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Changes between Version 1 and Version 2 of Proto/cModules/c0DesignSandbox/a0LinkQualityEstimate


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Timestamp:
Nov 30, 2014, 5:35:34 AM (9 years ago)
Author:
wontoniii
Comment:

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  • Proto/cModules/c0DesignSandbox/a0LinkQualityEstimate

    v1 v2  
    1414
    1515* Link quality is being estimated as 'average_rtt/2' for both wired and wireless links. This is causing two issues:
    16 *# For very low latency wired links, with delays less than 1 ms, the rtts show significant deviation (> 10%) due to scheduling variability in the software stack. This causes unnecessary stores, even when there's no congestion or losses.
    17 *# For wireless links, the rtt metric does not consider losses.
     16  * For very low latency wired links, with delays less than 1 ms, the rtts show significant deviation (> 10%) due to scheduling variability in the software stack. This causes unnecessary stores, even when there's no congestion or losses.
     17  * For wireless links, the rtt metric does not consider losses.
    1818* ETT is considered a reasonable metric for link quality in wireless mesh networks, and is computed as ((S/B) * ETX) ; where S is packet size, B is link data rate, and ETX is Expected Transmission Count.
    19 ** ETX is calculated as 1/(df*dr), where df and dr are delivery ratios in the forward and reverse directions.
    20 ** The delivery ratios can be estimated using the periodic probe messages, with the neighbor reporting it's observations in the probe ACK.
    21 ** Data packet transfers can also be used to asses the link probabilities. However, this may also include losses due to congestion. While this may be desirable, the approach may interfere with separate mechanisms to deal with congestion explicitly.
    22 ** bandwidth can be estimated using the packet-pair (or packet train) approach which involves sending 2 (or more) unicast packets in quick succession and measuring reception rate at the peer.
     19  * ETX is calculated as 1/(df*dr), where df and dr are delivery ratios in the forward and reverse directions.
     20  * The delivery ratios can be estimated using the periodic probe messages, with the neighbor reporting it's observations in the probe ACK.
     21  * Data packet transfers can also be used to asses the link probabilities. However, this may also include losses due to congestion. While this may be desirable, the approach may interfere with separate mechanisms to deal with congestion explicitly.
     22  * bandwidth can be estimated using the packet-pair (or packet train) approach which involves sending 2 (or more) unicast packets in quick succession and measuring reception rate at the peer.
    2323* For wireless links, the transmission time dominates the packet delivery latency for common radio-radio distances. But for wireline links, the propagation delay can easily dominate. Losses in wireline are also very rare under normal (not congested) conditions. Therefore, if we plan on combining link quality metrics across wired and wireless for path quality, ETT may not be the right metric. Then, what else? Rtt/2? But we start to include processing delays here, which may dominate both propagation and transmission delays.