11A/B/G NETWORKS ARE KNOWN AS SINGLE INPUT/SINGLE OUTPUT (SISO), W...

802.11a/b/g networks are known as single input/single output (SISO), which allows for

performance to degrade as a result of multipath, poor reception because of obstacles and

RF interference sources. 802.11n MIMO networks can take advantage of multipath to help

increase throughput at range providing much higher throughput at the same range.

MIMO Channels

IEEE 802.11n MIMO networks can operate in both the 2.4 GHz ISM and 5 GHz UNII

bands and are capable of either 20 or 40 MHz–wide channels. Even while operating in

40 MHz channel width mode, many frames are still transmitted with a 20 MHz chan-

nel width. The 20 and 40 MHz channel widths are defined by the IEEE for transmission

of OFDM modulated data. As one would expect, wider channels mean more data can be

transmitted over the RF medium simultaneously. Therefore, wider channels allow higher

data throughput. Think of this like cars traveling on a two-lane or a four-lane highway.

A 20 MHz–wide channel can be looked at as the two-lane highway and a 40 MHz wide

channel the four-lane highway. More cars can pass through four lanes in the same amount

of time than can pass through two lanes. The 20 MHz or 40 MHz channels can be used in

either the 2.4 GHz or 5 GHz frequency ranges. Because of the limited amount of frequency

space in the 2.4 GHz ISM band, there is only one 40 MHz–wide channel without any

adjacent-channel overlap.

range ISM band. 802.11g-compliant devices are backward compatible with 802.11b-

compliant devices. However, this backward compatibility comes at a price: reduced data

throughput. Because of protection mechanisms, ERP-OFDM devices used in 802.11g will

suffer in performance when an HR/DSSS device is in the same radio or hearing range of the

ERP-OFDM device.

HR/DSSS and ERP-OFDM systems have many common features:

Both operate in the 2.4 GHz ISM band.

Û

N

Both have three non-overlapping channels.

Both are subject to interference from other devices operating in the same frequency range.

HR/DSSS and ERP-OFDM (802.11b-compliant and 802.11g-compliant) devices are

backward compatible. ERP-OFDM is rated at 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Actual

throughput in an environment relatively free of interference will be about 18 to 22 Mbps. If

a DSSS or HR/DSSS device is introduced in the radio range of the ERP-OFDM device, the

throughput will decrease significantly because of protection mechanisms. How much of an

impact this makes depends on many factors in the environment. Typically, the decrease in

throughput is about 25 percent to 30 percent.

Adjacent-Channel and Co-channel Interference

Adjacent-channel and co-channel interference (two or more RF signals interacting with

each other and causing a degradation of performance) is a concern in the design, develop-

ment, and deployment of IEEE-based 802.11 wireless networks. This type of interference

will have an impact on the amount of actual throughput between devices over a wireless

network. As mentioned earlier in this chapter, the 2.4 GHz ISM band has only three non-

overlapping channels. Careful channel planning is required when designing or implementing

a wireless network. This type of planning will minimize issues such as poor throughput as

a result of adjacent and co-channel interference. Channel planning involves designing wire-

less networks so that overlapping RF cells are on different (non-overlapping) channels—for

example, channels 1, 6, and 11 in the 2.4 GHz ISM band. This will help optimize performance

and minimize degradation of throughput because of adjacent and co-channel interference. RF

energy propagates in several directions simultaneously. A well-designed wireless network

will account for a three-dimensional propagation. In other words, in a three-story building,

the RF from an access point on the second floor building may pass through to the first and

third floors; therefore interference could be an issue if the network is not planned properly.

WLAN/WPAN Coexistence

Wireless personal networks (WPANs) typically consist of portable devices such as personal

digital assistants (PDAs), cell phones, headsets, computer keyboards, and mice. As mentioned

in Chapter 4, the performance of IEEE 802.11 wireless LANs can be affected when co-located

Early Bluetooth devices can cause significant interference while operating in close prox-

imity of IEEE 802.11 wireless LANs. Bluetooth was designed to hop at a rate of 1,600 times

per second across the entire 2.4 GHz band, potentially causing significant interference with