TCP Performance in Wireless Channels
Alessandro Anzaloni, Alexander Bento Melo · SpaceOps 2002 Conference · 2002
TCP (Transmission Control Protocol) is a reliable transport protocol, tuned to perform well in traditional networks where packet losses occur mostly because of congestion. However, networks with wireless and other lossy links also suffer from significant non-congestion related losses due to reasons such as bit errors. In this paper, we present the evaluation of the performance degradation due the presence of a multiple access scheme and packet losses. The evaluation use a range of packet loss probabilities and the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) – IEEE 802.11 as multiple access protocol. In order to reduce the performance degradation a solution based on end – to –end protocols where the sender is aware of the wireless links was adopted. 1Introduction Reliable transport protocols such as TCP are tuned to perform well in traditional networks where packet losses occur mostly because of congestion. However, networks with wireless and other lossy links also suffer from significant non – congestion –related losses due to reasons such as bit errors and handoffs. TCP reply to all losses using congestion controls algorithms, resulting in low end-to-end performance in wireless communications (for details see [1] , [2]) . The objective of this paper is to evaluate the performance degradation in a typical mobile scenario where packet losses are mainly due to the presence of multiple users trying to access simultaneously the channel and high bit error probability. To achieve the objective the paper comprises the following section: Section 2 where solutions to reduce the TCP performance degradation in wireless links are described (in this paper the end-to-end protocol solution was utilized). Section 3 describes the simulation scenarios considered in our work as well as the main characteristics of the OPNET simulator and the TCP implementation. Section 4 shows the simulation results firstly defining the performance parameters used in the analysis and secondly the behavior of the TCP in the presence of the multiple access scheme and lossy links. Finally section 5 shows the main simulation results and points out our suggestions for future works in TCP wireless issues. 2TCP over Wireless Networks Several attempts to improve the performance of TCP in wireless links have been undertaken. The possible improvement to TCP fall into three categories: those, which can apply to link – layer protocols, TCP itself (also referred as end – to end protocol solution), and application – layer protocols. In the following is described the end-to end solution that uses fast retransmit, fast recovery and selective acknowledgment (readers interested in more details related to link and application layer solutions can read [3] and [4] respectively). Jacobson [5] proposed two procedures, named fast retransmit and fast recovery in order to improve the TCP links performance. Fast retransmit utilize the rule where if a TCP entity receives a segment out of order, it must immediately issue an ACK for the last in order segment that was received. TCP will continue to repeat his ACK with each incoming segment until the missing segment arrives to “plug the hole “ in its buffer. When the hole is plugged, TCP sends a cumulative ACK for all of the in –order segments received so far. The fast recovery technique avoids the initial exponential slow – start process cutting the congestion windows in half, and then proceed with the linear increase of the 2 congestion window. This technique consider that when multiple ACKs have returned indicates that data segments are getting through fairly to the other side and the connection doesn’t ‘t needs the slow – start process. Finally considering that fast retransmit it does not always prevent the retransmission those segments that have already been received [6] proposed a selective acknowledgement( sack) scheme which would allow a node to inform another node of all segments it has receivednot just those that have been received sequentially. This has the potential of preventing unnecessary retransmissions when a “hole “appears in the data. This is especially important for mobile nodes that communicate over slow (e.g. wireless) links. 3The simulation scenarios The simulations were conducted utilizing the OPNET software running in a DELL computer ( dual Pentium III 600 Mhz) and Windows NT operational system. The OPNET was used to simulate the TCP algorithms (fast retransmit, fast recovery and selective acknowledgement), the medium access control (IEEE 802.11) and the module to discard packets (according with a given probability of discard) .To evaluate the performance three simulation scenarios were considered: aPerformance considering only one user in the presence of the lossy channel (utilizing the module to discard packets). This scenario will be referred as a scenario in section 4 and it is represented in Figure 3.1. Figure 3.1: Scenario considering only one user. bPerformance considering several users attempting to access simultaneously a non-lossy channel (utilizing the IEEE 802.11). This scenario will be referred as B scenario in section 4 and it is represented in Figure 3.2. Figure 3.2: Scenario considering several users. cPerformance considering multiple access and lossy channel. This scenario will be referred as C scenario in section 4 and it is represented in Figure 3.2. The results of the simulation of these three scenarios are presented in the following section.