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21 Wireless Local Area Networks
Multi-input multi-output (MIMO) technology (see Chapter 23) is used in
802.11n to evolve the existing OFDM physical interface presently implemented
with legacy 802.11a/g. MIMO harnesses multipath with a technique known as
space division multiplexing (SDM). The transmitting WLAN device splits a data
stream
into multiple parts, called spatial streams, and transmits each spatial stream
through separate antennas to corresponding antennas on the receiving end. The
current 802.11n provides for up to four spatial streams, even though compliant
hardware is not required to support that many.
Doubling the number of spatial streams from one
to two effectively doubles
the raw data rate. There are trade-offs, however, such as increased power
consumption and, to a lesser extent, cost. The 802.11n specifi cation includes an
MIMO
power-save mode, which mitigates power consumption by using mul-
tiple paths only when communication would benefi t from the additional per-
formance. The MIMO power-save mode is a required feature in the 802.11n
specifi cation.
There are two features in the specifi cation that focus on improving MIMO
performance: (1) beam-forming and (2) diversity. Beam-forming is a technique
that focuses radio signals
directly on the target antenna, thereby improving range
and performance by limiting interference. Diversity exploits multiple antennas by
combining the outputs of or selecting the best subset of a larger number of anten-
nas than required to receive a number of spatial streams. The 802.11n specifi ca-
tion supports up to four antennas.
Another optional mode in the 802.11n effectively doubles data rates by
doubling the width of a WLAN communications channel from 20 to 40 MHz.
The primary trade-off is fewer channels available for other devices.
In the case of
the 2.4-GHz band, there is enough room for three nonoverlapping 20-MHz chan-
nels. A 40-MHz channel does not leave much room for other devices to join the
network or transmit in the same air space. This means intelligent, dynamic man-
agement is critical to ensuring that the 40-MHz channel option improves overall
WLAN performance by balancing the high-bandwidth
demands of some clients
with the needs of other clients to remain connected to the network.
One of the most important features in the 802.11n specifi cation to improve
mixed-mode performance is aggregation. Rather than sending a single data
frame, the transmitting client bundles several frames together. Thus, aggregation
improves effi ciency by restoring the percentage of time that data is being transmit-
ted over the network.
The 802.11n specifi cation was developed with
previous standards in mind
to ensure compatibility with more than 200 million Wi-Fi (802.11b) devices cur-
rently in use. An 802.11n access point will communicate with 802.11a devices on
the 5-GHz band as well as 802.11b and 802.11g hardware on 2.4-GHz frequen-
cies. In addition to basic interoperability between devices, 802.11n provides for
greater network effi ciency in mixed mode over what 802.11g offers.
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Table 21.12 lists the major components of 802.11n. Table 21.13
compares
the primary IEEE 802.11 specifi cations.
