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380 SIGNALING IN CELLULAR MOBILE TELECOMMUNICATIONSThe RELEASE COMPLETE message (Section 12.9.3) indicates that the other partyalso will stop sending and receiving information.After the network has sent or received the RELEASE COMPLETE, it sends aCHANNEL RELEASE message (Section 12.8.1) and then releases TACH. Onreceipt of the message, the MS also releases TACH.12.9 LAYER 3 MESSAGES ON THE Um INTERFACEETSI has defined some seventy GSM layer 3 messages on the Um interface. Theyare grouped according to the sublayers at the MS (Fig. 12.7-2). This sectiondescribes the messages mentioned in the previous section.Figure 12.9-1 shows the general message layout. The protocol discriminatorspecifies the message as a RR, MM, or CM message. Within each protocol, the particularmessage is indicated by the message type. Messages can include mandatoryand optional information elements IEs. The descriptions include the most importantIEs in the messages. For other IEs and details of IE coding, see [21].12.9.1 Messages for Radio Resources Management (RR)Assignment Command. This message is sent (on SDCCH) from the network tothe MS and indicates that a TACH has been assigned. Included is the channeldescription IE, which specifies the RF carrier channel number (ARFCN) and timeslot of TACH. Also included is the channel mode IE, which indicates the communicationmode on the channel (signaling, speech, or data).Assignment Complete. This is a response to the assignment command, sent bythe MS (on SDCCH). It indicates that the mobile wi1l tune to the assigned channel.Channel Release. This message is sent by the network—on the SDCCH orTACH that is currently assigned to the MS. It indicates that the network has releasedthe channel. It includes the release cause IE, which gives the reason for the release(normal release, release caused by failure, etc.).Figure 12.9-1. Layout of layer 3 messages on the Um interface. (From GSM 04.08 Version4.10 0. Courtesy of ETSI.)