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There are currently two kinds of mobile wireless networks. The first is known as infrastructure networks with fixed and wired gateways. Typical applications of this type of "one-hop" wireless network include wireless local area networks (WLANs). The second type of mobile wireless network is the infrastructure-less mobile network, commonly known as the ad hoc network. An ad hoc network is usually a self-organizing and self-configuring "multi-hop" network which does not require any fixed infrastructure. In an ad hoc network, all nodes are dynamically and arbitrarily located, and are required to relay packets for other nodes in order to deliver data across the network.
An ad-hoc wireless network, also called peer-to-peer, allows computer on the network to communicate with each other without using a router. Any system that has a wireless network interface card can transmit and receive information from other systems with wireless cards installed.
Wireless devices which are in ad hoc mode in a wireless computer network, communicate with each other directly. Wireless devices which are within each other's reach, discover and communicate with each other in a peer-to-peer way when they operate in an ad hoc mode.
In most cases, systems connected in a local, peer-to-peer environment cannot access other networks. To access the Internet, for instance, you need a piece of equipment that connects the systems to the local network - for instance, a cable/DSL modem, a host with a shared Internet connection, or a router or access point connected to another Ethernet network.
Ad hoc networks are suited for use in situations where infrastructure is either not available, not trusted, or should not be relied on in times of emergency. A few examples include: military solders and equipments in the battlefield, sensor networks for various research purposes, emergency rescue after an earthquake or flood, and temporary offices such as campaign headquarters.
2. Routing Protocols
As which is well known, routing is the most fundamental research issue in ad hoc networking. Routing protocols for ad hoc networks must deal with limitations such as high power consumption, low bandwidth, high error rates and arbitrary movements of nodes et al. Generally, current routing protocols for ad hoc networks can be categorized as:
re-active (source-initiated on-demand driven).
The pro-active routing protocols are the same as current Internet routing protocols such as the RIP (Routing Information Protocol), distance-vector, OSPF (Open Shortest Path First) and link-state. They attempt to maintain consistent, up-to-date routing information of the whole network. Each node has to maintain one or more tables to store routing information, and response to changes in network topology by broadcasting and propagating. Some of the existing pro-active ad hoc routing protocols are:
DSDV (Destination Sequenced Distance-Vector, 1994),
WRP (Wireless Routing Protocol, 1996),
CGSR (Cluster-head Gateway Switch Routing, 1997),
GSR (Global State Routing, 1998),
FSR (Fisheye State Routing, 1999),
HSR (Hierarchical State Routing, 1999),
ZHLS (Zone based Hierarchical Link State, 1999),
STAR (Source Tree Adaptive Routing, 2000).
Re-active routing protocols try to eliminate the conventional routing tables and consequently reduce the need for updating these tables to track changes in the network topology. In contrast to pro-active routing protocols which maintain all up-to-date at every node, routes are created only when desired by the source node in re-active protocols. When a source requires to a destination, it has to establish a route by route discovery procedure, maintain it by some form of route maintenance procedure until either the route is no longer desired or it becomes inaccessible, and finally tear down it by route deletion procedure. Some of the existing re-active routing protocols are:
DSR (Dynamic Source Routing, 1996),
ABR (Associativity Based Routing, 1996),
TORA(Temporally-Ordered Routing Algorithm, 1997),
SSR (Signal Stability Routing, 1997),
PAR(Power-Aware Routing, 1998),
LAR (Location Aided Routing, 1998),
CBR (Cluster Based Routing, 1999),
AODV (Ad hoc On-Demand Distance Vector Routing, 1999).
In pro-active routing protocols, routes are always available (regardless of need), with the consumption of signalling traffic and power. On the other hand, being more efficient at signalling and power consumption, re-active protocols suffer longer delay while route discovery. Both categories of routing protocols have been improving to be more scalable, secure, and to support higher QoS. Meanwhile, some protocols that combine the good properties of both pro-active and re-active protocols were proposed, such as ZRP (Zone Routing Protocol, 1999). Some of the above routing protocols have implementations for test.
