WiFoo..The.Secrets.of.Wireless.Hacking [Electronic resources]

Andrew A. Vladimirov

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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

Figure 5-14. Dual-Radio AP

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Receiver Desensitization

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).

Figure 5-15. Receiver Desensitization

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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.

This issue can also result from placing two single-band APs in close proximity. There should be, at minimum, approximately 5 feet between any two antennas attached to different 802.11 radios to provide adequate separation and receiver performance.

You can use a true dual-radio architecture to migrate from one technology to another or to just add bandwidth by permitting some users on one technology and other users on another technology. However, because these architectures have different specifications and ranges, you must consider a few items during the network design stages. If you want the cell sizes the same for both technologies, you have to adjust power levels or antenna selections appropriately.

A number of dual-band APs have come on the market over the past year. These were designed with the intention of providing support for both 2.4 GHz and 5 GHz. Some of these devices use two separate radios offering simultaneous support for both bands, whereas others use a single radio that can be set up to operate in either band, in which case it supports only one band at a time. It is imperative to understand the difference between the two types of systems. An AP with the single radio, although supporting both bands, does not lend itself to migrating easily from one technology to another and does not permit scaling by adding clients on both bands.

AP Radio Styles

N connector (or some variation of one of these).

Figure 5-16. Physical Security of an AP Radio

802.3af) for Power over Ethernet (PoE), there are several common ways to implement PoE (see splitter goes at the AP end, and has the circuitry to separate the Ethernet from the power. The splitter then has two output cables, one for Ethernet and one for power.

Figure 5-17. PoE Examples

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If you plan to power your AP from your network switches, investigate the power options of the switch (does it support 802.3af, or some other vendor's specific scheme?) and the AP to confirm compatibility. Also be aware that some switches might not have enough power to support dual-band APs, resulting in the need to use a power injector or a third-party power module.