A SURVEY ON TCP OVER MOBILE AD-HOC NETWORKS

This chapter is a survey on TCP performance in mobile ad-hoc networks. We flrst describe the problems of standard TCP in ad-hoc networks, and then present the design space and existing solutions to improve TCP throughput. Particularly, we use a detection-response framework to categorize difierent approaches and analyze the possible design options. 1. Introduction. A mobile ad-hoc network (MANET) is a special type of wireless networks. It consists of a collection of \peer" mobile nodes that are capable of commu- nicating with each other without help from a flxed infrastructure. The interconnections between nodes are capable of changing on a continual and arbitrary basis. Nodes within each other's radio range communicate directly via wireless links, while those that are far apart use other nodes as relays in a multi-hop routing fashion. The typical applications of MANETs include conferences or meetings, emergency operations such as disaster rescue, and battlefleld communications. Compared with the traditional wired internet, MANETs have two fundamental difier- ences, its usage of wireless channel and frequent node mobility. Both of these two charac- teristics have signiflcant impact on the TCP performance. 1.1. Wireless Channel. In a MANET, all nodes share the wireless medium and communicate through radios. The problem of contention and collision in such wireless networks is much more serious than in the wired environment. Currently, IEEE 802.11 (13) is the de facto Medium Access Control (MAC) protocol for Wireless LANs. It coordinates the medium access by Distributed Coordination Function (DCF), also known as CSMA/CA. It can operate in two modes, ad hoc mode and infrastructure mode. A central base station is needed to operate in infrastructure mode, while the ad hoc mode is for the non-centralized type of communication as in MANETs. However, it is doubted by some researchers (23) whether IEEE 802.11 works well in multi-hop ad hoc networks, particularly when TCP is used as the transport layer protocol. Existing research work (11) has shown that TCP performance decreases drastically as the hop count becomes larger and larger. Another problem with wireless channel that hurts TCP performance is, the high bit error rate of wireless links. Nowadays, the wired links are already so stable that we can ignore the link errors. But it is not yet the case as far as wireless links are concerned. Designed for wired networks originally, TCP, particularly its Congestion Avoidance and Control (14) mechanism, does not consider link errors as a possible reason for packet errors or losses. This can signiflcantly degrade the performance of TCP over wireless networks including MANETs. 1.2. Node Mobility. The node mobility we discuss here is on a continual and arbitrary basis, i.e., the nodes can keep moving in an arbitrary speed, toward an arbitrary direction. A typical scenario is people communicating with each other while driving their cars. It is a nature of MANETs. Due to the limited communication range of radios, such constant node mobility imme- diately leads to frequent link breakages and therefore route changes. Many ad-hoc routing