| Designing routing protocols for Wireless Mesh Networks is an active area for recent years. And routing metrics are critical for the routing protocols. The researchers used to focus on finding paths with minimum hop count. However, such routes may include slow or lossy links and this would lead to low throughput. A routing protocol with link quality aware metric will give a better performance.The characteristics of WMN sare similar to the ad-hoc networks, so the routing protocols developed for the ad-hoc networks can be used in WMNs. These routing protocols can be classified into different categories according to different criteria.When classified by the manner in which they react to the network topology changes, routing protocol can be divided into proactive (table-driven) protocols and reactive (on-demand) protocols. If we classified by the organization of the network, routing protocols can be grouped into flat protocols and hierarchical protocols. The proactive protocols propagate topology information periodically and find routes continuously, while the reactive protocols find route on demand. But the availability of bandwidth in 802.11 networks is limited, so theon-demand methods can help to conserve it. Regarding to the characteristics of the wireless mesh networks, on-demand routing protocols outperform proactive protocols in terms of packet delivery ratio, routing overhead, and energy efficiency. The reactive method works well in a wireless environment in case of mobile nodes and continuously changing topology. Therefore, most of the researches are focused on the on-demand routing protocols such like DSR and AODV.When the nodes have several routes to transmit the data packet, how to choose the least cost path comes to be a critical problem. In order to compute the least cost path, we must use one of the performance metrics. There are many performance metrics that can be used to represent the link cost now, and new ones continue to be created to improve the path selection rules. The most popular used metrics to calculate the link cost are:Expected Transmission Count (ETX), it represent the loss rate of broadcast packets between the two nodes in transmission process. The Hop Count, it is the number of the link hops between the transmission nodes.But the Hop Count metric may introduce the long distance wireless connection which is harmful for the WMNs. The ETX metric can detect the link quality of the route, so it is very useful for the mesh networks with limited mobility but high data flow. As the Hop Count metric, the ETX metric also prifered the fewer hops, it may result in the long distance wireless connection which may be the bottleneck of the network.. In this condition, I invent a new routing metric A-ETX, this metric considered both the link quality and the number of the hops.In the last part of this paper we do the simulation in NS-2. Firstly, wecompare the throughput of this 3 routing metrics with different data flows. The result shows that as the data flows increased, the ETX metric and the A-ETX metric have better performance than the Hop Count metric. And as the data flows increased on, the A-ETX metric have better performance than ETX metric. Secongdly, we compare the throughput with different mobility. The result shows that the Hop Count metric has the best perfprmance and the performances of the ETX metric and the A-ETX metric are almost the same. |
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