Dear Fabrice, Yes, I agree that both ETX of 15 and 1 are not appropriate. An ETX threshold value should be a good idea to avoid the cases that parent frequently changes or not change at all.
Thanks for the good advice! Regard, Tengfei On Sat, May 28, 2016 at 9:24 AM, Fabrice Theoleyre <theole...@unistra.fr> wrote: > Dear Tengfei, > > Thank you Tengfei for pointing out this problem. > > However, it may perform poorly in dense networks: a node has to test > iteratively each neighbor. > > Let me describe what I imagine: > -> a node selects one neighbor P as parent (default ETX = 1) > -> after sending many packets, the ETX of P value is readjusted (probably > not 1, let’s assume 1.1). > -> other (non tested) neighbors have a default value of 1 -> the node > changes its parent. > > In conclusion, RPL may take a long time before converging. Did I > misunderstand something? > > Both solutions (default ETX of 15 and 1) seem presenting drawbacks. We may > use a default ETX value which represents a good (but not perfect) radio > link. In other words, we should not test other neighbors if the ETX of our > parent is currently over this threshold value. > For instance, is an ETX of 1.5 reasonable? (PDR = 66%) > > Best regards, > Fabrice > > > > > Le 28 mai 2016 à 04:48, Tengfei Chang <tengfei.ch...@gmail.com> a écrit : > > Dear all, > > The OpenWSN community recently has a question related to the calculation > of RANK in minimal draft. and I would like share with you. > > In minimal draft, the link quality is measured by ETX=(numTX/numTXAck). > > 1. At the beginning of the network, numTxACK is zero. we need to give a > default value of ETX for this case in the draft. > 2. what the value should be? > In OpenWSN, this value is set to 15, which assume the link is very bad. > This assumption would cause an issue in following example case. > > *EXAMPLE:* > Image there are four nodes in the network, node 1 is the dagroot. > The topology of the network is linear at beginning. > > Node 1 <- Node 2 <- Node 3 <- Node 4 > > After a while, four node get their own RANK. Let assume numTx and numTxACK > are 100 and 75 as the same with the example in minimal draft. > Node RANK > 1 255 > 2 768 > 3 1280 > 4 1792 > https://tools.ietf.org/html/draft-ietf-6tisch-minimal-15#page-17 > > Then we move node 4 towards node 2 so that node 2 will be one neighbor of > node 4. when node 4 heard the DIO of node 2, because node 4 doesn't have > any packet send to node 2, so the numTxACK to node 2 is 0. > If we use 15 as the default ETX value, then RANK calculated based on node > 2 is > 768 + (3*15-2)*256=11776 > This is much larger than 1792 which is calculated based on node 3's rank. > So node 4 will still stick to node 3 as its parent. > Also since there will be no packet send to node 2 from node 4, the > numTxACK will keep to zero. > > If the link quality from node 4 to node 2 is the same with other links', > apparently, node 2 has a shorter path to node 1 then node 3. > > So Here are the proposal: > 1 define a default linkcost value (ETX) for the case when numTxACK is zero > 2. set its value to 1, which assumes the link quality (ETX) is 1, > > What do you think? > > Tengfei > > > -- > Chang Tengfei, > Pre-Postdoctoral Research Engineer, Inria > _______________________________________________ > 6tisch mailing list > 6tisch@ietf.org > https://www.ietf.org/mailman/listinfo/6tisch > > > -- Chang Tengfei, Pre-Postdoctoral Research Engineer, Inria
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