GSM手机经典接入流程

打电话的时候，先由MS通过RACH发起入网申请，网络再通过AGCH对MS的入网申请作出响应，分配一条SDCCH（这一过程是立即支配程序，用于位置更新或收发短消息），SDCCH也可以叫专用呼叫信道，网络通过SDCCH分配TCH。如果用户要打电话就需要进一步分配一条TCH（这一过程是支配程序，传送话音等）。接电话和打电话差不多，接的话网络多了一个寻呼的过程。
1. The MS powers up and searches its supported bands for the ARFCNwith the strongest signal.This signal is assumed to be the beaconof a nearby cell.
2. The MS searches (with a correlator) for the SCH on the selectedARFCN. (If no SCH isfound, the MS continues to try other ARFCNsaccording to their power.)
3. The MS decodes the SCH to get BTS timing and syncs its localframe clock with that of theBTS. (The content of the SCH message isdescribed in GSM 04.08 Section 9.1.30.)
4. Having the correct frame timing, the MS can now demultiplex theBCCH. The BCCH iden-tifies the carrier and provides detailedinformation on the services offered by the BTS andthe multiplexingconfiguration of the CCCH. This information is carried in a seriesof SystemInformation Messages, described in several subsections ofGSM 04.08 Section 9.1.
5. The MS starts decoding the CCCH and sends access requests on theRACH. Each RACHmessage occupies a single radio burst and contains arandom tag. The MS sends up to 8RACH bursts separated by randomdelays of 1-2 seconds while checking the CCCH for aresponse. Thecontent of the RACH burst is documented in GSM 04.08 Section9.1.8.
6. The BTS receives a RACH burst from the MS and responds with achannel assignmentmessage (Immediate Assignment, GSM 04.08 Section9.1.18) on the CCCH. In this messagethe BTS echoes back the tag andtiming from the corresponding RACH burst so that the MScanrecognize it. The message assigns the SDCCH for atransaction.
7. The MS receives the immediate assignment on the CCCH and changesits multiplexing andpossibly its ARFCN to match the parameters ofthe message.
8. The MS switches over to the assigned SDCCH, waits (with a brieftimeout) to detect anSDCCH in the idle state. Upon verifying theSDCCH, the MS sends a Location UpdatingRequest (GSM 04.08 Section9.2.15) containing some form of mobile identity.
9. Assuming the BTS accepts the MS, it responds with a LocationUpdating Accept (GSM 04.08Section 9.2.13). The MS and BTS thenclose the SDCCH and release the associated radioresources.
10. The MS is now “camped” on the BTS and continues to monitor theCCCH for Paging Requestmessages (GSM 04.08 Sections 9.1.22-9.1.24)associated with incoming (mobile-terminated)calls or text messagedeliveries.