Since a wired ad hoc network would be synonymous with a LAN, the term ad hoc networks almost always means ad hoc wireless networks and the two terms are will be used interchangeably throughout this series of articles.

The term Mobile Ad Hoc NETworks (MANETs) refers to ad hoc networks in which the nodes forming the ad hoc network are mobile. Most ad hoc networks allow their nodes to be mobile and are therefore MANETs.

In other words, these networks are formed on an as-needed basis and do not require the existence of any infrastructure. This property makes ad hoc wireless networks suitable for use in various scenarios like disaster recovery, enemy battlefields or in areas where user density is too sparse or too rare to justify the deployment of network infrastructure economically. Figure 8.1 below shows some examples of ad hoc wireless networks.

Figure 8.1 Examples of ad hoc networks

The scenarios and examples shown in Figure 8.1 above present a small subset of scenarios where ad hoc networks may be useful. An ad hoc network may operate in a standalone fashion or may be connected to a larger network like the Internet. Since ad hoc networks have such varied areas of use, it is instructive to classify them based on certain features.

First, ad hoc networks may be classified on the basis of their geographical coverage. Therefore we have ad hoc personal area networks (PANs), ad hoc local area networks (LANs) and ad hoc wide area networks (WANs).

Second, ad hoc networks may be classified based on whether or not nodes in the network are capable of acting as routers. To understand this classification, realize that the wireless networks that we are looking at always used the fixed, static, wired infrastructure for routing.

In traditional wireless networks (TWNs), call routing was achieved by dedicated routing switches of the PSTN and the core GSM network (which consisted of MSCs and GMSCs). Furthermore, since both the PSTN and the core GSM network are wired networks which are static (that is, their network topology almost never changes), it is relatively easy to proactively distribute the network topology information to the routing switches.

This in turn allows each routing switch to precompute and maintain routes to other switches, thus facilitating routing. Similarly, in wireless local area networks (WLANs), packet routing is achieved by using Layer 2 switches and IP routers.

Again, since these routing devices are connected by a wired infrastructure and are static, it is relatively easy to proactively distribute the network topology information to the routers and switches.

Ad hoc networks have two major limitations: a) there are no dedicated routing devices (since there is no infrastructure available) and b) the network topology may change rapidly and unpredictably as nodes move. In the absence of any routing infrastructure, the nodes forming the ad hoc networks themselves have to act as routers.

A MANET may therefore be defined as an autonomous system of mobile routers (and associated hosts) connected by wireless links - the union of which forms an arbitrary graph.

Given the central importance of routing in ad hoc networks, it is not surprising that routing forms a basis for classifying ad hoc networks into two groups: single-hop ad hoc networks and multihop ad hoc networks.

Single-hop ad hoc networks are ad hoc networks where nodes do not act as routers and therefore communication is possible only between nodes which are within each other's Radio Frequency (RF) range. On the other hand, multihop ad hoc networks are ad hoc networks where nodes are willing to act as routers and route or forward the traffic of other nodes.