ghts reserved. Any use is subject to the Terms of Use as given at the website. TDMA Wireless Telephony (GSM) Key Radio Concepts The other cell-wide signaling channel is the paging and access-grant channel (PAGCH). Its reverse link is the random-access channel (RACH), where the mobile tele- phone requests access to the system. There are two signaling modes on the traffic channel (TCH). The slow associated con- trol channel (SACCH) uses parts of the TCH not associated with speech coding, and the fast associated control channel (FACCH) uses parts of the TCH normally used for speech. For this reason, the SACCH is called out-of-band signaling, and the FACCH called in-band. Both of them are part of the same digital stream, but the FACCH takes bits from the voice channel stream and relies on the error-correction component of the GSM speech coder to fix it. 7.4.1 GSM call setup In GSM call setup, we think of the base station system (BSS), all the base stations served by a single BSC, as a single entity. An important difference between AMPS and GSM is that GSM has much more comprehensive specification of interfaces other than the air interface. Any AMPS mobile telephone should work on any AMPS system, but the interfaces between base stations and switches are often proprietary, requiring that both components be made by the same manufacturer. In contrast, any GSM BSC should work with any GSM BTS and any GSM MSC, at least in theory. (We would still recommend testing equipment for compatibility before investing a lot of money based on the GSM compatibility claim.) When the mobile telephone initiates a call, it uses the RACH to get the BSS to as- sign it a stand-alone dedicated control channel (SDCCH), which is sent over the access- grant channel (AGCH). The SDCCH is used until a voice traffic channel is assigned. Once the mobile telephone is set with the BSS, a service request message is sent to the MSC, which informs its home location register (HLR) or its visitor location register (VLR) that the mobile telephone is requesti