GSM EDGE network architecture

- a summary, overview or tutorial about the basics of the enhancements required for the GSM EDGE network architecture including the GGSN and SGSN.

In order that the GSM EDGE upgrade can be implemented, additions are required within the EDGE network architecture to be able to cater for the packet data that is carried by the system. The additional network entities required are the same as those used for GPRS and also for UMTS.

With the introduction of the new entities within the network, it was still necessary for the new EDGE network elements and those from the existing GSM elements to work along side one another. Accordingly the introduction of GPRS and EDGE technology saw the addition of some new entities within the over network architecture.

The two main elements that are required by the GSM EDGE network architecture are the GGSN and SGSN. These enable the network to be able to cater for the packet data that is passed over the network.


GSM EDGE network architecture upgrades

Although in practice a variety of elements are required within the network architecture, the main new network architecture entities that are needed for the EDGE upgrade are:

  • SGSN:   GPRS Support Node - this forms a gateway to the services within the network.
  • GGSN:   Gateway GPRS Support Node which forms the gateway to the outside world.
  • PCU:   Packet Control Unit which differentiates whether data is to be routed to the packet switched or circuit switched networks.

A simplified view of the GSM EDGE network architecture can be seen in the diagram below. From this it can be seen that it is very similar to the more basic GSM network architecture, but with additional elements.

GSM EDGE network-architecture
GSM EDGE network architecture


SGSN

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
  • Authentication
  • Attach/detach
  • 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.


GGSN

The GGSN, Gateway GPRS Support Node is one of the most important entities within the GSM EDGE network architecture.

The GGSN organises the inter-working between the GPRS / EDGE 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.


PCU

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 EDGE 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.


GSM EDGE 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 EDGE will require new investment and operators are keen to keep this to the minimum. The upgrades for the EDGE network are not as large as starting from scratch and rolling out a new network.

The EDGE 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.

By Ian Poole


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