1. OPPORTUNISTIC NETWORK
Opportunistic communication has emerged as a new communication model to cope with nowadays network problems. Opportunistic network exploits the variation of channel conditions. It provides a degree of freedom in the time domain and thus network performance is increased. Opportunistic network is a network where the network contacts are discontinuous or where link performance is vastly or extreme.
1.1 Opportunistic Concepts
In opportunistic networks such as MANETs, the com??munication is multihop, with intermediate nodes acting as routers that forward the messages addressed to other nodes. In this case, however, forwarding is not used because intermediate nodes such as mobile relays store the messages. This is done when there is no forwarding opportunity. In the transmission range, there are no other useful nodes for any contact opportunity with other mobile devices to forward information. That is why, the forwarding model is referred as ‘store, carry, and forward.’ Opportunistic networks is often compared to the delay-tolerant networks. The DTN archi??tecture and protocols are currently under study in the Internet Research Task Force’s Delay Tolerant Networking Research Group which is concerned with the interconnection of heterogeneous networks. The DTN approach is based on building an overlay atop dis??connected networks to combat network disconnections through persistent storage. The overlay provides func??tionalities similar to the Internet layer even if nonstop connectivity may never be available. Opportunistic networking is a more general concept and does not assume any compatibility with the Internet architecture, nor any a priori knowledge regarding the network topology, areas of disconnections, or future link availability. In opportunistic networks, route computations differ from those in traditional Internet- or MANET-routing algorithms. Specifically, forwarding and routing protocols are merged because routes are actually built while mes??sages are forwarded. Indeed, routes must be computed on the fly and hop by hop as each message progresses toward its destination. Nodes carrying messages to be forwarded opportunistically evaluate if any other node they contact could provide a good next hop toward the destination, then hand over the message if so.
1.2 ADVANTAGES OF OPPORTUNISTIC COMMUNICATION
1. Potentially high capacity.
2. Low cost.
3. Localized communications.
4. Fully decentralized operation.
5. Independence of any infrastructure.
6. Varying capabilities of the available networking technologies.
1.3 Trust, security, and privacy
Establishing trust and security for an interaction be??tween a priori unknown peers in an opportunistic network is challenging. However, social network structures offer a basis to enhance trust and security provision by capital??izing on ‘communities’ of devices that have commonality between them, either physically or logically. The idea of using social network structures and proper??ties for enhancing network security is not novel. There are several proposals based on using social networks to fight e-mail spam and defend against attacks. However, the use of social networks in completely decentralized networks is a completely new and challeng??ing task because, in such an environment, legacy security solutions based on centralized server or online trusted authorities becomes infeasible. In this case, a natural di??rection to pursue exploits electronic social networks and the trust and security relationships naturally embedded in human interactions.
1.4 CHALLENGES FOR OPPORTUNISTIC NETWORKS
1.4.1 Intermittent connectivity
In opportunistic networks the contact between pairs of devices provides the critical resource for collaboration. The connectivity problem is overstated by the lack of knowledge about the location, time, and communication bandwidth of such contacts. Hybrid routing prototcols that employ context, a profile, or a history of mobile users and devices should be investigated for adoption into opportunistic networking environment. It will be necessary to develop middleware mechanisms that mask delays and hide the complexity of the opportunistic paths from applications. The information acquired must be evaluated for caching, purging, ad dissemination because resources such as memory and bandwith are limited.
1.4.2 Delay tolerance
Successful implementations of DTN applications have demonstrated the usefulness of opportunistic networks. Delay tolerance is the key to distributed opportunistic computing. First, protocols for the creating of delay-tolerant opportunistic communication paths should be developed. Second, delay-tolerant information acquisition and dissemination requires new cache consistency mechanisms to mask delays and the underlying network. Third, execution of remote services in opportunistic environments requires novel mechanisms for service discovery, service execution, and management.
Potentially, many kinds of devices will likely come in contact opporunistically- cell phones, handheld and notebook computers, sensors, cameras, and RFID-enabled objects. These devices can be supported by diverse communication capabilities and the radio frequencies at which the communicate. Contact interoperability amoung these pairs of hererogeneous devices is a major challenge.
1.5 APPLICATIONS OF OPPORTUNISTIC COMMUNICATION
1.5.1 Infomobility services and intelligent transportation systems
Vehicular ad hoc networks exploit vehicle-to vehicle communications, as well as the communication with roadside infrastructure, to implement cooperative systems and to increase traffic efficiency and safety. Other applications include tourist information and assistance such as parking availability notification and maps, and entertainment such as gaming and streaming video. Opportunistic communication is to also allow access to the Internet from moving vehicles such as for smart phones, the most popular mobile devices. Such vehicles could have access to the internet from roadside wireless access point. This communication is opportunistic because vehicles lose connectivity as they go past the access point.
1.5.2 Internet Connectivity to Rural Areas
The use of opportunistic communication is to bring Internet connectivity to rural areas. In developing countries and rural areas deploying the infrastructure required to enable conventional
Internet connectivity is typically not cost-effective. Since, there is fixed routing in conventional Internet connections. Opportunistic networks represent an easy to deploy and a very cheap solution for Internet connectivity. Typically, rural villages have few collection points that store messages for a temporary basis. They are then addressed to the Internet for wireless mobile social networking communication
1.5.3 Crisis management
Legacy communications networks are not designed to withstand unplanned and unexpected disruptive events and are unsuitable to reliably support communication services for first responders. Opportunistic networking techniques can be adopted for interconnecting surviving parts of the telecommunication infrastructures, and services can be deployed for specific applications.
‘ An introduction to wireless mobile social networking in opportunistic communication http://airccse.org/journal/ijdps/papers/4313ijdps06.pdf
‘ Paolo Santi, ‘Mobility Models for next generation wireless networs, adhoc, vehicular and mesh networks’
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