Path Discovery Mechanism/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