GPRS Details for Network Managers

To better understand GPRS, we take a quick tour beginning with the mobile PC and traversing through the network. First, we have a notebook computer connected to a GPRS-capable cellphone or modem, either through a serial cable or other type of connection such as Universal Serial Bus (USB) or local wireless link. Or perhaps the connection device is in the form of a PC Card. The GPRS phone or modem communicates with GSM base stations, but unlike circuit-switched data calls which are connected to voice networks by the mobile switching center, GPRS packets are sent from the base station to what is called a Serving GPRS Support Node (SGSN). The SGSN is the node within the GSM infrastructure that sends and receives data to and from the mobile stations. It also keeps track of the mobiles within its service area. The SGSN communicates with what is called the Gateway GPRS Support Node (GGSN), a system that maintains connections with other networks such as the Internet, X.25 networks or private networks. See figure two. A GPRS network can use multiple serving nodes, but requires only one gateway node for connecting to an external network such as the Internet.

Figure three: GPRS system.

When the mobile station sends packets of data, it is via the SGSN to the GGSN, which converts them for transmission over the desired network, which could be the Internet, X.25 networks or private networks. IP packets from the Internet addressed for the mobile station are received by the GGSN, forwarded to the SGSN and then transmitted to the mobile station.

To forward IP or X.25 packets between each other, the SGSN and GGSN encapsulate these packets using a specialized protocol called the GPRS tunnel protocol (GTP) which operates over the top of standard TCP/IP protocols. But the details of the SGSN and GGSN are both invisible and irrelevant to the user who simply experiences a straightforward IP or X.25 connection that just happens to be wireless.

An interesting aspect of GPRS is how it achieves its high speeds to over 100 kbps when circuit-switched data today is limited to 9600 or 14.4 kbps. GPRS uses the same radio channel as voice calls, a channel that is 200 kHz wide. This radio channel carries a raw digital radio stream of 271 kbps which for voice calls is divided into 8 separate data streams, each carrying about 34 kbps. After protocol and error correction overhead, 13 kbps is left for each voice connection or about 14 kbps for data. Circuit-switched data today uses one voice channel. GPRS can combine up to 8 of these channels, and since each of these can deliver up to 14 kbps of data throughput, the net result is that users will be able to enjoy rates over 100 Kbps. But not all eight-voice channels have to be used. In fact, the most economical phones will be ones that are limited to 56 kbps. The GPRS standard defines a mechanism by which a mobile station can request the amount of bandwidth it desires at the time it establishes a data session.