9. Zhiwen Lin (zlin@sfu.ca), Wangang Zeng(wgzeng@cs.sfu.ca), and Meihua Judy Zhan (mjzhan@cs.sfu.ca)

Improving TCP Performance with Periodic Disconnections over Wireless Links

In theory, transport protocol such as TCP should be independent of the technology of the underlying network layer. In practice, it does matter because most TCP implementation have been carefully optimized based on assumptions that are for wired netwrok but which fail for wireless networks. The principal problem is the congestion control algorithm. Nearly all TCP implementations nowadays assume that timeout are caused by congestion, not by lost packets. Consequently, when a timer goes of, TCP slows down and sends less vigorously. The idea behind this approach is to reduce the network load and thus alleviate the congestion. But wireless transmission links are highly unreliable. They lose packets all the time. Many papers have been written proposing methods for improving TCP performance over wireless links, such as Berkley Snoop, Indirect TCP, WTCP etc. All these papers have, however, concentrated on only one problem associated with wireless links --- a perceived high BER over the wireless links.

While a high BER has significant implications for protocol performance, other limitations of the wireless environment are equally or more important than high BER. Frequent disconnection is one of these problems which are caused mainly by handoff, and physical obstacles blocking radio signals.

A solution proposed by Kevin Brown and Suresh Singh, M-TCP, was designed to work well in the presence of frequent disconnection events and over low bit-rate wireless links subject to dynamically changing bandwidth. In addition, it maintains end-to-end TCP semantics. The protocal structure may be viewed as a three-level hierarchy. At the lowest level are the mobile hosts(MH) who communicate with mobile support station(MSS) nodes in each cell. Several MSSs are controlled by a machine called the Supervisor Host(SH). The SH is connected to the wired network and it handles most of the routing and other protocol details for the mobile users. In additon it mantains connections for mobile users, handles flow-control and is responsible for maintaining the negotiated quality of service.

We'll simulate this M-TCP solution on OPNET and compaire its performance with other TCPs.

[References]

*1. J. Border, M. Kojo, J. Griner, G. Montenegro, Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, June 2001, http://www.ietf.org/rfc/rfc3135.txt.

*2. G. Montenegro, S. Dawkins, M. Kojo, V. Magret, N. Vaidya, "Long Thin Networks", RFC 2725, January 2000, http://www.ietf.org/rfc/rfc2757.txt.

*3. Kevin Brown, Suresh Singh, "M-TCP: TCP for Mobile Cellular Networks", ACM, July 1997, http://www.acm.org/sigcomm/ccr/archive/1997/oct97/ccr-9710-brown.pdf.

*4. Kevin Brown, Suresh Singh, "A Network Architecture for Mobile Computing", Proc. IEEE INFOCOMM'96, S.F. CA, March 1996, http://www.it.kth.se/~jiang/doc/literature/bibdata/Bro96.pdf.

*5. S. Singh, "Quality of Service Guarantees in Mobile Computing", J. Computer Communications, Vol. 19, pp. 359-371, 1996. *6. K. Seal and S. Singh, "Loss profiles: A Quality of Service Measure in Mobile Computing," Journal of Wireless Networks, vol. 2, no. 1, pp. 45-61, 1996.

*7. Ajay Bakre and B.R. Badrinath, "I-TCP: Indirect TCP for Mobile Hosts", In Proc. of 15th Int'l Conf. on Distributed Computing Systems (ICDCS), May 1995. http://users.ece.gatech.edu/~siva/ECE4894/list/7.pdf. Last visit Feb. 24th, 2002.