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12.7 INTRODUCTION TO THE GSM CELLULAR SYSTEM 369Data Link Layer (Layer 2). This consists of the Message Transfer Part level 2(MTP2) of Signaling System No. 7 (SS7)—see Chapter 8. MTP2 is responsiblefor the reliable transfer of signaling messages between the MSC andthe BSS. Details can be found in [l8].Message Layer (Layer 3). The Base Station System Application Part (BSSAP) ispresent at the MSC and BSS [19]. It is a user of the signaling connection controlpart (SCCP) of SS7 (Chapter 15). This is one of the few applications ofconnection-oriented SCCP. A signaling connection is established whenevera dedicated channel (SDCCH, FACCH, or FACCH) has been assigned to a MS.BSSAP consists of two parts: the Direct Transfer Application Part (DTAP) andthe BSS Management Application Part (BSSMAP).We start with BSSMAP at BSS. The RR at a BSS is involved in the allocation,encryption, and release of dedicated radio channels and in the transmission andreception of RR messages on the common-control radio channels. The BSSMAPat BSS and MSC handles the transfer of RR-related BSSMAP messages. At aBSS, RR and BSSMAP communicate with each other. Figure 12.7-3 illustrates afew RR–BSSMAP interactions. In example (a), a MS sends a RR message—onthe RACH—to request a dedicated radio channel. RR at BSS then allocates achannel, returns a RR message on PAGCH that includes the identity of thechannel, and informs its BSSMAP, which then composes a message that informsthe MSC and sends it to BSSMAP at the MSC. In example (b), the MSC sends aBSSMAP message requesting the transmission of a RR message on a commoncontrolradio channel. The requested RR message may be a paging message,Figure 12.7-3. Transfer of RR messages. (From GSM 04.07 Version.10.0. Courtesy of ETSI.)