Internet Access over GPRS
Introduction
I will explain how the General Packet Radio Service (GPRS) technology is used for providing mobile/wireless access to the Internet. We explain the fundamental GPRS concepts, protocols, and procedures and demonstrate the main functionality provided by the GPRS network. The key procedures examined are the registration procedure, the routing/tunneling procedure, and the mobility management procedure, all of which enable mobile/wireless IP sessions.
GPRS is a bearer service of the Global System for Mobile (GSM) communications, which offers packet data capabilities. The key characteristic of the data service provided by GPRS is that it operates in endto- end packet mode. This means that no communication resources are exclusively reserved for supporting the communication needs of every individual mobile user. On the contrary, the communication resources are utilized on a demand basis and are statistically multiplexed between several mobile users. This characteristic renders GPRS ideal for applications with irregular traffic properties (such as Web browsing), because, with this type of traffic, the benefits of statistical multiplexing are exploited; that is, we obtain high utilization efficiency of the communication resources. A direct effect of this property is the drastically increased capacity of the system in the sense that we can support a large number of mobile users with only a limited amount of communication resources. The increased capacity offered by GPRS, combined with the end-to-end packet transfer capabilities, constitute the main factors that drive the use of GPRS in providing wide-area wireless Internet access.
GPRS Overview
In general, a GPRS network can be viewed as a special IP network, which offers IP connectivity to IP terminals on the go. To provide such a mobile connectivity service, the GPRS network must feature additional functionality compared with standard IP networks. However, from a high-level point of view, the GPRS network resembles a typical IP network in the sense that it provides typical IP routing and interfaces to the external world through one or more IP routers.
By using shared radio resources, mobile users gain access to remote Packet Data Networks (PDNs) through a remote access router, which in GPRS terminology is termed Gateway GPRS Support Node (GGSN). You can think of access to a remote PDN as being similar to a typical dial-up connection. Indeed, as discussed in Section 3.3, a user establishes a virtual connection to the remote PDN. However, with GPRS a user may “dial up” to many remote PDNs simultaneously and can be charged by the volume of the transferred data, not by the duration of a connection. GPRS can offer both transparent and nontransparent access to a PDN. With transparent access, the user is not authenticated by the remote PDN, and he or she is assigned an IP address (private or public) from the address space of the GPRS network. On the other hand, with nontransparent access the user’s credentials are sent to the remote PDN and the user is permitted to access this PDN only if he or she is successfully authenticated. In this case, the user is typically assigned an IP address (private or public) from the address space of the PDN he or she is accessing. Note that, irrespective of the type of access to a PDN, a user is always authenticated by the GPRS network before being permitted access to GPRS services (this is further discussed in Section 3.2.3). The nontransparent access is particularly useful for accessing secure intranets (e.g., corporate networks) or Internet Service Providers (ISPs), whereas the transparent access is most appropriate for users who do not maintain subscriptions to third-party ISPs or intranets. As illustrated in Figure 3.1, the GPRS network forms an individual subnet, which (from an address-allocation point of view) contains all users who use transparent access to remote PDNs. External PDNs perceive this subnet as being a typical IP network.
GPRS Bearers
GPRS network effectively provides a GPRS bearer — that is, it provides a communication channel with specific attributes between the MS (the terminal) and the GGSN (the router). Over the GPRS bearer, the MS may send IP packets to the GGSN, and it may receive IP packets from the GGSN. As explained below, the GPRS bearer is dynamically set up at the beginning of an IP session (when the user “dials” to a specific PDN), and it can be tailored to match the specific requirements of an application. In other words, it can be set up with specific Quality of Service (QoS) attributes, such as delay, throughput, precedence, and reliability.
The PCU communicates with the Serving GPRS Support Node (SGSN) over a frame relay interface (Gb). The SGSN provides mobility management functionality, session management, packet scheduling on the downlink, and packet routing/tunneling. The interface between the SGSN and the GGSN (Gn) is entirely based on IP, typically on IPv4. The GGSN provides mainly routing and optionally screening functionality and can be considered to be a remote access router interfacing with the external PDNs. The fact that we have two IP layers within the GGSN implies that some sort of IP-to-IP tunneling is applied across the Gn interface.
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