Performance analysis of general packet radio service protocol stack

Abstract

Second?generation cellular systems, including Global System for Mobile Communications (GSM), provide primarily voice communications to mobile users. Recently, there has been a growing interest in packet data communications over wireless networks to provide fast and efficient Internet access to mobile users. Mobile radio channels exhibit poor bit?error?rate performance due to channel imperfections including time?varying distance, shadow losses, and multipath fading. Commonly used higher?layer protocols (e.g., transport layer protocols like TCP) have been designed to perform well in wireline networks where channel error rates are very low. Thus, the lower layers in wireless protocol stacks must be designed to address such higher?layer performance concerns. In particular, efficient and robust data link layer (LL) and media access control (MAC) layer protocols are crucial to system performance in wireless data communications. This thesis addresses the performance analysis of various layers in a wireless protocol stack and suggests techniques to enhance performance. We consider the protocol stack proposed in the General Packet Radio Service (GPRS) system. GPRS is a wireless system that provides packet data communications to mobile users using GSM cellular infrastructure. The radio link control/medium access control (RLC/MAC) layers in the GPRS protocol stack are responsible for the way in which GSM/GPRS radio resources (frequency?time slot pairs) are shared among multiple mobile users. The uplink (mobile?to?base station) channel resources are shared based on a request?reservation mechanism. Performance of the GPRS RLC/MAC layers under various radio channel and traffic load conditions influences the overall GPRS network performance. Several studies have investigated the performance of the GPRS RLC/MAC layers, but mainly through simulations. A new contribution in this thesis is the modeling and analytical evaluation of the performance of the RLC/MAC protocols in GPRS, considering the uplink request?reservation mechanism. Using the theory of Markov chains, we derive expressions for the uplink throughput and delay performance of the GPRS MAC protocol. Our analysis quantifies the throughput?delay performance as a function of system traffic load, number of random access channels (to carry resource requests from mobiles), and number of traffic channels (to carry data traffic) on the uplink. The results can be used to choose the optimal mix of random access channels and traffic channels for a given traffic load. Analytical performance results closely match simulation results. Another key contribution of this thesis is the proposal and performance analysis of slot?level retransmission at the RLC layer to enhance performance. A packet scheduled for transmission over the GPRS air interface is formatted into one or more logical link control (LLC) frames, which are then segmented into several RLC blocks. Radio resource allocation in GPRS is performed in units of RLC blocks. Each RLC block occupies four physical?layer time slots. A selective?repeat (SR) automatic repeat request (ARQ) mechanism is provided at the RLC layer to recover RLC block errors using a block check sequence (BCS) and block?level retransmission. The drawback of block?level retransmission is that even if an error occurs in only one slot, the entire block of four slots must be retransmitted, which degrades performance. In this thesis, through analysis and simulations, we show that slot?level retransmission significantly improves RLC layer performance, particularly when channel error rates are high. In addition, slot?level retransmission enables resource allocation at the individual time?slot level, achieving finer granularity and greater flexibility. Unrecovered block errors at the RLC layer are handled by another ARQ mechanism at the LLC layer, which performs retransmission of erroneous LLC frames. A related question is how many retransmission attempts should be allowed at the RLC and LLC layers. Our performance results show that it is more beneficial to allow a larger number of retransmission attempts at the RLC layer than at the LLC layer. We also study the performance of transport layer protocols over GPRS. Transmission Control Protocol (TCP) is an end?to?end transport layer protocol that provides reliable, in?sequence delivery of packets. We evaluate the performance of TCP on the uplink in GPRS through simulations. Both block?level and slot?level retransmission mechanisms at the RLC layer are considered. An ON?OFF traffic model, such as that representing web and email traffic, is used. The OFF?period distribution is modeled using a Pareto distribution. Results show that TCP performance improves with slot?level retransmission compared to block?level retransmission. The effects of various TCP parameters, such as the fast?retransmit threshold, on throughput performance are also evaluated.