Interfaces and Protocols The previous section introduced GSM network architecture, and this section introduces the SS7/C7 protocols that are used. It also discusses interfaces, because different protocols are used | Interfaces and Protocols The previous section introduced GSM network architecture and this section introduces the SS7 C7 protocols that are used. It also discusses interfaces because different protocols are used on different interfaces. The SS7 C7 protocols MTP SCCP TUP ISUP are protocols that were used before digital wireless networks were available. The final part of this section introduces SS7 C7 protocols that were specifically developed for GSM. Table 12-2 summarizes the interfaces and protocols that are used in GSM. Table 12-2. GSM Interfaces and Protocols Interface Between Description Um MS-BSS The air interface is used for exchanges between a MS and a BSS. LAPDm a modified version of the ISDN LAPD is used for signaling. Abis BSC-BTS This is a BSS internal interface that links the BSC and a BTS it has not been standardized. The Abis interface allows control of radio equipment and radio frequency allocation in the BTS. A BSS-MSC The A interface is between the BSS and the MSC. It manages the allocation of suitable radio resources to the MSs and mobility management. It uses the BSSAP protocols BSSMAP and DTAP . B MSC-VLR The B interface handles signaling between the MSC and the VLR. It uses the MAP B protocol. Most MSCs are associated with a VLR making the B interface internal. Whenever the MSC needs to access data regarding an MS that is located in its area it interrogates the VLR using the MAP B protocol over the B interface. C GMSC-HLR or SMSG-HLR The C interface is between the HLR and a GMSC or a SMSC. Each call that originates outside of GSM such as an MS terminating call from the PSTN must go through a gateway to obtain the routing information that is required to complete the call and the MAP C protocol over the C interface is used for this purpose. Also the MSC can optionally forward billing information to the HLR after call clearing. D HLR-VLR The D interface is between the HLR and VLR and uses the MAP D protocol to exchange data related to the location
