Two main advantages, simple yet very valuable, lay the basis for the introduction and the widespread diffusion of wireless local area networks (WLANs) in application areas such as office automation and home networking. These are their ease of installation because of the absence of wires and their capability of supporting communications among movable terminals.
In most cases WLAN systems are based on single-hop operation ; that is to say, a pair of terminals, whenever out of the reciprocal range of radio coverage, can connect to each other only through the use of an infrastructure providing access point devices and centralized control and management facilities.
Significant studies have been made recently of multi-hopping operations , generalizing the concept of peer-to-peer interconnection between terminals, out of immediate visibility.
For example, let us consider two terminals, A and B, not directly capable of interconnection, accessing another terminal, C, to exchange information: this “third node†is able to reach both nodes A and B and relay all messages not addressed to it.
It is then easy to extend this relaying concept to all elements in the network, more appropriately defined as network nodes able to support any communication between the source and the destination through an arbitrary number of wireless intermediate steps, forming multi-hop paths.
In case a fixed infrastructure and a centralized management are not in place, these networks, commonly called ad hoc networks (AHNs), should qualify as self-configuring and self-organizing, and should exploit their enhanced communication capabilities by letting nodes concur to implement the necessary networking functions for automatic operation and minimizing or even completely avoiding any manual setup.
Such characteristics may lead both to very high levels of scalability, because network management relies on the ability of each node to use local resources only, and to strong reliability, which is dependent on the nodes’ capacity to react to any anomalous event or failure by giving rise to automatic reconfiguration procedures.
Both the above properties, combined with the introduction of node redundancy in AHNs, result in very strong robustness of the overall system.
At first glance, it could appear sufficient to reuse all already established WLAN technologies to implement AHNs just by segmenting paths in more single-hop connections, each one managed as a WLAN operating in ad hoc (AH) mode. On the other hand, this approach, fully based on local management of the transmission resources, disregards interaction between contiguous WLANs.
Application Scenarios
AHNs originated in the early 1970s from the Packet Radio Network (PRNET) Project sponsored by the U.S. Department of Defense (DoD). In the early 1980s, concepts that evolved at PRNET were adopted by the Survivable Adaptive Radio Networks (SURAN) Project.
Both initiatives had as their goal laying the foundations for a packetswitched network (similar to the Internet), fully wireless and suitable for military applications such as communications among soldiers and fighting vehicles in hostile battlefield environments without the availability of any networking infrastructure.
Only recently, following the massive diffusion of mobile user terminals (cellular phones, pagers, PDAs, etc.), the research community has started to look at civilian applications for AHNs, especially where such solutions could well complement the existing commercial systems.
Important examples may be found in the projects MANET, WINS, and TERMINODES. All these activities testify to the growing interest of academic and industrial researchers in AHN. Among the many areas benefiting by AHN implementations, the field of environmental control and monitoring is worth mentioning.
In such a case the network nodes are based on specialized sensors that are able to react to particular events, to make local computations, and to exchange data with other instrumentation or control machines (machine-to-machine interfacing).
Ambient parameters can be monitored and measured, and results can then be supplied to the users (human-to-machine interfacing). Previous descriptions correspond to what is widely known as the Wireless Sensor Network (WSN).
The main factors suggesting the adoption of AH networking in such systems are their capability to establish infrastructureless wireless communications in difficult or even inaccessible locations, and their effectiveness to increase the robustness of the overall system in all cases of critical events by always having some running nodes able to perform networking functions instead of partially, or even fully, damaged or exhausted network members.
In a completely different scenario, a challenging possible application of the AH mode is its access to the Internet anytime, anywhere , in the sense of allowing users to connect through their own terminals to the worldwide network in total autonomy, without locational constraints.
In the AH mode of operation, the last-mile connection might be implemented by a multi hop path to the nearest available access point or IP gateway. These are widely known as mesh-based mobile networks. Valuable civilian applications may be foreseen in case of emergencies, disastrous events, rescue operations, and communications in Third World countries.
Additionally, a significant example of specialized scenarios is the car network, devoted to traffic control â€" possibly in combination with a global positioning system (GPS) â€" or used for advanced intervehicle communications (i.e., multimedia communication, multiplayer gaming, etc.).
Also, in this case, AH networking fits well the requirements of a collaborative system in which all (or many) elements have the twofold role of user and supplier of a number of services.
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