tailieunhanh - GSM Networks : Protocols, Terminology, and Implementation - Chapter 10

Giao diện A- Trên bình diện vật lý, các giao diện A-bao gồm một hoặc nhiều PCM liên kết giữa MSC và BSC, mỗi với một dung lượng truyền dẫn của 2 Mbps. Trâu, mà thường là nằm giữa MSC và BSC, đã được đưa vào xem xét khi kiểm tra giao diện này. Do đó, các giao diện A-có thể được tách ra thành hai phần. • Phần đầu tiên là giữa các trạm BTS và Trâu, nơi mà truyền- | 10 The A-Interface On the physical level the A-interface consists of one or more PCM links between the MSC and the BSC each with a transmission capacity of 2 Mbps. The TRAU which typically is located between the MSC and the BSC has to be taken into consideration when examining this interface. Consequently the A-interface can be separated into two parts. The first part is between the BTS and the TRAU where the transmitted payload still is compressed. Figure shows a possible channel configuration for three trunks. As on the Abis-interface a single traffic channel occupies only two of the eight bits of a PCM channel. That is why it is possible to transport four fullrate traffic channels on one PCM channel. The exceptions are TSs where signaling information is carried. Signaling information requires the entire 64 Kbps of a channel . TS 16 in Figure . The second part of the A-interface is the one between the TRAU and the MSC. There all data are uncompressed. For that reason every traffic channel requires the complete 8 bits or occupies an entire 64-Kbps PCM channel. The position of a signaling channel may be different before and after the TRAU as Figure shows. Dimensioning An examination of the channel configuration in Figure makes it obvious that the transmission resources between the BSC and the TRAU are used only 171 172 GSM Networks Protocols Terminology and Implementation Figure Possible channel configuration between the BSC and the MSC. The A-Interface 173 inefficiently. Only 2 bits of the PCM channel are actually occupied while the remaining 6 bits stay vacant. In that respect the A-interface between the BSC and the TRAU can be compared to the Abis-interface in a star configuration. It is possible by means of multiplexing to transport the data of several trunks from the BSC over only one physical 2-Mbps link before the data are actually handed over to the TRAU for decompression. That allows a savings of about two-thirds of the .