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Using LInk Quality and ETX metrics

Two of the metrics provided on the BBHN routers are Link Quality (LQ) and Expected Transmission Count(ETX).  I've been doing some reading up on what these metrics mean, and how they are useful in monitoring and evaluating routers in a mesh network.  The description of the requirements are listed here  http://www.olsr.org/docs/README-Link-Quality.html .  


The following information is a bit of a read, but I think its worthwhile to understand these concepts if you're running a BBHN.   The main point that is useful is how to use LQ and ETX when you're setting up and using BBHN. 


First, these metrics are visible on the "Mesh Status" web page that's a part of every router.  


The Link Quality metric is a simple measure of (surprise) packet  transmission quality between two nodes,  How it is calculated is a BBHN router is constantly sending HELLO packets to all of its neighbor nodes in a mesh. It counts how many of these packets are successfully acknowledged.    This metric is the percentage of packets that are successfully transmitted and an acknowledgement received.    The neighboring router is also sending HELLO messages, so the return link is measuring LQ as well (thats called the Neighbor LInk Quality - NLQ).   Since these are both probability numbers, the overall Link Quality for a round trip between two nodes is calculated as LQ * NLQ,   As an example, if the transmit LQ is 60% and the receive NLQ is 70%, then the round trip probability is 42%.   (i.e. 42 packets out of 100 would make it through a round trip).     This is not as bad as it sounds, since IP networks will retransmit dropped packets, so you would a reduced throughput speed, but not necessarily lost packets from an application level.    


Under minimal traffic conditions, like what we see with BBHN today, the LQ is a pretty good indicator of the wireless conditions between two nodes.   You can watch LQ and adjust your antenna to get to a maximum number, or use it to as a rough metric to compare antennas.   Since it will also vary with power and background noise, its a relative indicator (and you'll see a variance even under good conditions).


The other metric is called Expected Transmission Count (ETX), and it is directly related to LQ.   ETX are used for remote node (i.e. nodes that are not directly connected to your router, but are more than 1 hop away and are connected to other routers in a mesh).   Since its the inverse,  ETX is described as the number of expected packets that you would need to transmit to get a packet received and acknowledged .     For Example, an ETX of 1 would mean that on average, you should transmit a packet and it should be received - no dropped packets.  An ETX of 2 would mean that you need to transmit 2 packets to get one received, as you would expect 1 to be dropped (i.e. 50 % probability).


The metric is measure of how good LQ is between your router and a remote node through multiple hops in the network, and its measured that way as its easy to calculate overall ETX by adding the ETX on each route between the route and a remote node.  ETX = ETX1 + ETX2 + ETX3 , etc. 


ETX is not useful for aligning antennas, as LQ is.  However, its is a good diagnostic tool for determining where an issue may be in the network if you are trying to get to a service on a remote node.    If you see a remote node with poor ETX  (like 10),  but the rest of the nodes have a ETX of 2-4, then there is an issue somewhere in the path between you and the remote node. 


So, in summary,   LQ , with the caveats listed, is a useful metric for things like aligning antennas. The other metric, not discussed here, is looking at Signal/Noise/Ratio (SNR) value.    ETX is a good metric for seeing if there are path issues between remote nodes. 


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