802.11 Wireless Network Site Surveying and Installation

Selecting the Access Point 131

Selecting the Access Point

As you have learned up to this point, there are several different architecture designs of

WLAN out there. You have intelligent or fat APs, and lightweight or thin APs with limited

intelligence and dependence on some controller. Just as selecting the proper architecture is

important to your system design, so is selecting the proper AP. There are several different

issues to review on the AP side as well. This section looks at the two major different AP

implementations: single- or dual-radio architecture and AP radio styles.

Single- or Dual-Radio Architecture

Most APs were designed to support a single-radio platform, having one radio per AP (see

Figure 5-13). This has been the most common AP design to date. Some APs were provided

with dual PCMCIA slots so that a second radio could also be operated (see Figure 5-14).

At the time of introduction, dual-radio platforms were actually intended to provide a

migration path from 900 MHz to 2.4 GHz. You could put one of each radio into the AP and

have support for both bands as you migrated away from 900 MHz. However, some vendors

promised double the bandwidth with the architecture by using two of the same radios in the

AP. This actually introduces a problem called receiver desensitization, which causes poor

performance of both radios.

Figure 5-13 Single-Band AP

132 Chapter 5: Selecting the WLAN Architecture and Hardware

Figure 5-14 Dual-Radio AP

Receiver Desensitization

Every radio receiver has a specification that defines the capability of the receiver to “hear

and understand” some minimal signal strength. This is called receiver sensitivity or receiver

threshold (see Figure 5-15). This value represents the lowest signal that a radio can receive

and still recover the information or data from the signal. In the case of most 802.11b WLAN

radios, this is on the order of –80 dBm to –85 dBm. (The more negative the number, the

smaller the signal.) The typical 802.11b transmitter has a transmit power of +15 dBm to

+20 dBm (or 100 dB stronger than the receive threshold).

Because some cross talk may occur between the different channels in the 802.11 band, the

receiver incorporates filters and circuitry to reduce interference from other channels in the

same band. With the available 802.11 chipsets (components that are used in the radio

portion of the devices), the best RF filtering (the capability to reject certain RF energy) that

you can obtain, even at opposite ends of the band, is perhaps 65 to 75 dB. Most receivers

have a sensitivity in the –80- to –90-dBm range, and with the signal level coming out of a

transmitter set to channel 1 at 15 to 20 dBm (depending on transmitter power capabilities).

The signal level measured at the radio adjacent to it on channel 11 is 65 to 75 dB lower.

This places the received signal from the unwanted channel 1 at 50 dBm to –60 dBm. This

value is stronger than the minimal signal level of the receiver by a large margin. If the AP

radio on channel 11 is trying to receive a signal from a distant client, and the signal level is

near the minimal receiver threshold, the energy present in the channel 11 area transmitted

from the channel 1 transmitter only a few inches away will have a stronger signal level and

mask out the desired signal from the desired channel 11 client. This effectively reduces the

coverage area any time the adjacent radio is transmitting.