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AD HOC NETWORKS

Wireless Ad Hoc Networks:Basic Concepts

By Gary BreedEditorial Director

Ad hoc networks,which are alsocalled mesh net-

works, are defined by themanner in which the net-work nodes are organizedto provide pathways fordata to be routed fromthe user to and from thedesired destination.

Actually, the two names ascribed to thesenetworks provide considerable insight. Ad hochas two definitions—the first can be either“impromptu” or “using what is on hand,” whilethe other is “for one specific purpose.” Forexample, members of an ad hoc committee(studying a specific issue) might discover thatthey are attending the same event and decideto have an ad hoc (impromptu) meeting.

Ad hoc networks follow both definitions, aswell. They are formed as they are needed(impromptu), using resources on hand, andare configured to handle exactly what is need-ed by each user—a series of “one specific pur-pose” tasks.

The term mesh network accuratelydescribes the structure of the network: Allavailable nodes are aware of all other nodeswithin range. The entire collection of nodes isinterconnected in many different ways, just asa physical mesh is made of many small con-nections to create a larger fabric.

Figure 1 provides a simple diagram illus-trating these concepts. This diagram is mod-eled after a wireless “hot spot,” where an adhoc network links users to a router withaccess to the Internet. In this example, twousers are highlighted, showing two pathsthrough several nodes to the router.

If one of the intermediate nodes were tofail (e.g. that user leaves the area), the net-work will automatically reconfigure itself,locating an alternate path from the user to therouter. Typically, all available nodes are alsonetwork users, each sharing the total datatransfer capacity of the particular hardwareand operating protocol being used.

The network could also connect users toother users directly, as would be done in anindustrial control and monitoring network.Since there is no need for central administra-tion of the network configuration, it is most

This fundamental tutorialintroduces the concept of

ad hoc, or mesh, network-ing, which uses whatever

resources are available tocreate communication

paths from a user to his orher desired destination.

Figure 1 · Basic structure of an ad hoc, ormesh, network. The path from the user’snode to the destination node is provided byother users’ devices acting as routers.

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From March 2007 High Frequency ElectronicsCopyright © 2007 Summit Technical Media, LLC

High Frequency Design

AD HOC NETWORKS

efficient to design the system for autonomous operation ofeach node. In an industrial environment, a situation suchas an alarm would be propagated through the networkand received directly by each node. Each node would beprogrammed to respond according to its particular func-tion—machine control, process monitoring, supervisorypersonnel or central office.

Self-Configuring and Self-Healing ProcessesFigure 2 shows how ad hoc networks determine their

configuration. In Fig. 2a each node identifies the nodesthat are available for communications, based on signalstrength, which is mainly related to distance, but is alsoaffected by obstructions or interference. Some nodes maybe beyond range, others may be detectable but have insuf-ficient signal strength for reliable communications.

Once the available nodes are identified, this informa-tion is communicated to other nodes, along with informa-tion about the desired destination (Fig. 2b). Using thelists of available connections, the network configurationalgorithm selects a particular routing for each user to itsdestination. This process requires system operating soft-ware to have good decision-making algorithms, based onpractical criteria for signal strength, path reliability overtime, and network configuration patterns.

Over time, or even near-continuously, the network will

change. Users may come and go, nodes may be in motion,or changes in the electromagnetic environment may alterthe propagation between nodes. As these changes takeplace, the network will update its configuration and iden-tify new paths from users to destinations, as illustrated inFig. 2c. This type of reconfiguration will be repeated overand over as the network changes. Note that this is thesame process used in the Internet, where system loadingand hardware issues require redirection of a user’s datathrough different routers.

Advantages of Ad Hoc NetworksThe principal advantages of an ad hoc network include

the following:

• Independence from central network administration• Self-configuring, nodes are also routers• Self-healing through continuous re-configuration• Scalable—accommodates the addition of more nodes • Flexible—similar to being able to access the

Internet from many different locations

Limitations of Ad Hoc NetworksWhile ad hoc networks are typically used where they

have the greatest emphasis on its advantages, there aresome limitations:

• Each node must have full performance• Throughput is affected by system loading• Reliability requires a sufficient number of available

nodes. Sparse networks can have problems• Large networks can have excessive latency (time

delay), which affects some applications

Some of these limitations also apply to conventionalhub-and-spoke based networks, or cannot be addressed byalternate configurations. For example, all networks areaffected by system loading, and networks with few nodesare difficult to justify in hard-wired solutions.

Key ApplicationsWLAN is the initial application that received a con-

certed development effort. Peer-to-peer networks of com-puter/PDA users have become common. Commercial wire-less Internet service providers (WISP) use repeater nodesto extend coverage to a large area, while user nodes canextend service in their locality.

Control systems (e.g. environmental controls) andindustrial process monitoring and control are becomingmajor applications for mesh networking. These environ-ments are difficult to serve with dedicated wiring, beingspread over a large area, often with difficult access.Sensor networks from small-scale (e.g. household securi-ty monitoring) to large scale (e.g. wildlife tracking) are

also being developed with ad hoc networking as the oper-ational structure.

Developers of these and other applications have deter-mined that ad hoc networks are the most efficient way to

maintain system-wide communications. Hopefully, thisbrief tutorial will help you appreciate the value of thisimportant networking technique for establishing andmaintaining wireless communications.

Figure 2 · Creation and adaptation of an ad hoc network: (a) Determination of available nodes, (b) Selection ofthe optimal routing, and (c) Reconfiguration when the network makeup changes.

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(a) (b) (c)