Ad Hoc Networks
Wright State University
EE – 737
Digital Spread Spectrum Systems
Ad Hoc Networks represent complex distributed systems that comprise wireless mobile nodes that can freely and dynamically self-organize into arbitrary and temporary, „„ad-
hoc‟‟ network topologies, allowing people and devices to seamlessly inter-network in
areas with no pre-existing communication infrastructure, e.g., disaster recovery environments. Here a brief overview of what is Ad Hoc Networks and how they work is provided. Then a look at the advantages of Ad Hoc Networks and also the issues faced by Ad Hoc Networks is provided. The last part of this paper is dedicated to identifying the areas of improvement in the field of Ad Hoc Networks.
Ad Hoc Networks is defined as a collection of mobile hosts forming a temporary network without the aid of any centralized administration or standard support services. In Latin, ad hoc literally means "for this," further meaning "for this purpose only," and thus usually temporary. Ad hoc networks represent complex distributed systems that comprise wireless mobile nodes that can freely and dynamically self-organize into arbitrary and temporary, „„ad-hoc‟‟ network topologies, allowing people and devices to seamlessly inter-network in areas with no pre-existing communication infrastructure. The concept of Ad Hoc Networking has been around for nearly 20 years but has received renewed interest in the last 18 to 24 months.
In Ad Hoc Networks the individual mobile hosts (nodes) act at the same time as both the router and the host.
An ad-hoc (or "spontaneous") network is a local area network or any other small network, especially one with wireless or temporary plug-in connections, in which some of the network devices are part of the network only for the duration of a communication session, whereas in the case of mobile or portable devices it is part of the network when in some close proximity to the rest of the network. In Latin, ad hoc literally means "for this,"
further meaning "for this purpose only," and thus usually temporary. Ad Hoc Networks are future alternative to the current trend of connections among wireless devices via fixed infrastructure-based service.
HOW AD HOC NETWORKS WORK:
Ad Hoc Networks are useful in areas that have no fixed infrastructure and hence need alternative ways to deliver services. Ad Hoc Networks work by having mobile devices connect to each other in the transmission range through automatic configuration, i.e., setting up an ad hoc network that is very flexible. In other words there is no intervention of any controller that goes ahead and gathers data from all nodes and organizes it. All data gathering and cross-node data transfer is taken care of by the nodes themselves. Ad Hoc Networks are a major goal towards the evolution of 4G (Fourth generation) devices. In the nodes of the Ad Hoc Networks, computing power and network connectivity are embedded in virtually every device to bring computation to users, no matter where they are, or under what circumstances they work. These devices personalize themselves to find the information or software they need. The strife is to make use of all
technologies available without making any major change to the user‟s behavior. There is
also work going on to make the seamless integration of various networks possible, i.e., integration of LAN, WAN, PAN and Ad Hoc Networks. But there is still a lot of work to be done to make this completely possible.
Node mobility in an ad hoc network causes frequent changes of the network topology. Figure 1 shows such an example: initially, nodes A and D have a direct link between them. When D moves out of A‟s radio range, the link is broken. However, the network
is still connected, because A can reach D through C, E, and F.
Spread Spectrum Techniques are used in the implementation of Ad Hoc Networks because spread spectrum helps to reduce interference from other sources. Also it helps in bandwidth reuse. The boundaries of Ad Hoc Networks are not absolute and hence it is possible that when certain nodes stray into the area of influence of certain transmitters these may get affected by their signals. The use of Spread Spectrum (SS) makes sure that this does not happen as the spreading code and the de-spreading code should ideally be the same. This same technique provides the method for frequency reuse.
Differences between Cellular Network and Ad Hoc Network:
Cellular network Ad Hoc Network
Fixed, pre-located cell sites and No fixed base stations, very rapid
base stations. deployment.
Static backbone network Highly dynamic network
topology. topologies with multi-hop.
Relatively benign environment Hostile environment (losses, noise)
and stable connectivity. and sporadic connectivity.
Detailed planning before base Ad hoc network automatically
stations can be installed. forms and adapts to changes.
PHYSICAL LAYER IN AD HOC NETWORKS:
; Data rates:
– 1 Mbps, 2 Mbps
; Transmission bands
– Transmission in license-fee 2.4 GHz band (in US, Europe 2.4000-2.4835
GHz) and in 5 GHz band
; Use of “spread spectrum” technique for 1 Mbps or 2 Mbps
; DSSS (direct sequence spread spectrum)
; FHSS (frequency hopping spread spectrum)
ADVANTAGES OF AD HOC NETWORKS
The major advantage of the Ad Hoc Networks is that it does not need any base station as is required in regular mobile networks. They can form a network in any place as required immediately which make them indispensable in battlefield and disaster relief situations. They are useful in areas that have no fixed network for internet coverage. Here they can be used to provide coverage. They can be used in areas where the available network has been destroyed.
ISSUES FACED BY AD HOC NETWORKS:
Security is a very major concern in the development of Ad Hoc Networks. The boundaries of the network are not well defined and hence it is possible for any node to go out of the network. It is also possible for an Ad Hoc Network having a large number of nodes to split into two networks. It is less reliable than wired media due to the inherent problem faced by any wireless network.
Due to the formation of Ad Hoc Networks by various devices that need not be having the same capacity it is possible that each device may have different capacity, functionality and protocols. Hence it is necessary to find a solution where all there varied devices can operate together. They also have asymmetric propagation metrics. Capacity constraints faced by these networks in the form of transmission range, wireless bandwidth is another concern. This is taken care of to an extent by the use of Spread Spectrum techniques. Errors and breakdown could also happen in these networks and it is imperative to have a solution or a backup plan for these exigencies.
Ad Hoc Networks also face a problem called the Hidden-terminal and exposed-terminal phenomena. In Hidden terminal situation, A and C are outside the transmission range of each other and cannot detect each oth