Figure 1a. Infrastructure-based wireless network

Figure 1b. Ad hoc wireless network

Figure 2. Multi-hop network

ABSTRACT

Mobile Ad Hoc Networks (MANETs) refer to a class of wireless network that can be formed dynamically and randomly without the need for infrastructural setups. Such networks are able to adapt and reconfigure themselves on the fly according to node mobility and changing network topologies. These characteristics are particularly attractive to the military user due to the inherent unpredictability of the tactical environment. MANET technology has its roots in defence, having been developed from military research efforts. This article presents an overview of MANET technology, its key characteristics and how it can be leveraged for the Third Generation Singapore Armed Forces. Experience gained and lessons learnt from an experiment initiated and funded by the Future Systems Directorate on MANET are also discussed.

Chim Yuen Chong
Raymond Seah Kwang Wee
Sim Soon Lian
Tan Jia Hui

INTRODUCTION

A Mobile Ad Hoc Network (MANET) is an interconnected system of wireless nodes which communicate over bandwidth-constrained wireless links. Each wireless node can function as a sender, a receiver or a router. When the node is a sender, it can send messages to any specified destination node through some route. As a receiver, it can receive messages from other nodes. When the node functions as a router, it can relay the packet to the destination or next router in the route. When necessary, each node can buffer packets awaiting transmission (He, 2003).

The nodes move randomly; hence at a given point in time, an ad hoc network exists between the nodes, giving rise to an arbitrary network topology. MANETs can be dynamically formed among any group of wireless users and require no existing infrastructure or configuration.

MANET CHARACTERISTICS

A MANET has several marked characteristics. First, it does not have a centralised infrastructure. It is unlike the traditional mobile wireless networks in which base stations, access points and servers have to be deployed before the networks can be used. Figure 1(a) illustrates how an infrastructure-based wireless network would operate1.
Instead, as shown in Figure 1(b), the ad hoc network is decentralised, with all mobile nodes functioning as routers and all wireless devices being interconnected to one another. Intuitively, this means that the MANET is also a self-configuring network in which network activities, including the discovery of the topology and delivery of messages, are executed by the nodes themselves.

The second characteristic of a MANET is that it has a dynamic topology. Nodes are free to move arbitrarily, causing the network topology to change rapidly and unpredictably over time. Alternative paths are automatically found, after which data packets are forwarded across the multi-hop paths of the network. MANETs use various routing mechanisms to accomplish this. This is further elaborated in Annex A.

Thirdly, a MANET operates on bandwidthconstrained variable-capacity links. Wireless links have significantly lower capacity than hard-wired links. As such, a MANET has relatively low bandwidth links, high bit error rates, and unstable and asymmetric links. This is in contrast to wired networks which are characterised by high bandwidth links, low bit error rates and stable and symmetric links. One effect of having a low link capacity is that congestion is typically the norm rather than the exception (Corson and Macker, 1999).

Fourthly, a MANET is often bound by energyconstrained operations (Corson and Macker, 1999). This is because its nodes are often hand-held battery-powered devices. Since the mobile nodes rely on these exhaustible means for energy, power conservation is important in a MANET system design.

Lastly, there is limited physical security. Mobile wireless networks are more prone to the physical security threats of eavesdropping, interception, denial-of-service and routing attacks as compared to fixed-cable networks (Corson and Macker, 1999). Hence, security techniques have to be applied to reduce these threats. Nodes prefer to radiate as little power as necessary and transmit as infrequently as possible. This will decrease the probability of detection and interception. In addition, the decentralised nature of network control will add robustness against failure as opposed to the centralised networks.

LIMITATIONS OF MANETS

There is a current and future need for dynamic ad hoc networking technology. This highly adaptive networking technology, however, still faces various limitations.

Throughput Drops with More Hops

Nodes that are in the transmission range of each other are able to send the data packets directly. However, when the node needs to send data to a non-neighbouring node, the data packets will have to be sent through a sequence of multiple hops, with the intermediate nodes acting as routers. This indicates an increase in the number of hops taken. Throughput will decrease rapidly when the number of hops is increased. This can be explained using the four-hop network illustrated in Figure 2.

When link 1-2 is active, link 2-3 cannot be active because a node cannot be transmitting and receiving at the same time. Link 3-4 is also inactive because communication by node 3 may interfere with node 2 (Holland and Vaidya, 2002). Thus, with more hops, there are in turn more idle nodes, which reduces the throughput.

Throughput Drops with Increasing Mobility

Highly mobile nodes will result in more overheads due to frequent topology changes. This is because of the increase in the number of routing packet transmissions due to the need to determine new routes after route failures. When the routing table is used, each node keeps a list of all available destinations as well as the number of hops required to reach each destination. Changes in the topology will be reflected in the routing table. Any routing table changes are relayed to all the other nodes. This imposes a larger overhead on the overall network. When the overhead is large, a lower percentage of the packet goes towards the transmission of data, resulting in a lower throughput.

Delay

Delay represents the average time duration which a packet takes from the time it leaves a source to the time it arrives at its destination. As seen above, there is a need to keep the nodes busy with the transmission and receiving of packets in order to increase the throughput of the network. This, in turn will mean that the queue of each node is always not empty, leading to a longer delay.