GSM Phase 2+ General Packet Radio Service GPRS: Architecture, Protocols, and Air Interface

The General Packet Radio Service (GPRS) is a new bearer service for GSM that greatly improves and simplifies wireless access to packet data networks, e.g., to the Internet. It applies a packet radio principle to transfer user data packets in an efficient way between mobile stations and external packet data networks. This tutorial gives an introduction to GPRS. The article discusses the system architecture and its basic functionality. It explains the offered services, the session and mobility management, the routing, the GPRS air interface including channel coding, and the GPRS protocol architecture. Finally, an interworking example between GPRS and IP networks is shown. The impressive growth of cellular mobile telephony as well as the number of Internet users promises an exciting potential for a market that combines both innovations: cellular wireless data services. Within the next few years, there will be an extensive demand for wireless data services. In particular, highperformance wireless Internet access will be requested by users. Existing cellular data services do not fulfill the needs of users and providers. From the user's point of view, data rates are too slow and the connection setup takes too long and is rather complicated. Moreover, the service is too expensive for most users. From the technical point of view, the drawback results from the fact that current wireless data services are based on circuit switched radio transmission. At the air interface, a complete traffic channel is allocated for a single user for the entire call period. In case of bursty traffic (e.g., Internet traffic), this results in a highly inefficient resource utilization. It is obvious that for bursty traffic, packet switched bearer services result in a much better utilization of the traffic channels. This is because a channel will only be allocated when needed and will be released immediately after the transmission of the packets. With this principle, multiple users can share one physical channel (statistical multiplexing). In order to address these inefficiencies, two cellular packet data technologies have been developed so far: cellular digital packet data (CDPD) (for AMPS, IS-95, and IS-136) and the General Packet Radio Service (GPRS). GPRS is the topic of this paper. It was originally developed for GSM, but will also be integrated within IS-136 (see [1]). We treat GPRS from the point of view of GSM. GPRS is a new bearer service for GSM that greatly improves and simplifies wireless access to packet data networks, e.g., to the Internet. It applies a packet radio principle to transfer user data packets in an efficient way between GSM mobile stations and external packet data networks. Packets can be directly routed from the GPRS mobile stations to packet switched networks. Networks based on the Internet Protocol (IP) (e.g., the global Internet or private/corporate intranets) and X.25 networks are supported in the current version of GPRS. Users of GPRS benefit from shorter access times and higher data rates. In conventional GSM, the connection setup takes several seconds and rates for data transmission are restricted to 9.6 kbit/s. GPRS in practice offers session establishment times below one second and ISDN-like data rates up to several ten kbit/s. In addition, GPRS packet transmission offers a more user-friendly billing than that offered by circuit switched services. In circuit switched services, billing is based on the duration of the connection. This is unsuitable for applications with bursty traffic. The user must pay for the entire airtime, even for idle periods when no packets are sent (e.g., when the user reads a Web page). In contrast to this, with packet switched services, billing can be based on the amount of transmitted data. The advantage for the user is that he or she can be "online" over a long period of time but will be billed based on the transmitted data volume. To sum up, GPRS improves the utilization of the radio resources, offers volume-based billing, higher transfer rates, shorter access times, and simplifies the access to packet data networks. GPRS has been standardized by ETSI (the European Telecommunications Standards Institute) during the last five years. It finds great interest among many GSM network providers. At the moment field trials are being carried out, and it is expected that GPRS will be implemented in various countries by the middle of 2000 (see, e.g., [2][3] for Germany). This article provides an introduction to GPRS. We assume that the reader is familiar with the basic concepts of cellular networks. A brief overview of the GSM system can be found in [4]. In addition, there exists a variety of books on GSM, e.g., [5][6][7]. The structure of the paper is as follows. First we describe the GPRS system architecture and discuss the fundamental functionality. We then describe the offered services and the Quality of Service parameters. Afterward we show how a GPRS mobile station registers with the network, and how the network keeps track of its location. An example of how packets are routed in GPRS is given. Next, the physical layer at the air interface is explained, and we discuss the concept of multiple access, radio resource management, and the logical channels and their mapping onto physical channels. We then consider GPRS channel coding, and follow this with a discussion of the GPRS protocol architecture. Finally, we give an example of a GPRS-Internet interconnection.