1.2 Routing Challenges in MWNs

The purpose of routing is generally to find a path or multiple paths from the source to the destination, maintain or update path(s) when the topology or link quality changes, and forward packets along the path(s). Routing protocol design in MWNs faces a great challenge mainly due to the following facts.

First, the wireless link is unreliable. The properties and quality of a wireless link may vary with the transmission power, transmission rate, distance and path loss between two nodes. Furthermore, channel fading (such as multipath fading and shadowing) results in fluctuations in the received signal strength and therefore intermittent link behavior. The difficulty in managing or controlling the link quality and reliability in wireless networks makes it very hard to find and maintain a good and stable path from the source to the destination.

Second, the wireless medium is broadcast in nature. Transmission on one link may interfere with the transmissions on the neighboring links. This broadcast medium contention brings fundamental constraints on the routing performance, such as throughput and delay. There is inevitable intra-path and inter-path contention due to the broadcast nature of the wireless medium. It is very challenging to achieve optimal routing performance even when there is a single flow, due to the complicate interdependence between the medium contention, route selection, and medium access control. When there are multiple flows (different source-destination pairs) in the network, optimizing overall routing performance becomes extremely hard.

Third, mobility is an inherent property and phenomenon in wireless networks. The node's mobility makes network topology change frequently, thus complicating the task of finding and maintaining a good path between the source and destination. Mobility also affects the link quality, which introduces further challenges in maintaining a timely good path.

Fourth, wireless-embedded devices, such as sensors and handheld devices, are typically battery powered. The lifetime of the battery imposes a limitation on the operation hours and connectivity of the network. Energy efficiency has been a critical concern in energy-constrained networks (e.g. wireless sensor networks). Finding paths that consume minimum energy to deliver the packets from the source to the destination is an important approach to save energy in wireless sensor networks, since the radio communication has been identified as the major source of energy consumption in such networks. However, finding minimum energy consumption path(s) is neither easy nor enough. It is hard mainly because of the unreliability of the wireless link, and it is not enough because we also have to achieve other performance goals, such as satisfying a delay constraint due to specific applications (e.g. surveillance). Choosing and maintaining path(s) that strike a good balance between energy consumption and performance is challenging, especially with the presence of unreliable wireless links.

A large body of research on routing protocols in MWNs has been motivated by the above challenges. Next, we will introduce the major routing protocols in the literature, and motivate the opportunistic routing.