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Cisco Aironet 1520, 1130, 1240 Series Wireless Mesh Access Points, Design and Deployment Guide, Release 6.0
OL-20213-01
Troubleshooting
Because there could be many nodes attempting to join or re-join the network after an expected or
unexpected event, a hold-off time of 16 minutes is implemented. This means that no nodes are
exclusion-listed during this period of time after system initialization.
This exponential backoff and advance algorithm is unique and has the following useful properties:
•
It allows a node to correctly identify the parent nodes whether it is a true honeypot or is just
experiencing temporary outage conditions.
•
It credits the good parent nodes according to the time it has enabled a node to stay connected with
the network and the crediting requires lesser and lesser time over period in order to bring the
exclusion-list conviction period to be very low for real transient conditions and not so low for
transient to moderate outages.
•
It has built-in hysteresis for encountering the initial condition issue where many nodes try to
discover each other only to find that those are not really meant to be in the same network.
•
It has built-in memory for nodes that can appear as neighbors sporadically so they are not
accidentally considered as parents if they were, or are supposed to be, on the exclusion-list database.
The node exclusion-listing algorithm is constructed to guard the mesh network against serious stranding,
which was observed in customers’ networks. It integrates into AWPP in such a way that a node can
quickly (re-)converge and find the correct network under many kinds of adversities.
Throughput Analysis
Throughput depends on packet error rate and hop count.
Throughput is calculated as:
Throughput = BR * 0.5 * 1/n * PSR
BR = Raw backhaul rate, i.e. 18, 24 Mb/s
n = Backhaul hop count
PSR = Packet success rate = (1.0-PER) = (0.0 .. 1.0)
Two assumptions apply to this calculation:
•
There is no other traffic on the mesh
•
1/n factor is based on all hops hearing each other.
Generally, the throughput numbers per hop are as shown in
.
Capacity and throughput are orthogonal concepts. Throughput is one user's experience at node
N
and
total area capacity is calculated over the entire sector of
N
-nodes and is based on the number of ingress
and egress RAPs, assuming separate non-interfering channels.
For example, 4 RAPs at 10 Mb/s each deliver 40 Mb/s total capacity. So, one user at 2 hops out, logically
under each RAP, could get 5Mb/s each of TPUT, but consume 40 Mb/s of backhaul capacity.
Table 23
Throughput Numbers Per Hop
Hops
Throughput
One
Approximately 14 Mb/s
Two
Approximately 8 Mb/s
Three
Approximately 3 Mb/s
Four
Approximately up to 1 Mb/s