Most legacy access points today make use of Omni-directional antennas. Omni-directional
antennas transmit and receive RF energy in all directions much like a light bulb. Sectorized
antennas (also known as “high-gain” antennas) focus RF energy into a single direction, thereby
intensifying the strength of the signal that is transmitted much like a flashlight directing a strong
beam of light in the direction that it is pointed. Another physical property of high-gain sectored
antennas is the ability to transmit further and “listen better” to the signals of wireless stations
(clients). Some sectored antenna systems have multiple radios and multi-sectored or sectorized
antennas that allow for 360 degrees of coverage.
Omni-directional Access Point Problems The vast majority of access point deployments
today consist of products that use omnidirectional antennas. For the most part, this
type of deployment has served the market well for home use and light use in the enterprise.
But as the number of wireless users has dramatically increased and the number of
access points deployed to meet the needs of users increases, the omni-directional antenna
becomes its own worst enemy in the battle to address improved performance.
No matter what type of access point is used (“fat” or “thin”) blasting RF energy in all
directions becomes a barrier to the performance needed for today’s wireless networks. This
problem consists of a number of issues that all limit high-performance deployments: cell size,
channel reuse, hidden nodes and multi-path. These problems become even more acute with
Omni-directional access points, and are described in the following sections.
Sectored Antenna System and Channel Re-use：A sectored approach is vastly superior to
that of omni-directional access point. As the following diagrams indicate, a sectored
approach creates sectors or “slices” of cell coverage. Each sector uses a unique channel
and by its very design, clusters clients together on the same channel under the area of the
sectorized antenna pattern. If a wireless station moves too far to the left or right of any one
sector, it will automatically re-associate to the next sector.
Cellular network operators have long understood this benefit and make use of sectored base
stations. The result of a sectorized approach is that the indoor wireless network can operate at
full wireless capacity and allows an adjacent Wireless LAN Array to use the same set of
channels. In fact, each Wireless LAN Array can provide up to 810Mbps per cell if all twelve
802.11a channels and three 802.11bg channels are used. This pattern can be used over and over.
Contrast this with 108Mbps in an Omnidirectional deployment, the Wireless LAN Array
provides eight times the capacity.
A sectorized approach is vastly superior to that of an Omni-directional access point in eliminating
Hidden Node issues because all wireless stations (clients) in a given RF sector are associated to the
same radio; so they are geometrically on the same side of the Wireless LAN Array. Since the clients
exist in the same sector, the hidden node problem is eliminated as all stations are able to hear each
other and correctly determine when the air is busy or idle. This eliminates the performance-robbing
issues found with legacy Omni-directional access points.
As previously discussed, Omni-directional access points inherently create large amounts
of performance-robbing multi-path. With a sectorized approach, this problem is greatly
reduced because RF energy is not blindly transmitted in all directions. RF signals are
transmitted in the direction of the wireless client within a given sector and not in the
direction opposite the wireless client that would otherwise come back as distorting multi-path.
Lastly, because a sectorized antenna system typically has higher gain antennas than an
Omni-directional system, it can transmit stronger signals to wireless clients and receive
weaker signals from others. These increased rate and range performance metrics throughout
the sectorized wireless cell results in the ability to use and sustain higher data rates at all
distances within the cell improving overall wireless bandwidth.
发表于 2011-6-18 20:21:59
