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
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.
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