Alleviating Effects of Mobility on TCP Performance in Ad Hoc Networks Using Signal Strength Based Link Management

Abstract

Mobility in ad hoc networks causes link failures, which in turn result in packet losses. TCP attributes these losses to congestion. This results in frequent TCP retransmission timeouts and degradation in TCP performance even at light loads. We propose mechanisms that are based on signal strength measurements to alleviate such packet losses due to mobility at light loads. Our key ideas are (a) if the signal strength measurements indicate that a link failure is most likely due to a neighbor moving out of range, in reaction, facilitate the use of temporary high power to re-establish the link and, (b) if the signal strength measurements indicate that a link is likely to fail, initiate a route re-discovery proactively before the link actually fails. We make changes at the MAC and the routing layers to predict link failures and estimate if a link failure is due to mobility. We also propose a simple mechanism that can help alleviate false link failures that occur due to congestion when the IEEE 802.11 MAC protocol is used. We compare the above proactive and reactive schemes and also demonstrate the benefits of using them together. We show that, in high mobility, the performance of a TCP session can increase by as much as 45 percent when our methods are incorporated.

This work was partially funded by the DARPA Fault Tolerant Networks (FTN) contract no: F30602-01-2-0535.