.FLYINGHEAD WIRELESS INFRASTRUCTURE
.TITLE Implementation of distributed hash tables in mobile ad-hoc networks
.AUTHOR Humayun Bakht
.SUMMARY In this article, Contributing Editor Humayun Bakht explores the implementation of distributed hash tables in mobile ad-hoc networks. This article could be of great interest to you if you have have some technical knowledge of the Internet and its associated technologies.
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Mobile ad-hoc networks are one of the latest developments, and an attractive addition, to the family of wireless networks. With the addition of mobile ad-hoc networks, we can broadly classify wireless networks into two types: wireless fixed and wireless ad-hoc networks. Wireless fixed networks are a collection of wireless nodes that communicate with each other with the aid of some fixed infrastructure. A wireless ad-hoc network operates in the absence of a centralized structure.
Mobile ad-hoc networks introduced a completely new art of networking and can be deployed in places where we lack supporting structures. Each and every bit of a mobile ad-hoc network is unique. While these networks are beneficial in many ways, they do require unique strategies to be developed for routine network controls. Connecting these networks with a larger network, such as the Internet, is a challenging issue.
.H1 Distributed hash tables
DHTs (Distributed Hash Tables) can be seen as a platform for building a variety of scalable and dynamic distributed applications for the Internet. Scalable applications include distributed storage systems and application level multi-cast. DHTs can be used to develop a common infrastructure for certain distributed applications. DHTs are similar as they have a file cabinet that spreads over numerous servers. Therefore, if one server goes down, it does not disturb the rest of the network communication. As in peer-to-peer or ad-hoc networks, there is no central server that contains a list of the stored data. Instead, each mobile node has a list, or a routing table, where data is stored in the system.
The current DHT structure poses many challenges, such as fault-tolerance, locating objects, scalability, availability, load balancing, and incremental deployment. It is possible to implement current DHT specifications in mobile ad-hoc networks. If their deployment in mobile ad-hoc networks is successful, it could provide a more efficient way to establish distributed applications and services for ad-hoc networking.
In mobile ad-hoc networks, applications such as file-sharing and resource discovery can benefit from the distributed insert/lookup convergence provided by DHTs. Research in this area suggests one of the two possible approaches — layered and integrated — that can be used for the deployment of DHTs in mobile ad-hoc networks provided using a proximity-aware DHT Pastry and DSR (Dynamic Source Routing) protocol.
In a layered approach (hence: "pastry"), a proximity-aware DHT Pastry is directly layered on the top of mobile ad-hoc networks very similarly to the way it’s normally layered in the Internet. Pastry maintains its leaf set and routing table entries without source routes while DSR maintains source routes passively with respect to the demand of Pastry routing state. In mobile ad-hoc environment, it’s difficult to accomplish straightforward layering. It requires some modifications to accomplish the shared medium access nature of ad-hoc networks.
In a layered approach, Pastry’s node joining process has been reformed to allow the use of expanding ring search for locating a node wants to rejoin the network. Another important factor is the network delay, since it’s affected by many factors and has a high variability in ad-hoc networks. A modified Pastry is needed to use a hop count metric for proximity.
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Finally, to reduce the cost of proximity probing, certain modifications need to be made to the basic structure of DSR. DSR can then use its cache to reply to pings from Pastry if there’s a cached path to the node that’s pinged. In the absence of such a cached path, DSR can initiate the route request process.
In the integrated approach, a DHT is supported by integrating Pastry and DSR at the network layer of mobile ad-hoc networks via one-to-one mapping between the IP addresses of the mobile nodes and their respective node IDs in the namespace. Through this type of integration, a routing structure could be developed between a DHT and an ad-hoc routing protocol. Among the above two discussed approaches, the layered design adopted the same procedure as it’s in use for the Internet. This scheme explores the existing routing infrastructure for mobile ad-hoc networks to the full extent.
DHTs were designed to support larger networks, but they could also benefit mobile ad-hoc networks. The above two discussed schemes could possibly be extended further to support implementation of DHTs at a better scale in mobile ad-hoc networks.
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