Layer 2 vs. Layer 3 Mechanisms for Improving TCP Performance in 802.11 Wireless LANs

Access points in 802.11 wireless networks suffer from performance problems
because of insufficient resources at layer 2---the DCF (Distributed
Control Function) access method provides equal access probability to
all devices in a wireless cell including the access point
itself. Consequently, performance degrades and contention between
uploads and downloads can lead to the familiar TCP unfairness problem.

In this paper, we study the measured performance of mechanisms at
different layers for improving TCP performance in 802.11 wireless
LANs. At layer 2, we consider the AAP (Asymmetric Access Point)
solution that keeps low buffer occupancy at the access point. At
layer 3, we consider LAS-ACK, an adaptation of the Least Attained
Service (LAS) policy for wireless LANs that aims at minimizing the
average queue size by giving priority to the shortest connections.
Using an experimental testbed, we demonstrate that AAP is a good
solution especially for multimedia (delay and jitter sensitive)
transfers as long as upload traffic is low. On the other hand,
LAS-ACK is very efficient at minimizing the durations of most upload
and download transfers as long as the distribution of flow sizes is
skewed enough. The price to pay for combining LAS-ACK and AAP is the
requirement of deploying LAS-ACK on all wireless stations and not
only at the access point. The resulting solution actually combines
the positive effects of both solutions as LAS-ACK is less sensible
to the distribution of flow sizes, while multimedia flows benefit
from the short queue size at the access point.