3. Quality of Service
Quality of Service (QoS) is being developed to meet the emerging requirements of heterogeneous applications in the Internet which is able to provide only best-effort service. QoS is a guarantee by the network to provide certain performance for a flow in terms of the quantities of bandwidth, delay, jitter, jitter, packet loss probability etc. QoS of fixed wireless networks is still an open problem. Moreover, ad hoc networks make the QoS appear an even more challenging problem than ever before, despite some of re-active routing protocols can be configured to return only paths that comply with certain desired QoS parameters. Bandwidth is seriously limited. RF channel characteristics often change unpredictably, along with the difficulty of sharing the channel medium with many neighbours, each with its own potentially changing QoS requirements. Routes are using links with different quality and stability, which are often asymmetrical.
There are numerous multi-layer attempts to improve the QoS problems from the service contracts to the MAC layer. A promising method for satisfying QoS requirements is a more unified approach of cross-layer or vertical-layer integration. The idea is different from many of the traditional layering styles to allow different parts of the stack to adapt to the environment in a way that takes into account the adaptation and available information at other layers. QoS routing policies, algorithms and protocols with multiple, including pre-emptive, priorities are to be researched in the future.
Due to the nature of ad hoc networks, QoS cannot be guaranteed for a long time because of the link quality variation. Methods to detect and report changes in the connection quality should be investigated in the future. For example, Perkins suggested an addition of a new ICMP message (QOS_LOST) to be defined to inform the endpoint that a new route discovery should be initiated.
Security is a critical issue of ad hoc networks that is still a largely unexplored area. Since nodes use the open, shared radio medium in a potentially insecure environment, they are particularly prone to malicious attacks, such as denial of service (DoS). Lack of any centralized network management or certification authority makes the dynamically changing wireless structure very vulnerable to infiltration, eavesdropping, interference etc. Security is often considered to be the major "roadblock" in commercial application of ad hoc network technology.
Traditional methods of protecting the data with cryptographic methods face a challenging task of key distribution and refresh. Accordingly, the research efforts on security have mostly concentrated on secure data forwarding. However, many security risks are related to peculiar features of ad hoc networks. The most serious problem is probably the risk of a node being captured and compromised. This node would then have access to structural information on the network, relayed data, but it can also send false routing information which would paralyze the entire network very quickly.
One of the current approaches to the security problems is building a self-organized public-key infrastructure for ad hoc network cryptography. Key exchange, however, raise again the scalability issue. Another common approach is secure routing, which has an appealing idea of dividing the data on N pieces which are sent along separate routes and, at the destination, the original message is reconstructed out of any (M - out - of - N) pieces of the message. Nevertheless, security is indeed one of the most difficult problems to be solved. Having received only modest attention so far, its "golden age" of research can be expected after the functional problems on the underlying layers have agreed on. Actually, challenging issues on ad hoc networks are far beyond the above four, name some: node cooperation, interoperation with the Internet, aggregation, multicast, as well as the theoretical limitation of ad hoc networks. Technologies such as smart antennas, software radios also bring new research problems along with impetus to ad hoc networks.
The future of ad hoc networks is really appealing, given the vision of "anytime, anywhere" communications. Before those imagined scenarios come true, huge amount of work is to be done in both research and implementation. At present, the general trend is toward mesh architecture and large scale. New applications call for both bandwidth and capacity, which implies the need for a higher frequency and better spatial spectral reuse. Propagation, spectral reuse, and energy issues support a shift away from a single long wireless link (as in cellular) to a mesh of short links (as in ad hoc networks). Research on "multi-hop mesh-based" architecture showed it a promising solution to the implementation of ad hoc networks. As the evolvement goes on, especially the need of dense deployment such as battlefield and sensor networks, the nodes in ad hoc networks will be smaller, cheaper, more capable, and come in all forms. Large scale ad hoc networks are another hot issue in the near future which can be already foreseen.
Ad hoc networks have indeed the potential to change how we see the communication and networking world today, from the indoor ad hoc networks that can connect smart appliances to the Internet, to the ultimate "anytime, anywhere" communications. In all, although the widespread deployment of ad hoc networks is still year away, the research in this field will continue being very active and imaginative.