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When a Client Wi-Fi device's RF power level is less than the power level of the nearest Access Point, and the client device approaches the edge of the access point's coverage area, a critical problem occurs - the Client "sees" the Access Point but it does not have the capability of connecting to it. An Access Point should never have a higher input power than the lowest power Client Adapter being supported.
There are advertisements for high-power 802.11 Access Points that claim that you'll have six times more power than a standard Access Point. The problem is that the limiting factor is whichever unit (Access Point or Client Wi-Fi radio) has the lowest power input. Whether you're using a built-in Wi-Fi wireless LAN network interface, a PCMCIA 802.11 NIC, or an external USB 802.11 radio, the key point is to understand that the power level of the client Wi-Fi interface must as great, or greater than the power level of the associated access point. The following discussion explains the reason.
The PCMCIA Wi-Fi Client Adapter Card
Many PCMCIA Wi-Fi adapter cards have a flat antenna coil encapsulated in an epoxy "nub" that sticks out on the end.
A typical add-on PCMCIA Client Wi-Fi adapter has a flat antenna coil inside the external "nub" that sticks out of the side of the notebook computer. This is generally the worst possible way to orient the antenna. Here's why... The antenna on the Access Point sticks up (called "vertically polarized") but the PCMCIA antenna is horizontal (called "horizontally polarized").
The Problem with Cross-Polarization
When a transmitting and receiving antenna are cross-polarized the reception capability can be reduced significantly.
When a horizontally polarized antenna attempts to receive signals transmitted by a vertically polarized antenna (hence, "cross-polarized antennas") a loss of up to 30dB or more can result. This means that a Client that could have connected (with properly aligned antennas) at a maximum bit rate (11 Mbps 802.11b or 54 Mbps 802.11g or 802.11a) would be forced down the the minimum bit rate (as low as 1 Mbps), and a connection that could have taken place at a minimum bit rate would not occur at all. Cross-polarization is a bad thing.
Correcting for Cross-Polarization
A PCMCIA Wi-Fi adapter with "flip up" antennas to help
minimize cross-polarization signal loss.
Internal antennas (as would be found inside an Intel Centrino-equipped notebook) can be much larger than the tiny, flat antenna in a PCMCIA "nub" and external USB-attached Wi-Fi radios can have fully upright antennas. "Flip-up" antennas on a PCMCIA card provide a vertically-polarized antenna on the otherwise horizontal notebook computer.
A Wi-Fi Challenge In An Office Suite
In this office suite there is a need to provide Wi-Fi access to the Conference Room (B). The decision is made to mount the Access Point in the computer closet (A).
The example that's being presented here takes some liberties with the relative values being discussed but the points being made are based on the laws of physics:
1) An Access Point should never have a higher input power than the lowest power Client Adapter being supported.
2) To allow more Clients to "hear" and Access Point, use a high-gain antenna, don't increase the power!
100mW Access Point Coverage Boundary
We now consider the "edge" of the coverage area inside which the signal from the Access Point is sufficiently strong for acceptable Client use. Notice that our example supposes that the Access Point (A) is unable to reach the Client notebook computer on the conference table (B).
The problem would manifest itself by the fact that a few people at one side of the conference room might have connectivity but, if you're sitting up front near the projection screen (B) you just don't have a strong enough signal from the Access Point - you can't connect to the Wi-Fi wireless LAN.
100mW Client Card Coverage Boundary
We assume, for the purpose of this example, that the Client and the Access Point both use an antenna with essentially the same characteristics. In this case it should be obvious that if the 100mW Access Point can't reach the Client then a 100mW Client can't reach the Access Point.
The RF signal transmitted by the Client notebook computer is not sufficiently powerful to overcome the attenuation caused by the walls of the building in exactly the same way that the 100mW signal from the Access Point couldn't reach the Client.
200mW Access Point Coverage Boundary
The network administrator in our example has not read this page or they would know that simply increasing the Access Point's power will not fix the problem.
On the left you see the Access Point (A) with the same antenna and in the same location, but now we're using a 200mW input power level. Notice that the signal coverage boundary for the Access Point (A) has now extended beyond the notebook computer in the conference room (B).
The problem is actually worse now than it was before. You see, now the Client (B) sees that an Access Point (a WLAN) appears to be available (whereas before the WLAN didn't even appear). The Client, however, still can't send a signal back to the Access Point.
100mW Access Point with 3dB Gain Antenna
The solution to the problem is to use a high-gain antenna with the 100mW Access Point. The rule is: "An Access Point should never have a higher input power than the lowest power Client Adapter being supported."
The picture shows that a high-gain antenna works because it's construction causes transmitted RF to be focused (notice that the boundary is beyond the Client at 'B') but the same construction acts to "reach out" and acquire RF energy from the air in reverse, as if a much larger antenna existed for reception (as shown by the blue lines at "C" - these are not antenna elements; they simply represent the way the antenna works. Hence: "To allow more Clients to "hear" and Access Point, use a high-gain antenna, don't increase the power! "
it's true that most new notebook
computers can be purchased with built-in Wi-Fi, legacy systems and desktop computers still need additional hardware for Wi-Fi connectivity. Connect802 has that hardware available for quick delivery. A 200 mW (23 dBm) 802.11 Access Point may be able to transmit over 1000 feet but that doesn't help a 30 mW Client Wi-Fi adapter talk back to the Access Point. Wireless LAN Access Points with high-power don't help a WLAN unless the Client WiFi adapters are at least equal in power input. Wi-Fi signal boosters (RF amplifiers) can help offset cable loss when a long antenna cable needs to be run, but a high-power Access Point can not help an under-powered Wi-Fi client adapter. A High Power Access Point may be specified with a 200 mW signal level. Unfortunately this won't help improve the effective WLAN coverage unless the clients are equally equipped. Some examples of products that fall into the high-power category are the Enterasys RoamAbout Access Point 30000, The Lucent ap500, and the Engenius 2511. Using these types of high-power 200 mW or 23 dBm units the person doing the RF design must carefully consider what's going to be done with that "extra" RF power.. compensate for a long antenna cable or send the signal to places where Clients can't talk back. Consider which Wi-Fi client adapter cards you'll need if you're using a 200 mW wireless access point and mesh router note/repeater. You're not going to find a high-power 200 mW PCMCIA Wi-Fi 802.11 adapter card. You're not going to find a 200 mW high-power USB Wi-Fi 802.11 radio. Most WiFi client cards operate closer to 30 mW or 50 mW, much less than you would expect when some of them are even specified to operate as 100 mW (20 dBm) Wi-Fi 802.11 client adapters.