tailieunhanh - GSM Networks : Protocols, Terminology, and Implementation - Chapter 8

Hệ thống tín hiệu số 7 Tín hiệu hệ thống số 7 (SS7) cung cấp trong OSI Layers 1-3 cơ sở cho các tín hiệu giao thông trên tất cả các giao diện NSS, cũng như trên giao diện-A. Mối quan hệ GSM chứ không phải là ẩn ở phần đầu của mô tả, nhưng nó sẽ trở thành nhiều hơn và rõ ràng hơn khi người sử dụng các bộ phận khác nhau như SCCP và TCAP / MAP được trình bày. SS7, cùng với tất cả các chức năng của nó và các bộ phận người sử dụng, hình. | 8 Signaling System Number 7 Signaling System Number 7 SS7 provides in OSI Layers 1 to 3 the basis for the signaling traffic on all NSS interfaces as well as on the A-interface. The relation to GSM is rather hidden at the beginning of the description but it becomes more and more obvious when the various user parts like SCCP and TCAP MAP are presented. SS7 together with all its functionality and user parts forms a much more complex signaling system than LAPD and LAPDm. For that reason this whole chapter is dedicated to SS7 or rather to its Layers 1 to 3. The SS7 Network A SS7 network consists of directly connected signaling points SPs as shown in Figure a SPs that are connected through signaling transfer points STPs as shown in Figure b or a combination of SPs and STPs as shown in Figure c . An SP is a network node that has user parts . SCCP ISUP that allow the processing of messages addressed to that SP. The MSC the BSC and the exchanges of the PSTN fall into this category. The functionality of the STP typically is related to those of the SP with the additional capability of being able to relay SS7 messages. Note that it is possible to have a designated STP that has no SP functionality that is one that can only relay messages as shown in Figure b . 125 126 GSM Networks Protocols Terminology and Implementation SP - SP Figure a Directly connected SPs. Figure b An STP that interconnects SPs. Figure c An STP with SP functionality that interconnects SPs. Message Transfer Part SS7 without user parts consists only of the OSI Layers 1 through 3. Those three layers essentially are represented by the message transfer part MTP . Parts of the SCCP actually are also part of Layer 3. The MTP of SS7 performs the following general tasks It provides all the functionality of OSI Layers 1 to 3 required to provide for a reliable transport of signaling data to the various SS7 user parts. When problems arise the MTP takes the necessary measures to .