11N SPECIAL IS THAT IN A PURE 11N ENVIRONMENT, YOU CAN GET SPE...

802.11n special is that in a pure 802.11n environment, you can get speeds up to 300 Mbps,

but most documentation says it will provide 100 Mbps. This is probably because the ex-

pectation is that other 802.11 clients will be present. 802.11n is, in fact, backward compat-

ible with 802.11b/g and a.

The backward compatibility and speed capability of 802.11n come from its use of multi-

ple antennas and a technology called Multiple-Input, Multiple-Output (MIMO). MIMO,

pronounced Mee-Moh, uses different antennas to send and receive, thus increasing

throughput and accomplishing more of a full duplex operation.

MIMO comes in three types:

■ Precoding

■ Spatial multiplexing

■ Diversity coding

Precoding is a function that takes advantage of multiple antennas and the multipath issue

that was discussed in Chapter 3, “WLAN RF Principles.” 802.11n uses transmit beam-

forming (TxBF), which is a technique that is used when more than one transmit antenna

exists where the signal is coordinated and sent from each antenna so that the signal at the

receiver is dramatically improved, even if it is far from the sender. This technique is some-

thing that you would use when the receiver has only a single antenna and is not moving. If

the receiver is moving, then the reflection characteristics change, and the beamforming

can no longer be coordinated. This coordination is called channel state information (CSI).

Spatial multiplexing takes a signal, splits it into several lower rate streams, and then sends

each one out of different antennas. Each one of the lower rate streams are sent on the

same frequency. The number of streams is limited to the lowest number of antennas on ei-

ther the transmitter or the receiver. If an AP has four antennas and a client has two, you are

limited to two.

Currently, the Wi-Fi Alliance is certifying 802.11n devices even though they are still in

draft status. The Wi-FI Alliance is doing this using the interim IEEE 802.11n draft 2.0.