Path Discovery Mechanism/Metrics

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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).

Table 1 - Links costs for each datarate in a PREQ
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.

Table 2 Possible combinations of link costs for mesh ttl5
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