Wireless Local Area Networks

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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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