Hệ thống con trạm gốc (BSS), kết nối tất cả các thuê bao mạng lõi, là kết nối với các MSCs thông qua một số của 2 Mbit / s E-1 kết nối. Giao diện này được gọi là A-giao diện. Như đã được hiển thị trong mục , các giao thức BSSMAP và DTAP được sử dụng trên giao diện | Global System for Mobile Communications GSM 11 The base station subsystem BSS which connects all subscribers to the core network is connected to the MSCs via a number of 2 Mbit s E-1 connections. This interface is called the A-interface. As has been shown in Section the BSSMAP and DTAP protocols are used over the A-interface for communication between the MSC the BSS and the mobile stations. As an E-1 connection can only carry 31 channels many E-1 connections are necessary to connect an MSC to the BSS. In practice this means that many E-1s are bundled and sent over optical connections such as STM-1 to the BSS. Another reason to use an optical connection is that electrical signals can only be carried over long distances with great effort and it is not unusual that an MSC is over 100 kilometers away from the next BSS node. As an MSC only has a limited switching capacity and processing power a PLMN is usually composed of dozens or even hundreds of independent MSCs. Each MSC thus covers only a certain area of the network. In order to ensure connectivity beyond the immediate coverage area of an MSC E-1s which are again bundled into optical connections are used to interconnect the different MSCs of a network. As a subscriber can roam into the area that is controlled by a different MSC while a connection is active it is necessary to change the route of an active connection to the new MSC handover . The necessary signaling connection is called the E-interface. ISUP is used for the establishment of the speech path between different MSCs and the MAP protocol is used for the handover signaling between the MSCs. Further information about the handover process can be found in Section . The C-interface is used to connect the MSCs of a network with the home location register HLR of the mobile network. While the A-and E-interface described previously always consist of signaling and speech path links the C-interface is a pure signaling link. Speech channels are not necessary
