Load Balancing; Site Survey And Drm - Extreme Networks Summit WM Technical Reference Manual

Modern digital communications, including WLAN, are designed to be resilient to multipath. The CCK
modulation used in 11b rates (1, 2, 5.5, and 11) is capable of eliminating the unwanted "ghosts" and
preserving only the strongest signal received. Even better, the OFDM modulation used in 11a and in
11g-specific rates (6, 9, 12, 18, 24, 36, 48, and 54Mbps) is capable of separating the "ghosts" and adding
them "in-phase", thereby using the whole energy of the received signal. To counteract "fading", both
CCK and OFDM use very wide signal bandwidths; "fading" is frequency dependent and therefor, at
any given moment there is a very small probability that all frequencies in the WLAN bandwidth would
be faded.
However, no modulation is completely immune to multipath effects, and there are field situations in
which the multipath and fading are so strong that the signal cannot be received in certain spots. Such
coverage holes are typically very small (few centimeters in diameter) case in which client antenna
diversity (if available) would be able to compensate. However, even with antenna diversity, there is a
chance that both antennas are in a coverage hole.

Load balancing

The final factor to be considered when deploying Wireless APs and adjusting their RF configuration is
the load balancing. A Wireless AP with a larger Tx power will cover a larger area and therefore will
likely end up having more associated clients than a Wireless AP with lower Tx power. Therefore, it is
very important to maintain a decent balance between Wireless APs, i.e. to avoid deployments where, for
example, the same room is serviced by Wireless APs that utilize maximum power but also by Wireless
APs that use minimum power.

Site survey and DRM

A site survey is necessary for installing and configuring large WLAN networks. A site survey cannot be
replaced by the DRM. However, site surveys are not sufficient in addressing how the WLAN network
will perform over time, because the RF environment is not guaranteed to remain as it was the moment
the site survey was performed. The RF environment can change due to a number of factors. The
changes can be static or dynamic. The most frequent cause for static RF environment changes is when
the site survey is done before the building is complete. The addition of walls, doors, furniture etc, can
cause static RF environment changes. Another frequent cause for static RF environment changes is
when an office layout is re-arranged by moving the furniture, cubicles, or even walls. The most
challenging RF environment changes are dynamic changes. For example:
In hospitals, a high number of people and equipment are moving (including lead-lined curtains and
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large metallic objects).
In education institutions, a high number of students are moving and there are a high number of
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wireless clients that are relocating (student laptops).
In offices, institutions, or other public places a high number of people are moving.
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In warehousing, a high number of large objects (inventory) are moving, some of them could be
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reflective (metallic) or absorbent (wood, plaster, and concrete).
DRM can significantly help reduce the impact of such changes in the RF environment. Automatic
Channel Selection (ACS) can be used to select the optimum channel arrangement based on the actual
situation in the field, considering all RF environment changes that have occurred. The Automatic
Transmit Power Control (ATPC) is an even more powerful tool because it can adjust, on the fly, the Tx
power on all Wireless APs in order to compensate for both static and dynamic RF environment changes.
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