A Survey on Improving TCP Performance over Wireless Networks

Abstract

As a result of the advancement of wireless technology and the proliferation of handheld wireless terminals, recent years have witnessed an ever-increasing popularity of wireless networks, ranging from wireless Local Area Networks (WLANs) and wireless wide-area networks (WWANs) to mobile ad hoc networks (MANETs). In WLANs (e.g., the Wi-Fi technology) or in WWANs (e.g., 2.5G/3G/4G cellular networks), mobile hosts communicate with an access point or a base station that is connected to the wired networks. Obviously, only one hop wireless link is needed for communications between a mobile host and a stationary host in wired networks. In contrast, there is no fixed infrastructure such as base stations or access points in a MANET. Each node in a MANET is capable of moving independently and functioning as a router that discovers and maintains routes and forwards packets to other nodes. Thus, MANETs are multi-hop wireless networks by nature. Note that MANETs may be connected at the edges to the wired Internet. Transmission control protocol (TCP) is a transport layer protocol which provides reliable end-to-end data delivery between end hosts in traditional wired network environment. In TCP, reliability is achieved by retransmitting lost packets. Thus, each TCP sender maintains a running average of the estimated round trip delay and the average deviation derived from it. Packets will be retransmitted if the sender receives no acknowledgment within a certain timeout interval (e.g., the sum of smoothed round trip delay and four times the average deviation) or receives duplicate acknowledgments. Due to the inherent reliability of wired networks, there is an implicit assumption made by TCP that any packet loss is due to congestion. To reduce congestion, TCP will invoke its congestion control mechanisms whenever any packet loss is detected. Since TCP is well tuned, it has become the de facto transport protocol in the Internet that supports many applications such as web access, file transfer and email. Due to its wide use in the Internet, it is desirable that TCP remains in use to provide reliable data transfer services for communications within wireless networks and for those across wireless networks and the wired Internet. It is thus crucial that TCP performs well over all kinds of wireless networks in order for the wired Internet to extend to the wireless world. Unfortunately, wired networks and wireless networks are significantly different in terms of bandwidth, propagation delay, and link reliability. The implication of the difference is that packet losses are no longer mainly due to network congestion; they may well be due to some wireless specific reasons. As a matter of fact, in wireless LANs or cellular networks, most packet losses are due to high bit error rate in wireless channels and handoffs between two cells, while in mobile ad hoc networks, most packet losses are due to medium contention and route breakages, as well as radio channel errors. Therefore, although TCP performs well in wired networks, it will suffer from