Power 
Management - Wireless Local Area Networks

21.8 Power
Management
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21 Wireless Local Area Networks
Sleeping stations are required to periodically wake up and retrieve messages from
the AP. Power management is more difﬁ cult for peer-to-peer IBSS conﬁ gurations
without central AP. In this case, all stations in the IBSS must be awakened when
the periodic beacon is sent. Stations randomly handle the task of sending out
the beacon. An announcement trafﬁ c information message window commences.
During this period, any station can go to sleep if there is no announced activity
for it during this short period.
21.9 IEEE 802.11b — High Rate DSSS
In September 1999 IEEE ratiﬁ ed the 802.11b
high rate
amendment to the
standard, which added two higher speeds (5.5 and 11 Mbps) to 802.11. The key
contribution of the 802.11b addition to the WLAN standard was to standardize
the physical layer support to two new speeds, 5.5 and 11 Mbps. To accomplish
this, DSSS was selected as the sole physical layer technique for the standard, since
frequency hopping cannot support the higher speeds without violating current
FCC regulations. The implication is that the 802.11b system will interoperate
with 1 Mbps and 2 Mbps 802.11 DSSS systems, but will not work with 1 Mbps
and 2 Mbps FHSS systems.
The original version of the 802.11 speciﬁ es in the DSSS standard an 11-bit
chipping, called a
Barker sequence,
to encode all data sent over the air. Each
11-chip sequence represents a single data bit (1 or 0), and is converted to a
waveform, called a symbol, that can be sent over the air. These symbols are trans-
mitted at a 1 million symbols per second (Msps) rate using binary phase shift key-
ing (BPSK). In the case of 2 Mbps, a more sophisticated implementation based on
quadrature phase shift keying (QPSK) is used. This doubles the data rate available
in BPSK, via improved efﬁ ciency in the use of the radio bandwidth.
To increase the data rate in 802.11b standard, advanced coding techniques
are employed. Rather than the two 11-bit Barker sequences, 802.11b speciﬁ es
complementary code keying (CCK). CCK allows for multichannel operation in
the 2.4 GHz band by using existing 1 and 2 Mbps DSSS channelization schemes.
CCK consists of a set of sixty-four 8-bit code words. As a set, these code words
have unique mathematical properties that allow them to be correctly distinguished
from one another by a receiver even in the presence of substantial noise and mul-
tipath interference. The 5.5 Mbps rate uses CCK to encode 4 bits per carrier,
while the 11 Mbps rate encodes 8 bits per carrier. Both speeds use QPSK modula-
tion and a signal at 1.375 Msps. This is how the higher data rates are obtained.
Table 21.11 lists the speciﬁ cations.
To support very noisy environments as well as extended ranges, 802.11b
WLANs use dynamic rate shifting, allowing data rates to be automatically adjusted
to compensate for the changing nature of the radio channel. Ideally, users connect
at a full 11 Mbps rate. However, when devices move beyond the optimal range for
11 Mbps operation, or if substantial interference is present, 802.11b devices will
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transmit at lower speeds, falling back to 5.5, 2, and 1 Mbps. Likewise, if a device
moves back within the range of a higher-speed transmission, the connection will
automatically speed up again. Rate shifting is a physical layer mechanism trans-
parent to the user and upper layers of the protocol stack.
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