At the end of our discussion in class on XORs in The Air: Practical Wireless Network Coding, I began to think about the feasibility of a commercial implementation of the idea behind the article. This article proposes the application of network coding techniques to wireless networking to opportunistically increase the amount of data that can be sent in one transmission. It holds great potential because transmission time is a scarce resource that can't be scaled up with added hardware. Furthermore, use of the technique is orthogonal to many other strategies for improving network efficiency. The idea involves nodes listening and recording temporarily all packets they can hear, including those not meant for them. Then, a transmitting node can XOR new packets with previously transmitted ones to send more than one packet simultaneously. Listeners can XOR the transmitted packet with packets they have stored in memory to obtain their desired payload.

The technique has several important technical hurdles to overcome in transforming itself to commercial success. The main issue seems to be that all nodes participating in a network that uses network coding require custom software in their network stack. This leads to significant deployment challenges; specifically, standard 802.11 nodes can't join the network unaided. This problem could be mitigated by requiring only infrastructure nodes in multi-hop networks to use network coding, but this would also limit the opportunities for optimization by network coding. Second, the benefits of network coding vary widely with factors such as network topology and current traffic and congestion levels, so that it may be difficult to predict how much the average network might benefit from network coding. Third, the technique increases the complexity of the network software stack, as nodes must now store extra network-specific state in the form of past packets. Lastly, network coding software might have a significant negative impact on node power consumption, as nodes are now required to listen to every packet that is broadcast in the network.

In light of the above hurdles, I believe the potential benefits of network coding as presented in the paper do not seem to be worth the costs. That is not to say the paper is not successful; it combined novel ideas and inspired a host of excitement new research in wireless networking. However, more work would need to be done to integrate network coding successfully into live wireless networks.

Overcoming the Challenges of Wireless Networking

Wireless networking poses unique challenges when compared to traditional networking. Two issues in particular are at the heart of these difficulties: every act of communication involves broadcasting and packet loss is much more frequent and dynamic. The broadcasting nature of wireless networks means that the medium of communication, air, is precious, as it must be shared by everyone. Also, it is very difficult for nodes to know whether or not adjacent nodes are available for communication, since an adjacent node may be busy communicating with a third node that the first node can't detect. Lastly, the packet loss characteristics of wireless communication in combination with TCP can lead to uneven resource availability and even full starvation, especially with multi-hop communication.

That said, researchers have applied their creative intellects to making the best of a fairly harsh networking environment. For instance, one paper proposed a change to a core multi-hop network to cause nodes in a network to work together to enforce fairness constraints at each hop of the network. While the algorithm could slightly reduce overall bandwidth utilization, it significantly reduced resource starvation and uneven resource usage between nodes. In another paper a group of researchers proposed that multiple packets be encoded together at once by XOR-encoding packets together. This method requires nodes to remember recent packets they have heard, as well as track other neighbor state, but could result in as much as 4x improvements in bandwidth usage. Lastly, we learned of a proposed technique to allow nodes to cooperate with another in sending a packet across a multi-hop network. When a node fires a batch of packets, all capable listeners are enabled to assist those packets in traversing the network, so that even if the original next-hop destination failed to hear some packets nearby neighbors could pass on the packets for it.

These innovative solutions showed me both just how difficult improving wireless networking and extending its use cases can be. At the same time, it showed me that difficult problems can provide fertile ground for creative thinkers to apply themselves.