1.1 Multihop Wireless Networks

A multihop wireless network (MWN) is a network of nodes (e.g. computers) connected by wireless communication links. The links are usually implemented with digital packet radios. Due to the limited transmission range of the radio, many pairs of nodes in MWNs may not be able to communicate directly; hence they may need other intermediate nodes to forward packets for them. Multihop wireless networks have broad military and civilian applications in many critical situations. They have received increasing attention in the past decade due to their broad applications and easy deployment at low cost without relying on existing infrastructure (Akyildiz and Kasimoglu 2004; Akyildiz et al. 2002, 2005; Cerpa et al. 2001; Chong and Kumar 2003; Estrin et al. 2002; Lorincz et al. 2004). Different names are used to refer to them in different scenarios.

Mobile ad hoc Networks (MANETs)

Generally speaking, a mobile ad hoc network (MANET) is a self-configuring network of mobile devices connected by wireless links. Each device in a MANET is free to move independently in any direction. So the node-to-node connection and network topology will change frequently. The primary challenge in MANETs is continuously to maintain the routing information at each node required to properly route traffic. The applications of MANETs include search-and-rescue operations. Such scenarios are characterized by a lack of installed communications infrastructure because all the equipment might already be destroyed or the region could be too remote. MANETs can also provide communications between autonomous vehicles, aircraft and ground troops in the battlefield where a fixed communication infrastructure is always unavailable and infeasible.

Wireless Sensor Networks (WSNs)

Wireless sensor networks (WSNs) (Akyildiz et al. 2002) are another variant of MWNs. They are normally used to monitor various physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants. A large-scale WSN typically consists of hundreds or thousands of small and cheap sensor nodes with wireless communication capabilities. These sensor nodes may form local clusters, and reactively or periodically report the sensing results to one or multiple base stations via multihop routing. The sensors are usually powered by batteries with limited capacity. Energy efficiency is therefore the primary concern and key challenge in WSNs. The sensors are typically static but some more powerful sensor nodes may have mobile capability (Hu and Evans 2004).

Wireless-Mesh Networks (WMNs)

Wireless-mesh networks (WMN) (Akyildiz and Wang 2005) are another type of MWNs, and are usually used to provide the last mile wireless broadband Internet access for the civilian users. They can also support enterprise networking, healthcare and medical systems, and security surveillance systems. They consist of mesh routers and mesh clients, where mesh routers have minimal mobility and form the backbone of WMNs. The integration of WMNs with other networks such as the Internet, cellular networks, IEEE 802.11 WLAN, IEEE 802.15, IEEE 802.16, and sensor networks can be accomplished through the gateway and bridging functions in the mesh routers. Mesh clients can be either stationary or mobile, and can form a client mesh (multihop) network among themselves and with mesh routers. Network capacity in WMNs is an important issue. The capacity of WMNs is affected by many factors such as network topology, node density, traffic patterns, number of radios/channels used for each node, transmission power level, carrier sensing threshold, node mobility, and environment (indoor/outdoor), etc. A clear understanding of the relationship between network capacity and the above factors provides a guideline for protocol development, architecture design, deployment and operation of the network.

Vehicular ad hoc Networks (VANETs)

In VANETs, every vehicle communicates with other vehicles (V2V) and with roadside infrastructures (V2I) by means of wireless communication equipment. The most important usage of these networks is to inform other vehicles in emergency situations such as car accidents, urgent braking or traffic jams. In such cases, a vehicle can inform other vehicles by broadcasting safety messages before facing the event. VANETs are a cornerstone of the envisioned Intelligent Transportation Systems (ITS). They will contribute to safer and more efficient roads in the future by providing timely information to drivers and concerned authorities. VANETs are similar to MANETs, but the key difference lies in that in VANETs, vehicles move in an organized fashion rather than randomly. The vehicles are restricted in their range of motion and their mobility can be predicted in the short term, because their movement should obey certain traffic rules.

Compared with traditional single-hop wireless networks, such as cellular networks and local area networks, MWNs have several advantages: 1. coverage extension and connectivity improvement; 2. reducing energy consumption; transmission over multiple short-range wireless links might require less transmission energy than that required over long-range single-hop links; 3. cost efficiency: they avoid wide deployment of cables and can be deployed in a cost efficient way; 4. robustness: in MWNs, multiple paths might exist between a pair of communication nodes, which can be used to increase robustness of the network.