TCP Performance in Coded Wireless Mesh Networks

Abstract

This paper investigates the benefit of network coding for TCP traffic in a wireless mesh network. We implement network coding in a real 802.11a wireless mesh network and measure TCP throughput in such a network. Unlike previous implementations of network coding in mesh networks, we use off-the-shelf hardware and software and do not modify TCP or the underlying MAC protocol. Therefore, our implementation can be easily exported to any operational wireless mesh network with minimal modifications. Furthermore, the TCP throughput improvement reported in this paper is due solely to network coding and is orthogonal to other improvements that can be achieved by optimizing other system components such as the MAC protocol. We conduct extensive measurements to understand the relation between TCP throughput and network coding in different mesh topologies. We show that network coding not only reduces the number of transmissions by sending multiple packets via a single transmission but also results in a smaller loss probability due to reduced contention on the wireless medium. Unfortunately, due to asynchronous packet transmissions, there is often little opportunity to code resulting in small throughput gains. Coding opportunity can be increased by inducing small delays at intermediate nodes. However, this extra delay at intermediate nodes results in longer round-trip-times that adversely affect TCP throughput. Through experimentation, we find a delay in the range of 1 ms to 2 ms to maximize TCP throughput. For the topologies considered in this paper, network coding improves TCP throughput by 10% to 85%.

New jitter-aware network coding approach for packet selection in wireless mesh network using Jacobsen

“…Available methods, e.g. Huang et al (2008), added artificial delays to packets in order to gain time for coding and to increase throughput while the methods cannot be adopted in many immediate applications. However, early sending of packets or adding delays to them, aimed at gaining more time for coding, can influence on the jitter of sent data.…”

NCAPQ: Network Coding-Aware Priority Queueing for UDP Flows over COPE

“…Some others proposed new coded wireless systems, based on the idea of COPE [11], [7]. In [12], the problem of rate mismatch between flows that are coded together is addressed, by delaying packets. In [3], higher MAC level packet prioritization is given to coded than to uncoded packets.…”

Network Coding-Aware Queue Management for Unicast Flows over Coded Wireless Networks

“…COPE [1] has also noticed the problem with TCP performance due to rate mismatch, which motivated this study. As discussed in the introduction, [3] addressed this problem by delaying packets to code with others. We propose a different solution via queue management and congestion control.…”

“…Rate mismatch between flows can significantly reduce the coding opportunities, as there may not be enough packets from different flows at an intermediate node to code together. One possible solution is to artificially delay packets at intermediate nodes [3], until more packets arrive and can be coded together. However, the throughput increases with small delay (due to more coding opportunities) but decreases with large delay (which reduces the TCP rate); the optimal delay depends on the network topology and the background traffic and also may change over time.…”