Path Discovery Mechanism/Metrics
Metrics
XO's metrics
For the XO's PDM the cost of a path will be the sum of the costs of best PREQs forwarded (and successfully decoded) by the nodes that form the path during the path discovery process. With 'best' PREQs we mean those transmitted in the highest data rates.
In a path formed by four nodes (A -> B -> C -> D) each of the participants have broadcast four PREQs with the costs and data rates listed in Table 1. If all nodes successfully receive the 54Mbps PREQ from its predessesor in the path, the cost of the path will be 39 (13 x 3). But it may be the case that some of the PREQs are lost, due to poor link quality or collision. Say, that C could not decode the 54 and the 36Mbps PREQs. In this case, the cost of the path will be 72 (13 + 46 + 13).
| datarate (Mbps) | link cost |
|---|---|
| 54 | 13 |
| 36 | 28 |
| 11 | 46 |
| 1 | 64 |
Given the costs in Table 1, it is easy to see that the second less expensive path is a two hops path formed by two 54Mpbs links (i.e. links that were able to transfer the 54Mbps PREQ). Even a four hops path will be preferred over a single hop path if the former consists of three good links whereas the latter is a marginal link that cannot work at rates superior to 1Mbps.
It makes sense to choose many fast links instead of a slow one. A 1500 bytes frame will take *** ms to be transmitted over a 1Mbps link, this is as long as ** times the time needed to transfer the same frame over a good link (one that supports 54Mbps).
For one side we may consider that deciding the quality of a link over the reception of a single frame may not be the most precise mechanism - a 54Mbps PREQ frame may not be received just because of a transient spectrum condition or even due to a congested medium. On the other hand, we have to consider that (a) the routes will expire after some seconds (current default is 10 seconds) and (b) sending more control traffic in order to make better inferences cost airtime.
Table 2 displays all the possible combinations of links and associated costs for a mesh not bigger than 5 hops.
| total cost | datarates for each link |
|---|---|
| 13 | 54 |
| 26 | 54,54 |
| 28 | 36 |
| 39 | 54,54,54 |
| 41 | 54,36 |
| 46 | 11 |
| 52 | 54,54,54,54 |
| 54 | 54,54,36 |
| 56 | 36,36 |
| 59 | 54,11 |
| 64 | 1 |
| 65 | 54,54,54,54,54 |
| 67 | 54,54,54,36 |
| 69 | 54,36,36 |
| 72 | 54,54,11 |
| 74 | 36,11 |
| 77 | 54,1 |
| 80 | 54,54,54,54,36 |
| 82 | 54,54,36,36 |
| 84 | 36,36,36 |
| 85 | 54,54,54,11 |
| 87 | 54,36,11 |
| 90 | 54,54,1 |
| 92 | 11,11 |
| 92 | 36,1 |
| 95 | 54,54,54,36,36 |
| 97 | 54,36,36,36 |
| 98 | 54,54,54,54,11 |
| 100 | 54,54,36,11 |
| 102 | 36,36,11 |
| 103 | 54,54,54,1 |
| 105 | 54,11,11 |
| 105 | 54,36,1 |
| 110 | 11,1 |
| 110 | 54,54,36,36,36 |
| 112 | 36,36,36,36 |
| 113 | 54,54,54,36,11 |
| 115 | 54,36,36,11 |
| 116 | 54,54,54,54,1 |
| 118 | 54,54,11,11 |
| 118 | 54,54,36,1 |
| 120 | 36,11,11 |
| 120 | 36,36,1 |
| 123 | 54,11,1 |
| 125 | 54,36,36,36,36 |
| 128 | 1,1 |
| 128 | 54,54,36,36,11 |
| 130 | 36,36,36,11 |
| 131 | 54,54,54,11,11 |
| 131 | 54,54,54,36,1 |
| 133 | 54,36,11,11 |
| 133 | 54,36,36,1 |
| 136 | 54,54,11,1 |
| 138 | 11,11,11 |
| 138 | 36,11,1 |
| 140 | 36,36,36,36,36 |
| 141 | 54,1,1 |
| 143 | 54,36,36,36,11 |
| 146 | 54,54,36,11,11 |
| 146 | 54,54,36,36,1 |
| 148 | 36,36,11,11 |
| 148 | 36,36,36,1 |
| 149 | 54,54,54,11,1 |
| 151 | 54,11,11,11 |
| 151 | 54,36,11,1 |
| 154 | 54,54,1,1 |
| 156 | 11,11,1 |
| 156 | 36,1,1 |
| 158 | 36,36,36,36,11 |
| 161 | 54,36,36,11,11 |
| 161 | 54,36,36,36,1 |
| 164 | 54,54,11,11,11 |
| 164 | 54,54,36,11,1 |
| 166 | 36,11,11,11 |
| 166 | 36,36,11,1 |
| 167 | 54,54,54,1,1 |
| 169 | 54,11,11,1 |
| 169 | 54,36,1,1 |
| 174 | 11,1,1 |
| 176 | 36,36,36,11,11 |
| 176 | 36,36,36,36,1 |
| 179 | 54,36,11,11,11 |
| 179 | 54,36,36,11,1 |
| 182 | 54,54,11,11,1 |
| 182 | 54,54,36,1,1 |
| 184 | 11,11,11,11 |
| 184 | 36,11,11,1 |
| 184 | 36,36,1,1 |
| 187 | 54,11,1,1 |
| 192 | 1,1,1 |
| 194 | 36,36,11,11,11 |
| 194 | 36,36,36,11,1 |
| 197 | 54,11,11,11,11 |
| 197 | 54,36,11,11,1 |
| 197 | 54,36,36,1,1 |
| 200 | 54,54,11,1,1 |
| 202 | 11,11,11,1 |
| 202 | 36,11,1,1 |
| 205 | 54,1,1,1 |
| 212 | 36,11,11,11,11 |
| 212 | 36,36,11,11,1 |
| 212 | 36,36,36,1,1 |
| 215 | 54,11,11,11,1 |
| 215 | 54,36,11,1,1 |
| 218 | 54,54,1,1,1 |
| 220 | 11,11,1,1 |
| 220 | 36,1,1,1 |
| 230 | 11,11,11,11,11 |
| 230 | 36,11,11,11,1 |
| 230 | 36,36,11,1,1 |
| 233 | 54,11,11,1,1 |
| 233 | 54,36,1,1,1 |
| 238 | 11,1,1,1 |
| 248 | 11,11,11,11,1 |
| 248 | 36,11,11,1,1 |
| 248 | 36,36,1,1,1 |
| 251 | 54,11,1,1,1 |
| 256 | 1,1,1,1 |
| 266 | 11,11,11,1,1 |
| 266 | 36,11,1,1,1 |
| 269 | 54,1,1,1,1 |
| 284 | 11,11,1,1,1 |
| 284 | 36,1,1,1,1 |
| 302 | 11,1,1,1,1 |
| 320 | 1,1,1,1,1 |