Tuesday, May 1, 2007

Positioning of wireless devices in a mobile ad-hoc network

WIRELESS INFRASTRUCTURE

By Humayun Bakht

As I discussed in my previous articles, we already know that there is no fixed infrastructure in a mobile ad-hoc environment. In order to support routing or exchange packets of information among various hosts in ad-hoc networks, each node has to rely on others for packet forwarding.

It then becomes necessary for each participating node to act as a "middle-man" intermediate node between any two participating devices of an ad-hoc network. Figure A shows how the main responsibility of these intermediate nodes is to receive and forward data packets from the source to the destination node or vice versa. We then come across several other related issues, such as what may happen if an intermediate node becomes selfish and refuses to forward data packets.

FIGURE A

This illustration shows how data moves between nodes.

It's true that the implementation of different routing mechanisms depends on how you position the wireless devices in an ad-hoc environment. You simply can't have any well-established routing strategy for a mobile ad-hoc network if you're not able to track a particular wireless device's movement within the network.

Right now, there are a number of different methods in use to determine the position of mobile nodes, including CI (Cell Identification), TA (Time Advance information), AOA (Angle of Arrival), and AGPS (Assisted Global Positioning System). We'll take a look at two of the more common methods.

One method used to track node positions in an ad-hoc environment is to use a GPS (Global Positioning System). There are several unresolved issues about the overall adaptability of this technique, especially when a fixed infrastructure doesn't exist and when the GPS is too weak to receive wireless signals. On top of that, there are extra costs incurred when you use this technique to monitor node movement in a mobile ad-hoc network.

The TOA (Time of Arrival) method is another strategy which is used to accomplish the same task. TOA is typically used to obtain the distance between two mobile nodes. As I noted earlier in this article, a node may be required to forward packets on behalf of another node. This consumes energy and thus reduces battery life without direct advantages. However, these strategies can help in minimize interruptions and therefore increase the quality of service offered by the network.

Positioning wireless devices in an ad-hoc environment is a fundamental and crucial issue for mobile ad-hoc network operations and management. The most common methods used to track node movement could use improvement. There's a solid need to develop new and efficient techniques which are not only simple to implement but also highly adaptable to support various routing techniques for mobile ad-hoc networks.