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 others transmissions, but B is in the transmission range of both. As shown below a collision may occur, for example, when the station A and station C start transmitting towards the same receiver, station B. This should be avoided.
In Exposed terminal situation, A transmission range covers B and C. Hence when A transmits to B, C thinks that it cannot transmit when actually it could transmit to D. This is a waste of resource which should also be avoided.
Route changes will occur due to router mobility, i.e., as the node themselves act as routers and certain nodes can leave the network in between.
Energy consumption and saving is a major are of interest. Advances in battery technology have not been at par with the development of Ad Hoc technology. Most existing solutions for saving energy in ad hoc networks revolve around the reduction of power used by the device. At the MAC level and above, this is often done by selectively sending the device into a sleep mode, or by using a transmitter with variable output power (and proportionate input power draw) and selecting routes that require many short hops, instead of a few longer hops. Beaconing is used by the nodes to let the other nodes know of its presence. The beaconing interval has to be short enough to let the other nodes know that the node is in the network yet long enough so as to save power.
Security is the major issue in wireless Ad Hoc Networks and actually ought to receive a complete analysis of it than being presented as a part of the study on Ad Hoc Networks. The use of wireless links renders an ad hoc network susceptible to link attacks ranging from denial of service, passive eavesdropping to active impersonation, message replay, and message distortion. Eavesdropping might give an adversary access to secret information, violating confidentiality. Active attacks might allow the adversary to delete messages, to inject erroneous messages, to modify messages, and to impersonate a node, thus violating availability, integrity, authentication, and non-repudiation. Nodes, roaming in a hostile environment (e.g., a battlefield) with relatively poor physical protection, have non-negligible probability of being compromised. Therefore, we should not only consider malicious attacks from outside a network, but also take into account the attacks launched from within the network by compromised nodes. Therefore, to achieve high survivability, ad hoc networks should have a distributed architecture with no central entities. Introducing any central entity into our security solution could lead to significant vulnerability; that is, if this centralized entity is compromised, then the entire network is subverted.
An ad hoc network is dynamic because of frequent changes in both its topology and its membership (i.e., nodes frequently join and leave the network). Trust relationship among nodes also changes, for example, when certain nodes are detected as being compromised. Unlike other wireless mobile networks, such as mobile IP, nodes in an ad hoc network may dynamically become affiliated with administrative domains. Any security solution
with a static configuration would not suffice. It is desirable for our security mechanisms to adapt on-the-fly to these changes.
Finally, an ad hoc network may consist of hundreds or even thousands of nodes. Security mechanisms should be scalable to handle such a large network.
The denial of a service can be caused by such legitimate ways as a radio jamming or battery exhaustion. An attacker can cause a radio jamming by jamming a wider frequency band and in that way using more power. The latter can be of real threat, because once a battery runs out the attacker can walk away and leave the victim disabled. This kind of technique is called the sleep deprivation torture attack. Symmetric key cryptography is used to provide authenticity and integrity. Integrity means that no node has been maliciously changed. The devices themselves should be able to detect security breaches and plug them.
Ad Hoc Networks is an area that is being widely researched these days and is a very fast growing area. Much work still is left to be done in this field for it to be commercially viable. It is the technology that is providing the stepping blocks to the evolution of 4G. Power Control is a major area of improvement and also they need to be made more secure. Ad Hoc Networks have started to be implemented in the field today in battlefields and also in disaster struck areas. As time goes by we can see more applications of Ad Hoc Networks.
There are a few areas that need to be given particular focus for improvement in Ad Hoc Networks.
Scalability: Currently the size of Ad Hoc Networks are small and work needs to be done to identify to what size can these networks grow and further try to increase the size of these networks to what is that of the Internet today.
Quality Of Service: A quality of service is defined for the network with no losses and attempts should be made achieve that. QoS parameters will involve bandwidth considerations and savings of bandwidth will be implemented. Also finding the shortest path so as to save power in the devices as the source of power is very limited. Care should be taken so as to have no collision losses.
Power Control: Reducing power to the communications interface and entering sleep
state are ways of extending battery life of mobile units. But these techniques make communication difficult. Hence some efficient technique should be developed to make this viable. Research should also focus on getting battery technology growth on par with Ad Hoc Network Technology growth.
Security: Security needs to be very widely investigated as they are imperative. Wireless networks are as such insecure and particularly so with Ad Hoc Networks. Implementations of current cryptography techniques are not good enough and also difficult.
Location Access: User location could be incorporated into routing.