GPRS technology tutorial [2]

- a summary, overview or tutorial of the basics of GPRS, General Packet Radio Service, giving the cellular network structure and the mobiles.

GPRS Cellular Network

Although designed to run alongside the GSM system, the core network structure updated for GPRS has several new elements added to enable it to carry the packet data. The network between the BSC and BTS is similar, but behind this there is a new infrastructure to support the packet data.

For GPRS, the data from the BSC is routed through what is termed a Serving GPRS Support Node (SGSN). This forms the gateway to the services within the network, and then a Gateway GPRS Support Node (GGSN) which forms the gateway to the outside world.

SGSN

The SGSN serves a number of functions for GPRS mobiles. It enables authentication to occur, and it then tracks the location of the mobile within the network, and ensures that the quality of service is to the required level.

For the network protocols there are two layers that are used and supported by GPRS, namely X25 and IP. In operation the protocols assign addresses (Packet Data Protocol or PDP addresses) to the devices in the network for the purpose of routing the data through the system. Thus the GGSN appears as a data gateway to the public packet network, and thus the fact that the users are mobiles cannot be seen.

In operation the mobile must attach itself to the SGSN and activate its PDP address. This address is supplied by the GGSN which is associated with the SGSN. As a result a mobile can only attach to one SGSN, although once assigned its address it can receive data from multiple GGSNs using multiple PDP addresses.

GPRS mobiles

Not all GPRS mobiles are designed to offer the same levels of service. As a result they are split into three basic categories according to their capabilities in terms of the ability to connect to GSM and GPRS facilities:

1. Class A: - This class describes mobile phones that can be connected to both GPRS and GSM services at the same time.



2. Class B: - These mobiles can be attached to both GPRS and GSM services but they can be used on only one service at a time. A Class B mobile can make or receive a voice call, or send and or receive a SMS message during a GPRS connection. During voice calls or texting the GPRS service is suspended but it is re-established when the voice call or SMS session is complete.



3. Class C: - This classification covers phones that can be attached to either GPRS or GSM services but user needs to switch manually between the two different types.

GPRS mobiles are also categorized by the data rates they can support. Within GSM there are eight time slots that can be used to provide TDMA, allowing multiple mobiles onto a single RF signal carrier. Within GPRS it is possible to use more than one slot to enable much higher data rates to be achieved when these are available. The different speed classes of the mobiles are dependent upon the number of slots that can be used in either direction. There are a total of 29 speed classes. Class one mobiles are able to send and receive in one slot in either direction, i.e. uplink and downlink, and class 29 mobiles are able to send and receive in all eight slots. The classes within these two limits are able to support sending and receiving in different combinations of uplink and downlink slots.

Further pages of this tutorial
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