Multihop Wireless Networks: Opportunistic Routing

7.1 A Brief Review of MORE

In the original opportunistic routing ExOR (Biswas and Morris 2005), the MAC layer is tied with routing to provide opportunistic gain. However, due to the strict scheduling on forwarders' access to the medium, it actually prevents spacial reuse to a large extent. If multiple packets are received by the same receiver, those packets may contain duplicate information that wastes the precious network bandwidth. In the case of a large network with highly lossy wireless medium, this problem is aggravated because the strict scheduling and coordination among forwarders is hard to realize due to multipath fading and packet collisions. Thus, MORE was proposed by Chachulski et al. (2007) to solve this challenge. By integrating random linear network coding1 at the intermediate forwarders, nodes forward the coded packets randomly instead of using the structured scheduling approach in ExOR.

Here we illustrate the motivation of MORE using a simple example. Consider the topology in Figure 7.1 with a single unicast flow. Traditional routing first determines the best path before transmission, which is “A → B → C.” However, there is always a chance for the destination node C to receive packets from source A directly. If, in one transmission, B and C both receive packet p1, this will cause a wasted transmission at node B using traditional routing because B does not know C's reception status. ExOR solves this problem by enforcing a strict transmission coordination for node ...