An Investigation on
General Packet Radio Service (GPRS)
For Wireless Internet Solution
Computer Science and Software, McMaster University
The General Packet Radio Service (GPRS) is a new bearer service for Global System for Mobiles (GSM), which significantly improves and simplifies wireless access to the Internet. It applies a packet radio principle to transfer user's data packets in an efficient way between mobile stations (MS) and Internet. This paper investigated the GPRS, described the system architecture and basic functionality, explained how the GPRS network works with external Internet, and discussed the limitations of GPRS.
It had been forecast that the trend taking place in fixed networks - whereby data traffic is overtaking voice traffic - will also spill over into wireless networks. This transition was expected to occur in fixed networks around year 2000. The transition in wireless networks will follow soon thereafter. However, one challenge to this trend is that the current wireless infrastructures must evolve to accommodate the anticipated data traffic while simultaneously carrying voice traffic in an efficient, high-speed manner.
General Packet Radio Service (GPRS) is being introduced to efficiently transport high-speed data over the current wireless network infrastructures (GSM). GPRS signaling and data traffic do not travel through the GSM network. The GSM network is only used for table look up to obtain GPRS user profile data. GPRS uses 1 to 8 radio channel timeslots which can be shared by multiple users. It packetizes the user data and transports it over Public Land Mobile Networks (PLMN) using an IP backbone. From there, it interfaces to other Public Data Networks (PDNs), including the Internet. As a result, GPRS provides several unique features:
speed: the ability to offer speeds of 14,400 bps to 115,000 bps, which allow for comfortable Internet access using wireless devices.
2. GPRS SYSTEM ARCHITECTURE
The GPRS network system architecture is illustrated in Figure 1. A GSM network mainly consists of four components.
Mobile Switching Center (MSC) is the central component of the NSS. Operates all switching functions for the mobiles within its jurisdiction. Interface between mobile and other (including fixed) network.
Network Databases : Home Location Register and Visitor Location Register together with MSC provides the call routing and roaming capabilities of GSM.
In order to integrate GPRS into the existing GSM network, two major new core network elements are introduced: the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support node (GGSN).
Serving GPRS Support Node (SGSN): An SGSN is responsible for the delivery of data packets from and to the mobile stations within its service area. SGSNs send queries to Home Location Registers (HLRs) to obtain profile data of GPRS subscribers. SGSNs detect new GPRS mobile stations in a given service area; and, finally, SGSNs process registration of new mobile subscribers and keep a record of their location inside a given service area.
Gateway GPRS Support Node (GGSN) : GGSNs are used as interfaces between the GPRS backbone network and the external Public Packet Data Networks. GGSNs maintain routing information that is necessary to tunnel the Protocol Data Units (e.g IP) to the SGSNs that service particular mobile stations. Other functions include network and subscriber screening and address mapping. One or more GGSNs may support multiple SGSNs.
In addition to the new GPRS components, following existing GSM network elements must also be enhanced in order to support GPRS.
Base Station System (BSS): must be enhanced to recognize and send user data to the SGSN that is serving the area.
Home Location Register (HLR): must be enhanced to register GPRS user profiles and respond to queries originating from SGSNs regarding these profiles.
Figure 1. GPRS Network System Architecture
As can be seen, there are a number of new standardised network interfaces introduced:
Gb Frame relay connection between the SGSN and the PCU within the BSS. This transports both user data and signalling messages to/from the SGSN.
Gn The GPRS backbone network, implemented using IP LAN/WAN technology. Used to provide virtual connections between the SGSN and GGSN.
Gi The point of connection between GPRS and the external networks, each referenced by the Access Point Name. This will normally be implemented using IP WAN technology.
Gr Interface between the HLR and SGSN that allows access to customer subscription information. This has been implemented using enhancements to the existing GSM C7 MAP interface.
3. INTERWORKING WITH THE EXTERNAL INTERNET
The GPRS transmission plane is described in Figure 2, which provids transmission of user data and its associated signaling.
Before a GPRS mobile station can use GPRS services it must obtain an address used in the packet data network (a PDP address) and create a PDP context. The context describes the characteristics of the connection to the packet data network (PDP type, PDP address, service precedence, reliability, delay, throughput and GGSN). With an active PDP context, packets from mobile station will be sent to its current SGSN first, then this SGSN encapsulates the IP packets, examines the PDP context, and routes them to appropriate GGSN. The GGSN decapsulates the packets and sends them out on the IP network. Similarly packets from the external packet data network will be routed to the GGSN first, which then queries the HLR and obtains the information where the MS is currently located in. It encapsulates the incoming packets and tunnels them to the current SGSN of the mobile user. The SGSN decapculates the packets and delivers them to MS. Each GGSN has an IP address and each mobile station has been assigned an IP address by its GGSN. Thus the MS's IP address has the same network prefix as the IP address of its GGSN.
In GPRS network, user's current locations are managed in two levels: Micro mobility management tracks the current routing area or cell of the mobile station. It is performed by the SGSN. Macro mobility management keeps track of the mobile station's current SGSN and stores it in the HLR, VLR, and GGSN.
Figure 2. GPRS Transmission Plane
4. LIMITATION OF GPRS
LIMITED CELL CAPACITY FOR ALL USERS There are only limited radio resources that can be deployed for different uses. For example, voice and GPRS calls both use the same network resources. The extent of the impact depends upon the number of timeslots, if any, that are reserved for exclusive use of GPRS. However, GPRS does dynamically manage channel allocation and allow a reduction in peak time signaling channel loading by sending short messages over GPRS channels instead.
SPEEDS MUCH LOWER IN REALITY: Achieving the theoretical maximum GPRS data transmission speed of 172.2 kbps would require a single user taking over all eight timeslots without any error protection. Clearly, it is unlikely, further the initial GPRS terminals are expected to be severely limited- supporting only one, two or three timeslots. The reality is that mobile networks are always likely to have lower data transmission speeds than fixed networks. RESULT: Universal Mobile Telephone System (UMTS) are introduced.
GPRS will provide a massive boost to mobile data usage and usefulness. That much seems assured from its flexible feature set, its latency and efficiency and speed. The only question is how soon it takes off in earnest and how to ensure that the technical and commercial features do not hinder its widespread use.
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Architecture, protocols and air interface", http://www.comsoc.org/pubs/surveys/3q99issue/bettstetter.html
 "Introduction to GSM", http://www.pulsewan.com/data101/gsm_basics.htm
 Trillium Digital Systems, Inc., "General Packet Radio Service (GPRS) White Paper",
 Im Donahue, BTCellnet Peter Lisle, and BTCellnet, "GPRS Network Infrastructure Dimensioning and Performance", http://www.gprscongress.com/pages/marketinfo.html