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Home > GPRS
Terminals/Handsets 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. Motorola's GPRS solution
introduces two new network nodes into the GSM PLMN (Public Land Mobile Network)
- the SGSN and the GGSN. 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.
NB: GC & GS Interfaces are optional 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, latency & jitter
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 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: 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.
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