FabFi Deployment Guide
From Fab Lab wiki
Contents |
Selecting a Site
Choosing a Reflector
Selecting the Best Channel
Pointing
Some tips for pointing:
- Tape a drinking straw to the reflector at the vertex of two of the slats (make sure it is firmly pressed into the corner) and look through the straw. What you see is what you are pointing at.
- From an SSH session, you can run the following command
while true; do wl rssi; sleep 1; clear; done;
to continually test the strength of a connection in real time. You can then watch the screen while moving the reflector to find the best pointing. This will ONLY be informative if the two fabfis you are pointing are on a channel with no other fabfis.[http:://www.jouercasino.eu casino en ligne francais]
Advanced Topics
Sharing Reflectors
When many FabFis are deployed in one place, it is often possible to point reflectors in a one-to-many configuration. In other words, use one reflector on one end of a link that talks to many reflectors on the other end. This is beneficial because it means less hardware and less radio noise.
The best way to determine if a one-to-many configuration is possible is to look at the Status>OLSR page on the administration tab of the remote node's web GUI
Above is a screen capture of the OLSR page for a remote node in Jalalabad. As you can see, it recognizes 4 neighbors, each with an associated LQ and NLQ number:
The Link Quality and the Neighbor Link Quality are values between 0 and 1 or, which is equivalent, between 0 and 100%. They represent the probability that a packet that our neighbor sends actually makes it to us (Link Quality) and that a packet that we send actually makes it to our neighbor (Neighbor Link Quality).
Let's now look at the probability for a successful packet round trip, i.e. the probability that we successfully send a packet to our neighbor and, on receiving it, our neighbor successfully replies with a response packet. For a successful round trip both packets must get through, the packet that we've sent and the response packet that our neighbor has sent. So, the success probability is NLQ x LQ, where NLQ is the Neighbor Link Quality of the link and LQ is its link quality. For example, if we have a NLQ of 60% and a LQ of 70%, the probability of a successful round trip is 60% x 70% = 0.6 x 0.7 = 0.42 = 42%. from [1]
The ETX number is 1 / (LQ * NLQ) and represents the expected number of transmissions required to send a single packet of data (smaller numbers are better). If your fabfi sees it's new dedicated pair reflector and an existing reflector with very similar ETX numbers (<.05 difference), and the existing reflector has a reliable path to the closest uplink, it is completely safe to use the exiting reflector as a pair to the new remote node.
In the example above, 10.102.0.5 is an existing reflector and 10.102.0.27 is the dedicated pair to fabfi28 (the node whose OLSR page is shown). Because the link quality is very similar between 5 and 27, we can conclude that 27 is not necessary.
In general, poorly pointed nodes are better at sending than receiving. In the above example we see that NLQ is lower than the LQ. We can guess, therefore, that 28 is pointed well, but 27 and 5 are not pointed precisely in the right direction. In the case of 5, imperfect pointing is to be expected because it is not set up to talk to 28, but in practice 27 is not much better than 5. As a result, 5 can be used in place of 27 and 27 can be removed.
- NOTE: Only nodes on the same wireless channel (or connected by a cable) will be visible to each other as neighbors.
- NOTE 2: There may be reasons for you LQ and NLQ numbers to decrease OTHER than pointing. it is always a good idea to check your signal strength at the command line with
iwconfig
This command will show you the signal strength of the last packet it received. for this to be informative you MUST ISOLATE the pair you are looking at on their own channel. If there are multiple devices on the same channel, you will not be able to tell which one you are hearing.
