Convergence /

Factors affecting GPRS throughput

By Steve Taylor and Larry Hettick
Network World Convergence Newsletter, 04/24/02

Today, we'd like to pass along the comments of Tim Hayward, who recently responded to our newsletters about mobile data with some interesting insight into General Packet Radio Service, or GPRS. Hayward is in the GPRS network development team at Vodafone New Zealand's Technology Department.

He writes:

GPRS users' throughput is affected by a range of factors, including:

1. Terminal capability. Currently the best capability is 4 x timeslot (TSL) on the downlink (4 x 13.3K bit/sec at the physical layer, dropping to 4 x 9.6K bit/sec if the radio conditions are poor) and 2 x TSL on the uplink.

2. Radio network capacity. Radio resource is always scarce. GPRS typically competes with voice, and usually voice has priority in a given cell site. So, during voice busy hours there may be very little resource available for GPRS. Many operators are dedicating at least 1 TSL to GPRS, with the ability to utilize extra TSLs when they are free of voice calls. The business case is not yet there to justify providing additional capacity solely for GPRS.

3. Radio network quality. GPRS is more sensitive to poor radio network environment than voice. Factors such as interference and lack of a single dominant cell can dramatically reduce GPRS performance. Of course, TCP reacts to this, so TCP-based applications will not work well in poor radio conditions or while mobile in urban environments. Small-packet UDP applications are currently best suited to GPRS, but round-trip times will typically vary between 600 [milliseconds] and several seconds. Early adopters of GPRS, using applications like courier dispatch and mobile sales force, usually use some communications server middleware to manage UDP messages between clients and back-end systems.

So, the user experience is a factor of all this - at best, 50K bit/sec on the downlink, but during the day in urban environments much less (say, 20K bit/sec to 30K bit/sec), and much less if there is a large number of users.

There are a number of radio network enhancements coming from most vendors over the next three to 12 months that will provide better control over how GPRS resources are allocated to terminals. But it is unlikely that performance will far exceed 50K bit/sec on the downlink in the near-mid term (six to 24 months). And of course, performance may be a victim of its own success if the number of users expands rapidly, but radio resource takes time to add into the network.