Most of the wireless and wireline data communications is bursty and it would allow for a much more efficient use of the scarce radio resources if the mobile would only occupy resources when actually sending or receiving data. If a mobile station can be made "stay connected" all the time, only consuming bandwidth when sending and receiving data, then there is no need to dial up and connect calls, with all the waiting for modems that this usually implies. This new feature "always online" will be provided by the new GSM Phase 2+ GPRS bearer service.
As an answer to bursty data traffic and high network utilisation the GPRS bearer services utilises packet switching technology. The main advantage of this packet switching technology is that no resources are tied up while they are not in use for sending and receiving data. GPRS introduces packet switching to the GSM network all the way from an server in an external IP network, to a mobile station, it integrates with existing GSM systems and reuses the GSM radio network infrastructure and the same transmission links between the GSM network nodes. A new multi slot combining traffic technology lets GPRS mobile stations use more than one timeslot when required delivering, within limits, bandwidth-on-demand.
GPRS can use one to four (theoretically eight, however due to hardware limitations in the mobile station only four) timeslots for uplink and downlink traffic. Depending on the (CS), each timeslot can carry a different amount of data. There are four different CS defined with different capacities. The capacity ranges from 9.05 kbit/s (CS 1) to 21.4 kbit/s (CS 4). In some articles, it may be read that GPRS will provide users with a bandwidth of 171.2 kbit/s. This is the theoretical maximum speed (21.4 * 8 = 171.2 kbit/s) and will never be achieved in a real network, because of the mobile station hardware limitations (only four timeslots) and the fastest scheme (CS 4) requires a radio channel with superior quality, which can only be achieved while users being stationary and very close to the base station.
For the GSM network operator, GPRS means that they can quickly roll out packet switched services on their existing GSM network (900 MHz and 1800 MHz). It makes it possible for an operator to use new billing models for data traffic, such as only charging for data transmitted not for the duration of the connection to the network. However, some network operators will not allocate permanent timeslots for GPRS users only. Instead, they will use the dynamic option of GPRS, that means that GSM voice traffic is prioritised and packet switched data traffic is only transmitted on timeslots not currently in use for voice traffic. By doing this the operators can sell the capacity which is now wasted.
GPRS is relatively easy to deploy within existing GSM networks, since most of the upgrades can be done by software upgrades in existing GSM nodes. On the hardware side there are two major new elements, GPRS Support Nodes (GSN), that have to be added. The Serving GPRS Support Node (SGSN), serving the mobiles stations and the Gateway GPRS Support Node (GGSN), provides the interworking function with external packet data networks.
The SGSN is responsible for handling packet data traffic in a geographical area. It provides authentication, mobility management, logical link management towards the mobile station, packet routing and transfer, plus connections to the Home Location Register (HLR), and the GPRS packet control unit (PCU). The PCU takes care of the radio-specific parts of the GPRS protocol stack. Architecturally the PCU is part of the Base Station Subsystem (BSS) but it can be sited with a BSC or a BTS.
The GGSN is the interface towards external IP and X.25 networks. It is acting as an access server and is responsible for routing incoming traffic to the correct SGSN, i.e. it is responsible for setting up a logical link to the mobile station (MS), through the SGSN. The GGSN also translates between data formats and translates signalling protocols and address information to enable communication between the different networks.
Class C devices operate exclusively GPRS services (although some devices may be manually set to handle GPRS or GSM, these are then seen as two different devices by the network, depending on the manual mode setting).
Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann