Friday, June 1, 2007

Network wide broadcasting, handling data loss in mobile ad-hoc networks


By Humayun Bakht

Mobile ad-hoc networks offer a unique art of network formation where mobile devices can communicate with each other without a pre-existing infrastructure. Ad-hoc networks have been considered to be the foundation for new technologies. The first and second generation mainly focused on military applications while the third generation, from 1992 onwards, is geared towards commercial applications such as Bluetooth and ad-hoc sensors networks.

One important issue is to try to reduce the packet or the data loss during an active transmission. In mobile ad-hoc networks, mobile link transmission errors, mobility, and network congestion are some of the major causes of data loss. Data loss due to transmission errors is mainly affected by the physical condition of the channel and the region where networks are deployed. These losses can't be reduced with the improvement in ad-hoc routing protocols.

Each mobile device in an ad-hoc network has to rely on others for forwarding data packets to other nodes in the network. Routing protocols of a mobile ad-hoc network is another way to transmit data from one device to another. NWB (Network-wide Broadcast) is considered to be one of the routing or data exchange related operations and it's used to discover routes for both unicast (one-to-one) and multi-cast (one-to-many) data exchange operations.

NWB can also be defined as a process through which one mobile device sends a packet to all other devices in the network. NWB in mobile ad-hoc networks provides important control and route establishment functionality to different protocols of mobile ad-hoc networks. It's especially important for paging, alarming, location updates, route discoveries or even routing in highly mobile ad-hoc environments. Network-wide broadcasting is normally achieved via flooding.

In a flooding or broadcasting task, a source mobile device floods or broadcasts the same message to all the devices in the network. Therefore, it becomes crucial for routing protocols to have some sort of scalable flooding mechanism that can help each device decide between an active or passive state, so that network can remain connected and its lifetime could be maximized. Some of the desirable properties of a scalable flooding scheme are reliability and power and bandwidth efficiency, which can be measured by savings in rebroadcasts.

Until recently, NWB was limited to blind flooding schemes, where each mobile device receiving the message for the first time will re-transmit it. This solution is considered to be less efficient due to collision, contention, and several redundancy problems. Some of the existing methods for flooding a wireless network intelligently are using omni-directional or directional antennas, with equal or adjusted transmission radius and for scheduling node activities and selective rebroadcast approach. These can play an important role in enhancing NWB.