Co-Channel Interference; Wireless Mesh Network Coverage Considerations; Cell Planning And Distance - Cisco aironet 1522 Design And Deployment Manual

Both the hidden node problem and the exposed node problem are inherent to wireless mesh networks
because mesh access points share the same backhaul channel. Because these two problems can affect the
overall network performance, the Cisco mesh solution seeks to mitigate these two problems as much as
possible. For example, the AP1520s have at least two radios: one for backhaul access on 5GHz channel
and the other for 2.4GHz client access. In addition, the radio resource management (RRM) feature
enables cell breathing and automatic channel change, which can effectively decrease the collision
domains in a mesh network.
There is an additional solution that can help to further mitigate these two problems. To reduce collisions
and to improve stability under high load conditions, the 802.11 MAC uses an exponential backoff
algorithm, where contending nodes back off exponentially and re-transmit packets whenever a perceived
collision occurs. Theoretically, the more retries a node has, the smaller the collision probability will be.
In practice, when there are only two contending stations and they are not hidden stations, the collision
probability becomes negligible after just three retries. Collision probability increases when there are
more contending stations. Therefore, when there are many contending stations in the same collision
domain, a higher retry limit and a larger maximum contention window are necessary. Further, collision
probability does not decrease exponentially when there are hidden nodes in the network. In this case,
RTS/CTS exchange can be used to mitigate the hidden node problem.

Co-Channel Interference

In addition to hidden node interference, co-channel interference can also impact performance.
Co-channel interference occurs when adjacent radios on the same channel interfere with the performance
of the local mesh network. This interference takes the form of collisions or excessive deferrals by
CSMA. In both cases, performance of the mesh network is degraded. With appropriate channel
management, co-channel interference on the wireless mesh network can be minimized.

Wireless Mesh Network Coverage Considerations

This section provides a summary of items that must be considered for maximum wireless LAN coverage
in an urban or suburban area, to adhere to compliance conditions for respective domains.
The following recommendations assumes a flat terrain with no obstacles (green field deployment).
Cisco always recommends a site survey before taking any real estimations for the area and creating a bill
of materials.

Cell Planning and Distance

RAP-to-MAP ratio is the starting point. For general planning purposes, the current ratio is 20 MAPs per
RAP.
Cisco recommends the following values for cell planning and distance in non-voice networks:
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Cisco Aironet 1520, 1130, 1240 Series Wireless Mesh Access Points, Design and Deployment Guide, Release 6.0
OL-20213-01
RAP-to-MAP ratio–Recommended maximum ratio is 20 MAPs per RAP.
AP-to-AP distance–A spacing of no more than of 2,000 ft between each mesh access point is
recommend. When you are extending the mesh network on the backhaul (no client access), use a cell
radius of 1,000 ft.
Hop count–Three to four hops
One square mile in ft (5280
–
approximately three or four hops. (See
2
), is nine cells and you can cover one square mile with
Figure 25 and
Site Preparation and Planning
Figure 26.)
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