCCH).
SACCH (Slow Associated Control Channel) 慢速随路控制信道
A bidirectional point-to-point channel used forauxiliary signaling during any transaction where a TCH/FACCH or SDCCH is present. TheSACCH is normally used to carry system information and measurement reports related tohandover planning and link status. It can also carry SMS during an in-progress call. Like theSDCCH and FACCH, the SACCH is modeled after the ISDN D-channel.
经典的通信流程 1. The MS powers up and searches its supported bands for the ARFCN with the strongest signal.This signal is assumed to be the beacon of a nearby cell.
2. The MS searches (with a correlator) for the SCH on the selected ARFCN. (If no SCH isfound, the MS continues to try other ARFCNs according to their power.)
3. The MS decodes the SCH to get BTS timing and syncs its local frame clock with that of theBTS. (The content of the SCH message is described in GSM 04.08 Section 9.1.30.)
4. Having the correct frame timing, the MS can now demultiplex the BCCH. The BCCH iden-tifies the carrier and provides detailed information on the services offered by the BTS andthe multiplexing configuration of the CCCH. This information is carried in a series of SystemInformation Messages, described in several subsections of GSM 04.08 Section 9.1.
5. The MS starts decoding the CCCH and sends access requests on the RACH. Each RACHmessage occupies a single radio burst and contains a random tag. The MS sends up to 8RACH bursts separated by random delays of 1-2 seconds while checking the CCCH for aresponse. The content of the RACH burst is documented in GSM 04.08 Section 9.1.8.
6. The BTS receives a RACH burst from the MS and responds with a channel assignmentmessage (Immediate Assignment, GSM 04.08 Section 9.1.18) on the CCCH. In this messagethe BTS echoes back the tag and timing from the corresponding RACH burst so that the MScan recognize it. The message assigns the SDCCH for a transaction.
7. The MS receives the immediate assignment on the CCCH and changes its multiplexing andpossibly its ARFCN to match the parameters of the message.
8. The MS switches over to the assigned SDCCH, waits (with a brief timeout) to detect anSDCCH in the idle state. Upon verifying the SDCCH, the MS sends a Location UpdatingRequest (GSM 04.08 Section 9.2.15) containing some form of mobile identity.
9. Assuming the BTS accepts the MS, it responds with a Location Updating Accept (GSM 04.08Section 9.2.13). The MS and BTS then close the SDCCH and release the associated radioresources.
10. The MS is now “camped” on the BTS and continues to monitor the CCCH for Paging Requestmessages (GSM 04.08 Sections 9.1.22-9.1.24) associated with incoming (mobile-terminated)calls or text message deliveries.