Improving wireless TCP/IP performance using the median filter algorithm

Improving wireless TCP/IP performance using the median filter algorithm

The estimation of Retransmission Timeout (RTO) in Transmission Control Protocol (TCP) affects the throughput of the transmission link. If RTO is just a little larger than Round Trip Time (RTT), retransmissions will occur too often, and this increases congestion in the transmission link. If RTO is much larger than RTT, the response to retransmit when a packet is lost will be too slow, and this will decrease the throughput in the transmission link (Comer
2006a).

Currently, the Jacobson/Karels Algorithm is widely used for the estimation of RTO in TCP implementations. The algorithm uses an Exponential Weighted Moving Average (EWMA) fi�lter
to estimate RTT and then determines the RTO from this. The EWMA �filter is good if the RTT follows a Gaussian distribution. In reality, traffic in the Internet is bursty and tends to follow a heavy-tailed distribution. Using an EWMA approach to estimate heavy-tailed distribution is
inadequate.

The median fi�lter has been recognized as a useful non-linear fi�lter due to its edge preserving
and impulse suppressing characteristics, so it is eff�ective in removing impulsive noise (Nodes & Gallagher Jr. 1982). The median fi�lter has been applied to many areas of signal processing, particularly in image processing to remove positive and negative impulsive noise. Thus, it can
perform well for heavy-tailed distributions.

In this project, the median �filter is applied to estimate the RTT over TCP links, under bursty
traffic conditions. Experiments and simulations are conducted to determine if the median fi�lter performs better than the EWMA fi�lter. In the experiments, consistent RTT and a small RTO are obtained, which are desirable factors for high connection throughput. In the simulations,
the median fi�lter not only delivers higher throughput, but also drops fewer packets during transmission than EMWA filter does. Both the experiments and simulations show that the median �filter outperforms the EWMA �filter under bursty traffic conditions.