the MS, which always contains some form of mobile ID, is echoed back to the MS for verification. Location updating The location updating procedure is defined in GSM 04.08 Sections 4.4.1 and 7.3.1. This procedure normally is performed when the MS powers up or enters a new Location area but may also be performed at other times as described in the specifications. In its minimal form, the steps of the transaction are: The MS and BTS perform the radio channel establishment procedure. On the newly established dedicated channel, the MS sends the MM Location Updating Request message containing either an IMSI or TMSI. The message also implies connection establishment in the MM sublayer. The network verifies the mobile identity in the HLR or VLR and responds with the MM Location Updating Accept message. The network closes the Dm channel by sending the RR Channel Release message. There are many possible elaborations on this transaction, including: authentication ciphering TMSI assignment queries for other identity types location updating reject Mobile-Originating Call (MOC) establishment This is the transaction for an outgoing call from the MS, defined in GSM 04.08 Sections 5.2.1 and 7.3.2 but taken largely from ISDN Q.931. In its simplest form, the steps of the transaction are: The MS initiates the radio channel establishment procedure and is assigned to a Dm channel, usually an SDCCH. This establishes the connection in the L3 RR sublayer. The first message sent on the new Dm is the MM Connection Mode service Request, sent by the MS. This message contains a subscriber ID (IMSI or TMSI) and a description of the requested service, in this case MOC. The network verifies the subscriber's provisioning in the HLR and responds with the MM Connection Mode Service Accept message. This establishes the connection in the L3 MM sublayer. (This is a simplification. In most networks MM establishment is performed with authentication and ciphering transactions at this point.) The MS sends the CC Setup message, which contains the called party number. Assuming the called party number is valid, network response with the CC Call Proceeding message. The network sends an RR Assignment message to move the transaction off of the SDCCH and onto a TCH+FACCH. Once the MS has acquired the timing on the TCH+FACCH, it responds on the new FACCH with the RR Assignment Complete message. From this point on, all control transactions are on the FACCH. When