Access Point Interference - Avaya Communication Server 1000 Installation And Commissioning Manual

Access Point interference

When more than three APs are deployed, the APs themselves are a significant source of
interference. This is known as cochannel interference. Therefore, it is important to consider
how channel reuse impacts network capacity.
To maximize the distance between APs operating on the same channel, tile the channels. To
scale capacity, add more APs in the same geographic region and at the same time, reduce the
transmit power of each AP.
However, the overall throughput increase does not increase proportionally with the number of
APs that are added because each individual AP loses throughput, even though the number of
APs per square foot is increasing. Note that the biggest loss of per-AP throughput occurs when
going from nonchannel-reuse to reusing channels. For more information about this subject,
see the whitepaper available from http://www.avaya.com/.
The goal is to achieve the required call density for the number of calls per square foot. Getting
the most calls per AP is not a useful objective of capacity planning. The parameters that must
be tuned to engineer a voice network for capacity are:
• channel reuse factor (that is, the number of channels in the channel plan)
• transmit power of each AP
• the radius of the cell (that is, based on the physical distance between APs)
Because of the complexity of this topic and the simulation data that is required, it is not possible
to discuss tuning all three variables or even two variables at a time. An example of a light to
medium office environment (mostly cube space but some walls) is provided instead.
Example
The channel reuse factor for 802.11b networks is fixed at three (three nonoverlapping channels
in the 2.4 GHz range), corresponding to channels 1, 6, and 11. The transmit power is fixed
at 50 mW, which establishes the radius of the cell.
Now the effects of cell size, based on the other fixed parameters, can be compared.
If the deployed cells have a radius of anywhere from 33 ft to 75 ft, the call capacity per square
foot is essentially the same. This means that packing cells in tighter than a 75 ft radius per
AP is a waste of money. This example shows that in a typical office environment with APs at
half power, you can deploy APs anywhere from 100 ft to 150 ft from each other. More walls
mean there must be less distance between APs, and lowering the power of the AP lessens
the required distance between APs, both of which also serve to increase the net call density.
Avaya WLAN IP Telephony Installation and Commissioning
Other network design considerations
November 2010 65