The Global System for Mobile (GSM) telecommunications is used in cellular telephone networks in many countries around the world. GSM offers a useful range of network services and standards, including facilities for data, as well as voice, transmission. Existing GSM networks are based on time-division multiple access (TDMA) digital communications technology.
Code-division multiple access (CDMA) is an improved digital communications technology, which affords more efficient use of radio bandwidth than TDMA, as well as a more reliable, fade-free link between cellular telephone subscribers and base stations. The leading CDMA standard is IS-95, promulgated by the Telecommunications Industry Association (TIA).
PCT patent application PCT/US96/20764, which is incorporated herein by reference, describes a wireless telecommunications system that uses a CDMA air interface (i.e., basic RF communications protocols) to implement GSM network services and protocols. Using this system, at least some of the TDMA base stations (BSSs) and subscriber units of an existing GSM network would be replaced or supplemented by corresponding CDMA equipment. CDMA BSSs in this system are adapted to communicate with GSM mobile switching centers (MSCs) via a standard GSM A-interface. The core of GSM network services is thus maintained, and the changeover from TDMA to CDMA is transparent to users.
Hybrid cellular communications networks, incorporating both GSM and CDMA elements, are also described in PCT patent publications WO 95/24771 and WO 96/21999, and in an article by Tscha, et al., entitled "A Subscriber Signaling Gateway between CDMA Mobile Station and GSM Mobile Switching Center," in Proceedings of the 2nd International Conference on Universal Personal Communications, Ottawa (1993), pp. 181-185, which are incorporated herein by reference. None of these publications deals with specific issues of data communications. Such issues include both the need for protocol compatibility and the different demands of voice and data communications. Although TIA has promulgated standards of CDMA over-the-air data transmission, such as IS-657 and IS-707, the GSM BSS-MSC interface is not suited to support such transmission.
Generally speaking, because GSM was developed primarily for circuit-switched transmission, it is not well suited for packet-switched data, as is commonly transmitted over the Internet. For this reason, the European Telecommunications Standards Institute (ETSI) has proposed a general packet data service (GPRS) to operate in conjunction with GSM cellular networks. GPRS is described in a number of GSM standards, including 02.60, 03.60 and 03.64, which are incorporated herein by reference.
When a subscriber unit (or mobile station--MS) in a GSM network with GPRS wishes to send and/or receive packet-switched data, the MS through the BSS with which it is in communication makes contact with a "serving GPRS support node" (SGSN), over a GSM-standard Gb interface. The data are transmitted and received by the BSS through the SGSN, separately from the voice channels that go through the MSC, to a packet data network (PDN), such as the Internet. The SGSN likewise maintains its own separate mobility management and security facilities. Unlike GSM-TDMA voice communications, GPRS allows dynamic time slot allocation in the air interface between the subscriber units and the SGSN.
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