Architecture and Evaluation of an Unplanned 802.11b Mesh Network

The paper tries to combine the best characteristics of multi-hop and single-hop network types; unplanned mesh network with wide coverage and acceptable performance. It evaluates such architecture using Roofnet which is a multi-hop 802.11b Internet access network with 37 nodes spread over four square Km of densely populated area. The main characteristics of Roofnet are

• Each Roofnet node involves PC with Internet access via Ethernet port, 802.11b card, omni-directional antenna and software; Linux, routing software, DHCP and Web server
• Each node has a unique Roofnet layer address (assigned automatically via Roofnet software) and IP address (assigned via DHCP server) that will be used inside Roofnet only
• If a Roofnet node success to connect to an ISP, it will advertise itself as Internet gateway and provide wireless sharing of Internet connection for other nodes using NAT. Otherwise it is default router for the host and it can connect to the Internet via the gateway with the best ETT metric (lowest ETT or highest throughput).
• ETT uses periodic broadcast of 1500-byte in a forward direction at each available 802.11 bit rate and 60-byte at 1Mbps in reverse direction
• Roofnet routing protocol Srcr uses Dijkstra's algorithm on the partial database of many link's metrics to find highest throughput route whenever the node learns of changed link metric
• The nodes learn the link's metric either from dummy queries from gateway or from the forwarded data packets which includes the link's current metric in the packet's source route.
• Roofnet uses SampleRate algorithm to select bit rate that provide highest throughput depending on the delivery probability, as it send data packet

Evaluation of Roofnet found that the average throughput over all links is 627 kbps and it decreases with number of hops and average RT delay of 39 msec. Roofnet users talk to Internet gateway with an average throughput of 1395 kbps and average RT delay of 22 msec. Also, it has been found that Srcr effectively selects short links of a few hundred meters and ignores long links