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IEEE 802.11b — High-Rate DSSS


In September 1999, IEEE ratified 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 PHY support to two new speeds, 5.5 and 11 Mbps. To accomplish this, DSSS was selected as the sole PHY technique for the standard, since frequency hopping (FH) cannot support the higher speeds without violating current FCC regulations. The implication is that the 802.11b system will interoperate with 1 and 2 Mbps 802.11 DSSS systems, but will not work with 1 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 transmitted at a one million symbols per second (Msps) rate using binary phase shift keying (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 efficiency 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 64 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 multipath interference. The 5.5 Mbps rate uses CCK to encode four bits per carrier, while the 11 Mbps rate encodes eight bits per carrier. Both speeds use QPSK modulation and a signal at 1.375 Msps. This is how the higher data rates are obtained.

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 transmit at lower speeds, falling
back to 5.5, 2, and 1 Mbps. Likewise, if a device moves back within the range of a higherspeed transmission, the connection will automatically speed up again. Rate shifting is a PHY mechanism transparent to the user and upper layers of the protocol stack.

Source of Information : Elsevier Wireless Networking Complete


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