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GPRS Benefits

GSM's new GPRS (General Packet Radio Services) data transmission technology is optimized for "bursty" datacom services such as wireless Internet/intranet and multimedia services. It is also known as GSM-IP (Internet Protocol) because it will connect users directly to Internet Service Providers.

One of the main benefits of this new packet-switched technology is that users are always connected, always on-line, and may be charged only for the amount of data that is transported. Voice calls can be made simultaneously over GSM-IP while a data connection is operating - depending on the phone Class and Type.

In a Class 8 device for example, there are four times as many receive channels as there are transmit channels, to accommodate the higher bandwidth demands of data reception.

A class B terminal means that in the idle mode, there is a choice of whether to make a voice call, which would be with a circuit switched connection or whether to transmit data, which would be sent in a packet format.

Users will also benefit from fast and easy up to 170 kbps data access to different services.

Two major new core network elements are introduced with GPRS: the SGSN and the GGSN. The SGSN monitors the state of the mobile station and tracks its movements within a given geographical area. It is also responsible for establishing and managing the data connections between the mobile user and the destination network.

The GGSN provides the point of attachment between the GPRS domain and external data networks, such as the Internet and corporate Intranets. Each external network is given a unique Access Point Name (APN) which is used by the mobile user to establish the connection to the required destination network.

The GSM Base Station Subsystem (BSS) has been adapted to support the GPRS connectionless packet mode of operation. A new functional node called the Packet Control Unit has been introduced (as part of the BSS) to control and manage the allocation of GPRS radio resources to mobile users.

The modifications to the radio infrastructure and the additional functionality introduced with GPRS mean that new Mobile Stations(MS) - typically handsets, PDA's, PCMCIA radio cards - are required.

Ericsson for example offers a robust IP end-to-end GPRS solution with open interfaces enabling integration into multi-vendor networks.

The company's GPRS solution also offers leading-edge security for wireless use of intranet and corporate LAN services.

GPRS is a smooth add-on to integrate into existing networks. For new operators, it's also attractive to launch GPRS networks to provide competitive datacom services.

GPRS roaming

GPRS roaming is a basic requirement for making future global mobile Internet services possible for GPRS subscribers in other operators' GPRS networks.

GRX, as specified in the IR.34 recommendations laid down by the International Roaming Expert Group (IREG) of the GSM Association, is a centralised IP routing network for interconnecting GPRS networks. GRX based GPRS roaming has now been implemented successfully for the first time in the world, with a solution that is fully compliant with the GSM Association recommendation. The tests were executed by combining Sonera's GPRS system and Nokia's packet core network by a backbone network solution that uses Sonera's GPRS Roaming Exchange. The solution is also designed to meet the roaming needs of future 3G networks.

Motorola's GPRS solution introduces two new network nodes into the GSM PLMN (Public Land Mobile Network) - the SGSN and the GGSN.

A number of new interfaces are added to connect the SGSN and GGSN to the appropriate GSM and non-GSM elements required to provide global packet data service.

Motorola's GPRS infrastructure solution is designed around a powerful IP routing engine, providing operators with a scalable and flexible solution that can tailor the packet switching capability in line with the predicted data subscriber growth.

The SGSN tracks packet capable mobile locations, performs security functions and access control. The GGSN interfaces with external packet data networks (PDNs) to provide the routing destination for data to be delivered to the subscriber's mobile terminal and to send mobile-originated data to its intended destination.

The GGSN is connected with SGSNs via an IP-based GPRS backbone network. The PCU performs radio functions and GPRS network functions. The PCU interfaces to the OMC-G, base station controller and SGSN.

A number of new standardised network interfaces have been introduced

Gb: A 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: The 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.

Gs: An optional interface that allows closer co-ordination between the GSM and GPRS networks.

Gc: An optional interface that allows the GGSN access to customer location information.

A number of other elements, not shown in the diagram above, are also introduced:

The Charging Gateway (CG) provides the means to collect and co-ordinates the billing information produced by the SGSN and GGSN before processing by the billing system.

The IP Domain Name Server (DNS) is needed to enable the user to establish a data session with the destination network. It provides the mapping between APNs and GGSN IP addresses.

Earlier in 1999, Motorola and Cisco Systems Inc., the worldwide leader in networking for the Internet, announced a strategic alliance to develop and deliver a New World framework for Internet-based, wireless networks. This collaboration will deliver the first all-IP platform for the wireless industry, which unites different standards for wireless services worldwide, and introduce an open, Internet-based platform for integrated data, voice and video services over cellular networks.

Reliability

Currently, the support of differentiated Quality of Service (QoS) is minimal. However, GPRS does make it possible to ensure the integrity of received data through the implementation of two reliable modes of operation: RLC Acknowledged and LLC Acknowledged.

RLC acknowledged mode is used by default to ensure that the data received by/from the MS is without error.

LLC acknowledged mode is an optional feature that may be provided. This protocol ensures that all LLC frames are received without error. However, use of this protocol has an impact on throughput since the correct receipt of all LLC frames has to be acknowledged.

Latency & jitter

Latency is the time taken for data packets to pass through the GPRS bearer, normally measured as a round-trip time. Jitter is the variability in this time.

In GPRS there are a number of factors contributing to the overall latency. These include:

  • the Mobile Station (MS)
  • radio resource procedures
  • the effective data throughput
  • the GPRS core network nodes.

Mobile Station (MS) delay is the time taken by the MS to process an IP datagram and request radio resource. This includes the delay from the PC to MS, and the MS processing time. This delay is typically less than approximately 100ms, with the possible exception of the processing associated with establishing the initial uplink radio channel. The time taken depends on the MS, and hence the supplier.

Radio resource procedures are the major source of delay in GPRS. In order for the MS to be capable of sending or receiving data, radio resource known as a Temporary Block Flow must be made available to the user. If a TBF is currently active then the MS may use it hence minimising the delay. However, if no TBF is established then the MS and network must exchange signalling messages in an attempt to establish a TBF. The time taken to successfully achieve an active TBF will depend on the availability of radio resources and will be different for the uplink and downlink directions. Once established, the TBF will generally remain active for as long as data is made available to the layer (i.e. for as long as there are LLC frames to transmit).

Effective data throughput (over-the-air delay) is the rate at which user data is physically transmitted between the MS and the SGSN over an active TBF. The delay associated with this throughput is directly related to the size of the IP datagram being sent. Smaller packets cause less delay. The delay is proportionally reduced when multiple timeslots are used. The effective throughput is also dependent on the number of re-transmissions resulting from the hostile radio environment (i.e. the RLC Block Error Rate). The time taken to re-transmit erroneously received information will affect the size of the delay.

Core network delay occurs as packets transit through the SGSN and GGSN. These nodes effectively operate as IP routers and as such will have a relatively low impact on the overall latency. However, under high load conditions the transit delay may increase.