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Wednesday, May 11, 2011

Opportunistic networks: when information routing becomes Autonomic

In opportunistic networks end-to-end communication between users does not require a continuous end-to-end path between source and destination. Network protocols are designed to be extremely resilient to events such as long partitions, node disconnections, etc, which are very features of this type of self-organizing ad hoc networks. This is achieved by temporarily storing messages at intermediate nodes, waiting for future opportunities to forward them towards the destination. The mobility of users plays a key role in opportunistic networks. Thus, providing accurate models of mobility patterns is one of the key research areas.

Opportunistic networks also aim at building networks out of mobile devices carried by people, possibly without relying on any pre-existing infrastructure. However, opportunistic networks look at mobility, disconnections, partitions, etc. as features of the networks rather than exceptions. Actually, mobility is exploited as a way to bridge disconnected “clouds” of nodes and enable communication, rather than a drawback to be dealt with. More specifically, in opportunistic networking no assumption is made on the existence of a complete path between two nodes wishing to communicate. Source and destination nodes might never be connected to the same network, at the same time. Nevertheless, opportunistic networking techniques allow such nodes to exchange messages. By exploiting the store-carry-and-forward paradigm, intermediate nodes (between source and destination) store messages when no forwarding opportunity towards the final destination exists, and exploit any future contact opportunity with other mobile devices to bring the messages closer and closer to the destination. This approach to build self-organizing infrastructure-less wireless networks turns out to be much more practical than the conventional MANET paradigm. It is clear that understanding the real mobility patterns of users is key in this networking environment, as mobility of users is one of the enabler of end-to-end communications.

Opportunistic networks share several concepts with Delay Tolerant Networks (DTNs). can be seen as a generalization of DTNs. Specifically, in opportunistic networks no a-priori knowledge is assumed about the possible points of disconnections, nor the existence of separate Internet-like sub-networks is assumed. Opportunistic networks are formed by individual nodes, that are possibly disconnected for long time intervals, and that opportunistically exploit any contact with other nodes to forward messages. The routing approach between conventional DTNs and opportunistic networks is therefore quite different. Since in DTNs the points of disconnections (and, sometime, the duration of disconnections) are known, routing can be performed along the same lines used for conventional Internet protocols, by simply considering the duration of the disconnections as an additional cost of the links. Since opportunistic networks do not assume the same knowledge about the network evolution, routes are computed dynamically while the messages are being forwarded towards the destination. Each intermediate node evaluates the suitability of encountered nodes to be a good next hop towards the destination. Consider the illustration below to understand the concept of Opportunistic networks.

A user at the desktop opportunistically transfers, via a Wi-Fi ad hoc link, a message for a friend to a user passing nearby, “hoping” that this user will carry the information closer to the destination. This user passes close to a train station, and forwards the message to a traveler going to the same city where the destination user works. At the train station of the destination city a car driver is going in the same neighborhood of the destination’s working place. The driver meets the destination user on his way, and the message is finally delivered.

Opportunistic networks actually exploit users’ mobility to bridge disconnections and partitions. Exploiting the social behavior of users to define the basic mechanisms of users’ movements is a very interesting direction, hence making use of autonomic concepts.

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