GPRS General Packet Radio Service Tutorial Includes:
GPRS basics Network architecture Mobile / multislot classes Radio interface Coding Channels: physical, logical, control GPRS protocol stack Operational states
GPRS provided a packet data capability for the 2G cellular systems, enabling the evolution of GSM to provide a data capability.
To allow the GPRS network to provide the packet data capability additional network entities are required to be added to the overall architecture - two of the main entities are the GGSN and SGSN.
A packet data network architecture is overlayed or added to the existing GSM architecture to provide the data capability. The existing GSM network architecture is used to carry the circuit switched voice calls as well as the network access, etc.
GPRS network architecture upgrades
With GPRS providing additional connectivity in terms of packet data, there are naturally a number of upgrades needed to the network architecture required. A number of new elements are needed for the network, but these can operate alongside the existing elements meaning that the GPRS capability is an upgrade to the network and not a completely new network structure.
The main new network architecture entities that were needed are:
- SGSN, Serving GPRS Support Node: The SGSN forms a gateway to the services within the network.
- GGSN Gateway GPRS Support Node: The GGSN, forms the gateway to the outside world.
- PCU, Packet Control Unit: The PCU detects whether data is to be routed to the packet switched or circuit switched networks.
From the diagram given above it can be seen that the GPRS network architecture added some extra elements to the GSM network to enable it to cary the packet data. The PCU added to the base station network routed the data according to whether it was packet or circuit switched.
The SGSN or Serving GPRS Support Node element of the GPRS network provides a number of takes focussed on the IP elements of the overall system. It provides a variety of services to the mobiles:
- Packet routing and transfer
- Mobility management
- Logical link management
- Charging data
There is a location register within the SGSN and this stores location information (e.g., current cell, current VLR). It also stores the user profiles (e.g., IMSI, packet addresses used) for all the GPRS users registered with the particular SGSN.
The GGSN, Gateway GPRS Support Node is one of the most important entities within the GPRS network architecture.
The GGSN organises the interworking between the GPRS network and external packet switched networks to which the mobiles may be connected. These may include both Internet and X.25 networks.
The GGSN can be considered to be a combination of a gateway, router and firewall as it hides the internal network to the outside. In operation, when the GGSN receives data addressed to a specific user, it checks if the user is active, then forwarding the data. In the opposite direction, packet data from the mobile is routed to the right destination network by the GGSN.
The PCU or Packet Control Unit is a hardware router that is added to the BSC. It differentiates data destined for the standard GSM network (circuit switched data) and data destined for the GPRS network (Packet Switched Data). The PCU itself may be a separate physical entity, or more often these days it is incorporated into the base station controller, BSC, thereby saving additional hardware costs.
GPRS network upgrading
One of the key elements for any network operator is the cost of capital expenditure (capex) to buy and establish a network. Capex costs are normally very high for a new network, and operators endeavour to avoid this and use any existing networks they may have to make the optimum use of any capital. In addition to the capex, there are the operational costs, (opex). These costs are for general maintenance and other operational costs that may be incurred. Increasing efficiency and reliability will reduce the opex costs.
Any upgrade such as that from GSM to GPRS will require new investment and operators are keen to keep this to the minimum. The upgrades for the GPRS network are not as large as starting from scratch and rolling out a new network.
The GPRS network adds to the existing GSM network. The main new entities required within the network are the SGSN and GGSN, and these are required as the starting point.
The base station subsystems require some updates. The main one is the addition of the PCU described above. Some modifications may be required to the BTS, but often only a software upgrade is required, and this may often be achieved remotely. In this way costs are kept to a minimum.
The GPRS network architecture can be viewed as an evolution of the GSM network carrying both circuit switched and packet data. The GPRS network architecture was also used as the basis for the 3G UMTS network. In this way network operators could evolve their networks through GPRS and possibly EDGE to the full 3G networks without having to replace and install more new equipment than was absolutely necessary.
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