Performance analysis of TCP over enhanced UMTS network

Abstract

The increase in demand and overwhelming response of 3G mobile networks has made it a technology which would drive the future of mobile communication. On the other hand the popularity of the Internet has also increased with type of services it is providing. Internet mainly relies on TCP/IP protocol which is the backbone of almost all NRT packet services.

The designer of TCP made it for wired networks only, so it lacks the capability to handle the wireless nature of the link. The performance decreases when the error in the wireless link occurs in burst. To recover error more reliably wireless link uses some recovery mechanism at the link layer like ARQ. The error recovery by doing retransmission at the link layer may increase the latency and adds to the overall RTT of packet. TCP treats these sudden increase in delays as a congestion indication, and starts it congestion recovering mechanism which indeed not required here. TCP lowers the number of packet to be transmitted by lowering the Congestion Window and hence causing a degraded performance. Many solutions have been proposed to alleviate the problem. But all have some flaws at their own level. Introduction of HSDPA in 3G UMTS system, has raised up some hopes regarding TCP performance. The technical features that HSDPA has brought proven to be a performance enhancer for TCP. The effect of ARQ delay is minimized by the introduction of a new error mechanism at the physical layer. This error mechanism uses the principles of chase combining and incremental redundancy to increase the probability of successful decoding of the data block. The TTI duration has been changed from 10ms to 2ms so that overall delay reduces to a great extent. More efficient algorithms have been proposed to cope with the time varying channel conditions which can be utilized to schedule users to maintain an optimal use of system resources. The thesis investigates the influence of these features and also the effect they imposes on TCP performance.