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mr-aodv - Improving Reactive Routing on Wireless Multi-rate...

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Improving Reactive Routing on Wireless Multi-rate Ad-hoc Networks Rafael Paoliello Guimar˜aes and Llorenc ¸ Cerd`a Polytechnic University of Catalonia Computer Architecture Department Jordi Girona 1-3, E-08034 Barcelona, Spain e-mail: { rafael.guimaraes, llorenc } @ac.upc.edu Abstract — Traditional ad-hoc routing protocols typically choose minimum hop paths for transmissions. In a multi-rate environment, where nodes may elect between several transmission rates in order to accommodate different channel conditions, these protocols usually choose paths that contain long range links, with low effective throughput and low reliability. In this paper, we propose a mechanism for reactive routing protocols that leads to the election of high throughput routes while not increasing significantly the signaling overhead. By allowing a node to have a complete knowledge of its one-hop neighborhood topology, we are able to increase significantly the performance of the network through the improvement of the route election process on multi- rate environments. I. I NTRODUCTION Wireless communications have been spread all over the world over the last years. Most of the commercially available wireless devices are based on the IEEE 802.11 standards family. Most of them, such as 802.11b [2], 802.11a [1] and 802.11g [3], allow the use of different transmission rates. The election of which transmission rate should be used depends on the wireless medium conditions. The worse the channel quality, the stronger the code that should be used and, consequently, the lower the achieved transmission rate. Since channel quality is directly related to distance between nodes, we may say that usually, the closer two nodes are from each other, the higher the transmission rate used between them. In 802.11a (and also in 802.11g), for example, the set of possible data transmission rates are 6, 9, 12, 18, 24, 36, 48, 54 Mbps while 802.11b supports 1, 2, 5.5 and 11 Mbps. In order to exploit this capability, some Medium Access Control (MAC) mechanisms are required, the Auto Rate Fallback (ARF) [8] protocol was the first to deal with this issue. The sender increases (or decreases) the transmission rate to be used in future transmissions based on the successes (or failures) in the previous ones. In other mechanisms, like the Receiver Based Auto Rate (RBAR) [6] protocol, the receiver measures the quality of the channel when it receives a Request To Send (RTS) message and selects the appropriate rate to be used under these conditions. It then informs the sender the rate to be used for data transmission through the Clear To Send (CTS) message. This work was supported by CAPES - Brazil, the Ministry of Education of Spain under grant CICYT TEC2004-06437-C05-05 and by the European NoE EuroNGI. However, in order to fully use the multi-rate capabilities on a wireless ad-hoc network, the routing protocol should also be aware of this information. There is no point on being able to transmit at so many different rates on the MAC layer, if at the end, the routing protocol always chooses routes based only on hop count. Traditional routing protocols, like the Ad-hoc On-
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