tailieunhanh - McGraw Hill - 2002 - W-CDMA and cdma2000 for 3G Mobile Networks_6

Tham khảo tài liệu 'mcgraw hill - 2002 - w-cdma and cdma2000 for 3g mobile networks_6', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 174 Chapter 5 the figure interfaces GSM to a private or public network such as a PSTN ISDN circuit-switched public data network packet-switched public data network and so on. Because the data communication protocols of a mobile station may be different from those of devices it is in communication with across a network entity IWF may perform protocol conversion rate adaptation and so on. Short messaging service A mobile station in GSM may transmit or receive short alphanumeric messages during both idle and active call states. GPRS This is a new service available with GSM Phase 2 that enables multiple users to transmit packet data over a single slot. In this section we will present a brief description of GPRS. General Capabilities and Features of GPRS In circuit-switched data services when a user wants to transmit or receive any data first a physical channel is set up using the normal GSM call control procedures. Because data usually comes in bursts separated by variable periods of inactivity the channel may remain idle for a considerable length of time depending upon the type of data services being used. One could of course release the channel during inactive periods of data and reestablish the connection when user data is ready. However this approach is not very efficient or practical because delays associated with call control procedures for setting up a physical channel are relatively long. A packet switching system where multiple users may transmit their data over the same physical channel using the so-called virtual circuits overcomes this problem by taking advantage of the statistical nature of the traffic arrival process. The virtual circuits may be either permanent or switched. But even when they are switched call control delays for setting up or tearing down a virtual circuit are usually very small. As we mentioned before GPRS is a new feature of GSM that provides the capability of packet mode transmission of user data and signaling information using the .