Packet data services over UMTS and its Evolution - Objective and Subjective Performance Investigations

The overall quality of a packet service over a UMTS networks depends on several factors that encompass the End-2-End entities, including networking components, protocols, end user applications and user interfaces. Entities are interdependent with each other and the overall quality can be constrained by one or more bottlenecks. For example, a very poorly designed interface to a service can downgrade the quality much lower, even if the bandwidth provided by the link is high.

This project addresses the performance evaluation of packet services over UMTS networks from an End-2-End QoS perspective. In concrete terms, the project tries to answer the question "How do network protocol algorithms, parameters, and network conditions affect the quality of packet data services over UMTS networks". In this formulation, quality refers to a measure based on the perceived end-2-end QoS of a certain service, which is varying based on the concerned application and user. For example, speech services can not tolerate delays but can live with some errors while file downloads demand an absolutely error free communication.

As it is impractical, if not impossible, to try to capture the workings of such a complicated system as a packet service provision over a UMTS network through a purely analytical way, the project work is mainly based on emulation studies. RESPECT (A Real-time Emulator for Service Performance Evaluation in Cellular neTworks) is developed to this end, with a detailed implementation of the UMTS link layer protocol. Applications such as HTTP and FTP can be run on top of RESPECT, which makes them experience the effects of a UMTS network in terms of packet drops and delays.

So far, FTP services were investigated under different network scenarios. The results indicate that though some of the link layer timers for polling and status reporting have an impact, the maximum link layer retransmission limit is the main factor that affects the overall performance. The delivery order of link layer segments to upper layers is also found out to be important for high bit rate connections, and the aggressive retransmission mechanisms that are employed by some TCP enhancements such as TCP FACK are found out to lead to poor performance if out of order delivery is employed by the link layer.

Preparations are being made to undertake usability studies, where users will evaluate their perceived QoS of different services (FTP, HTTP and streaming) under different network conditions. RESPECT has also been extended to suppor a heterogenous network comprised of GPRS, UMTS and WLAN, and results, mainly concerning the effect of inter-system handover, would be available soon. (Oumer Teyeb, Malek Boussif, Troels B. Sørensen, Preben E. Mogensen, Jeroen Wigard, Nokia Networks)