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Introduction
General packet radio service (GP RS) is a standard from the European Telecommuni-cations Standards Institute (ETSI) on pack-et data in GSM systems. GPRS has also been accepted by the Telecommunications In-dustry Association (TIA) as the packet-data standard for TDMA/136 systems. By adding GP RS functionality to the public land mobile network (PLMN), operators can give their subscribers resource-efficient ac-cess to external Internet protocol-based (IP) networks.
GPRS offers air-interface transfer rates up to 115 kbit/s—subject to mobile terminal capabilities and carrier interference. More-over, GPRS allows several users to share the same air-interface resources and enables op-erators to base charging on the amount of transferred data instead of on connection time. In the initial release, GPRS uses the same modulation as GSM (GMSK). The subsequent evolution of packet-based services in GSM introduces EDGE technology.1
GP RS introduces two new nodes (Fig-ure 1) for handling packet traffic:
?the serving G P RS support node (SGSN); and
?the gateway G P RS support node (GGSN).
These nodes interwork with the home loca-tion register (HLR), the mobile switching center/visitor location register (MSC/VLR) and base station subsystems (BSS).
The GGSN, which is the interconnection point for packet data networks, is connect-ed to the SGSN via an IP backbone. User data—for example, from a GPRS terminal to the Internet—is sent encapsulated over the IP backbone.
The SGSN, in turn, is connected to the BSS and resides at the same hierarchical level
in the network as the MSC/VLR. It keeps track of the location of the GPRS user, per-forms security functions and handles access control—that is, to a large extent, it does for the packet data service what the MSC/VLR does for circuit-switched service. In the GPRS standard, three new types of mobile terminal have been defined: ?Class A terminal, which supports simul-taneous circuit-switched and packet-switched traffic;
?Class B terminal, which supports either circuit-switched or packet-switched traf-fic (simultaneous network attachment) but does not support both kinds of traffic simultaneously; and
?Class C terminal, which is attached either as a packet-switched or circuit-switched terminal.
The terminal types are further differentiated by their ability to handle multi-slot operation. Since class A and class B terminals sup-port both circuit-switched and packet-
GPRS—General packet radio service H?kan Granbohm and Joakim Wiklund
By adding GPRS to the GSM network, operators can offer efficient wire-
less access to external IP-based networks, such as the Internet and cor-
porate intranets. What is more, operators can profit from the rapid pace of service development in the Internet world, offering their own IP-based
services using the GPRS IP bearer, thereby moving up the Internet value
chain and increasing profitability.
End-users can remain connected indefinitely to the external network
and enjoy instantaneous transfer rates of up to 115 kbit/s. Users who are
not actually sending or receiving packets occupy only a negligible amount
of the network’s critical resources. Thus, new charging schemes are
expected to reflect network usage instead of connection time.
Ericsson’s implementation of GPRS enables rapid deployment while
keeping entry costs low—the two new nodes that are added to the net-
work can be combined and deployed at a central point in the network.
The rest of the GSM network solely requires a software upgrade, apart
from the BSC, which requires new hardware.
The authors describe Ericsson’s implementation of GPRS. In particular,
they explain the role of the two new GPRS support nodes and needed
changes to Ericsson products in the PLMN.
GPRS
texts (the PDP context defines important parameters, such as the access point name, quality of service, the GGSN to be used, and so on, for connection to the external packet data network);
?to interwork with the radio resource man-agement in the BSS; and
?to generate charging data.
Gateway GPRS support node
Like the SGSN, the GGSN is a new com-ponent (also based on the AXB 250 plat-form) in the PLMN. It accommodates the interface to external IP-based networks. The access-server functionality in the GGSN is defined according to standards from the In-ternet Engineering Task Force (IETF). The main functions of the GGSN are
?to function as a border gateway between the PLMN and external networks;
?to set up communication with external packet data networks;
?to authenticate users to external packet networks;
?to route and tunnel packets to and from the SGSN; and
?to generate charging data.
Changes to a PLMN with Ericsson products
Cell plan
GPRS introduces a new set of logical chan-nel types that have been optimized for pack-et data. The physical radio resources in a cell may be dedicated to GPRS or shared with the circuit-switched service, in which case this service takes precedence. If dedicated resources have not been assigned in a cell, then GPRS broadcast and control signaling is handled via ordinary control channels (BCCH, RACH, and so on).
GPRS does not use location areas (LA). In-stead, a routing-area (RA) concept has been introduced.
In the first GP RS release, and in cases where GP RS traffic does not constitute a sig-nificant part of network traffic, operators are advised to use the same cell parameters and borders as for their circuit-switched sys-tems. Later, as GP RS traffic grows, the GPRS service and the circuit-switched ser-vice might need different cell parameters and borders.
GPRS can be introduced by defining ei-ther shared or dedicated resources on exist-ing transceivers (TRX). New transceivers and frequencies can also be set aside specif-ically for GPRS.Billing and customer administration systems
With the introduction of GP RS, current customers’ subscriptions will be enhanced and new customer categories will appear—possibly including those with GPRS-only subscriptions. These and other changes will have an impact on the operator billing and customer administration (BCA) systems. The call detail records (CDR) generated by the SGSN and the GGSN indicate to which external packet network the connec-tion was set up, the volume of data that was transferred, the quality of service offered, the date and time of connection, and the dura-tion of the session. This information, which differs from what CDRs of circuit-switched services currently provide, will affect exist-ing billing systems. In all likelihood, oper-ators will not base charges for GPRS services on the duration of a session, as is the case with circuit-switched services. Instead, charges will be based on a flat fee or on vol-umes of data transferred. Operators may also want to offer subscribers of both circuit-switched and packet-switched services a single, consolidated invoice with itemized charges for each service.
To moderate the impact on billing sys-tems, Ericsson’s billing gateway (BGW) can be connected between the SGSN and the GGSN (which generate CDRs) and the billing systems. The functionality of the billing gateway entails
?storing CDRs during long GPRS sessions (sessions last for as long as the PDP con-text is active);
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GPRS
Circuit-switched