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TABLE 7.1. Comparing Different Link Estimation Metrics Property Metric ETX RTT Packet pair EDR RNP ELR Contention No Yes Yes Yes No Yes Packet Loss Yes No No Yes Yes No Varying Data Rates No No No No No Yes Varying Packet Sizes No No No No No Yes
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ELR accounts for the temporal variations of the link quality by sampling different neighbors adaptively. The drawback of ELR is the inability to calculate the the packet loss information since unicasts are associated with packet retries that cannot be obtained from the MAC hardware in realistic deployment. Table 7.1 summarizes the properties of the different link quality estimation metrics. It is to be noted that some metrics may re ect some components affecting link quality, but in a very coarse form. For example, RTT may capture the packet drops because the packet drops will lead to a higher RTT. ELR cannot capture the required number of retransmissions (even though theoretically it is possible) because it uses unicast packets and the packet loss due to retransmission cannot be captured. We do not explicitly consider such secondary effects in the table. 7.4.6 Route-Quality Estimation Once the individual link qualities are estimated, the next problem is how to combine them to reach a measure of an overall route quality. The quality of the route depends upon the quality of the individual links present in the route. The multihop route can be imagined as pipe, with each link representing a part of the pipe with constant diameter which represents the link quality. The overall throughput of the route is the amount of data that can be pushed in the pipe. It can be seen that the throughput is affected by the bottleneck link which has the least diameter. Multiple routes may exist between a given source and destination. The routing protocol is responsible for the choice of the route which can support maximum throughput for the connection. Yang, Wang, and Kravets [43] suggest that isotonicity is an important property for combining the link quality into a single route quality. Isotonocity refers to the property that the cost of a route strictly increases when it is pre xed with additional hops. For example, consider two routes between node A and node B, say R1 and R2 as shown in Figure 7.2. Let the combined link metrics of R1 and R2 be represented as r1 and r2 . Let there be another path that is pre xed to route A . . . B, say from node X to A denoted by X . . . A . . . B. Consider the two routes between X and B in X . . . A . . . B, one passing through R1 and the other through R2 . Let us denote them by X . . . R1 and X . . . R2 , respectively, and denote their metrics by xr1 and xr2 , respectively. The isotonicity property says that if
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Figure 7.2. Isotonicity.
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we have the metric r1 r2 , then we obtain xr1 xr2 . In essence, this property says that each part of the route should have an equal value when included in a larger route. 7.4.7 Limitations of Link-Quality-Aware Routing In this section, we summarize the disadvantages of the link-quality estimation techniques. The majority of the link-quality estimation mechanisms require probe packets (broadcast or unicast) to sample the link quality. The accuracy of the estimates obtained via probes is limited because the actual data packets can be transmitted in different data rates and have different packet sizes; both these factors affect the perceived quality of the channel. The use of broadcast control packets fails to capture the link quality at different data rates. Capturing the link quality for all the data packet sizes makes the measurement technique infeasible. The second disadvantage of such measurement techniques is the inability to predict the future load of the networks. Let us assume that the link quality re ects the data packet transfer and that the variance of link quality is captured at a ne level. Based on the metrics, a route with a set of links is selected and the data packets ow through this route. The transmission of these data packets will change the interference patterns and packet loss rates at other links. Thus, the past estimation of the link qualities are invalid and the measurements have to be reinitiated at the nodes that are affected. Zhang et al. [42] discuss the disadvantages of measurement of packet losses using a broadcast medium (other than the known issues of variable data rate and packet sizes). The number of packets lost can be measured in a broadcast environment. However, the MAC layer uses retransmissions if the packet transmission is unsuccessful in unicast packets. The discrepancy arises due to the inability of the routing layer to know the number of retransmissions that have occurred at the MAC layer. The MAC protocol, which is implemented in the hardware, does not expose such information to the above layers. Hence, measurements such as ETX, which use broadcast packets for measuring packet loss, cannot be easily transformed to measure the packet loss rate of unicast traf c. In summary, the link-quality-aware routing protocols discussed above are based on a methodology for estimating the quality of links. The links quality of the links forming a route are then combined to produce an estimate of the quality of the route.
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