Congestion Control Technique for Wireless Networks

Abstract

Congestion control in wireless networks has been extensively investigated over the years and several schemes and techniques have been developed, all with the aim of improving performance in wireless net-work. With the rapid expansion and implementation of wireless technology it is essential that the congestion control problem be solved. This paper presents a congestion control schemes which are different in slow start threshold calculation, bandwidth estimation, and congestion window manipulation. A comprehensive comparison of these approaches is given in relation to assumptions, bandwidth estimation, congestion window size manipulation, performance evaluation, fairness and friendliness and improved throughput. I. Introduction The congestion control in wireless networks has been extensively investigated over the years and several schemes and techniques have been developed, all with the aim of improving performance in wireless network. With the rapid expansion and implementation of wireless technology it is essential that the congestion control problem be solved. The Transmission Control Protocol (TCP) has been widely used in today's Internet. The protocol supports reliable data transport by establishing a connection between the transmitting and receiving ends. The transmitter starts a timeout mechanism when sending a packet to the receiver. The transmitter constantly tracks the round-trip times (RTTs) for its packets as a means to determine the appropriate timeout period. At the receiver, each received packet is acknowledged implicitly or explicitly to the transmitter. If the transmitter does not receive an acknowledgment for a given packet when the corresponding timeout period expires, the packet is deemed to be lost and subject to retransmission. A congestion window with dynamically adjusted size is used by the protocol to regulate the traffic flow from the transmitter to the receiver. Although TCP was initially designed and optimized for wired networks, the growing popularity of wireless data applications has lead third generation wireless networks such as CDMA2000 and UMTS networks to extend TCP to wireless communications as well. The initial objective of TCP was to efficiently use the available bandwidth in the network and to avoid overloading the network (and the resulting packet losses) by appropriately throttling the senders' transmission rates. Network congestion is deemed to be the underlying reason for packet losses. Consequently, TCP performance is often unsatisfactory when used in wireless networks and requires various improvement techniques. A key factor causing the unsatisfactory performance is that the radio link quality in wireless networks can fluctuate greatly in time due to channel fading and user mobility 1.1 Problems of TCP in Wireless Scenario TCP protocol may experience performance degradations over wireless networks, due to non-congestion related packet loss and varying round trip times Unmodified standard TCP performs poorly in a wireless environment due to its inability to distinguish packet losses caused by network congestion from those attributed to transmission errors. Packet loss or reception of out of- order packets indicates failures. To remove such failures ,TCP implements flow control and congestion control algorithms based on the sliding window. The performance of TCP is generally lower in wireless networks than in fixed .Thesis explained by the fact that TCP cannot distinguish problems that typically occur in wireless networks from congestion. The congestion control algorithms in TCP are based on the assumptions that data is lost mainly due to congestion and that data loss duet transmission errors is rare .Therefore, data loss is interpreted as a signal of congestion in the network. Even in a wireless network, where data loss may not be related to congestion, TCP considers it as a congestion case.TCP segments may be lost if the radio conditions are poor and the link layer protocol provides a low reliability. After some retransmission attempts the link layer protocol gives up and leaves further error recovery to TCP. Handover events may also lead to data loss. A whole window of data may be lost due to handover. Data loss due to an unreliable link layer or a handover may cause a timeout event followed by slow start or three Daypack followed by fast retransmit and fast recovery. In either case, the congestion control action taken by TCP is unnecessary. Directly after the loss event ,the radio quality may become high again, and after handover data may be transmitted without problems to the new base station.TCP may also misinterpret a sudden increase in the round trip time as data loss. If the delay is long enough for the retransmission timer to expire before an acknowledgments received, then TCP misinterprets the delay as an indication of data loss due to congestion.