FACCH. The MS starts alerting (ringing, etc.) and sends the CC Alerting message to the network. When the subscriber answers, the MS sends the CC Connect message to the network. The network response with the CC Connect Acknowledge message. At this point, the call is active. As in the MOC, the TCH+FACCH assignment can happen at any time, with the three common techniques being early, late and very early assignment. Call clearing The transaction for clearing a call is defined in GSM 04.08 Sections 5.4 and 7.3.4. This transaction is the same whether initiated by the MS or the network, the only difference being a reversal of roles. This transaction is taken from Q.931. Party A sends the CC Disconnect message. Party B responds with the CC Release message. Party A responds with the CC Release Complete message. The network releases the RR connection with the RR Channel Release message. This always comes from the network, regardless of which party initiated the clearing procedure. SMS transfer on Um GSM 04.11 and 03.40 define SMS in five layers: L1 is taken from the Dm channel type used, either SDCCH or SACCH. This layer terminates in the BSC. L2 is normally LAPDm, although GPRS-attached devices may use Logical link control (LLC, GSM 04.64). In LAPDm SMS uses SAP3. This layer terminates in the BTS. L3, the connection layer, defined in GSM 04.11 Section 5. This layer terminates in the MSC. L4, the relay layer, defined in GSM 04.11 Section 6. This layer terminates in the MSC. L5, the transfer layer, defined in GSM 03.40. This layer terminates in the SMSC. As a general rule, every message transferred in L(n) requires both a transfer and an acknowledgment on L(n-1). Only L1-L4 are visible on Um. Mobile-Originated SMS (MO-SMS) The transaction steps for MO-SMS are defined in GSM 04.11 Sections 5, 6 and Annex B. In the simplest case, error-free delivery outside of an established call, the transaction sequence is: The MS establishes an SDCCH using the standard RR establishment procedure. The MS sends a CM Service Request, The MS initiates multiframe mode in SAP3 with the normal LAPDm SABM procedure. The MS sends a CP-DATA message (L3, GSM 04.11 Section 7.2.1), which carries an RP-DATA message (L4, GSM 04.11 Section 7.3.1) in its RPDU. The network responds with a CP-ACK message (L3, GSM 04.11 Section 7.2.2). The network delivers the RPDU to the