IEEE 802.11b

From Hill2dot0
Jump to: navigation, search

This high rate extension to the original 802.11 wireless LAN standard has enabled it to support a raw bit rate of up to 11 Mbps. Unlike the first release of 802.11, which supported infrared, frequency hopping spread spectrum (FHSS), and direct sequence spread spectrum (DSSS), this higher speed version only supports the use of DSSS.

802.11b still uses the 2.4 GHz ISM band but now uses a spreading technique called complementary code keying (CCK). When operating at the 1 and 2 Mbps rate, an 11-chip Barker code, which is a simple binary code, is used to provide the spreading. However, for the high rate extension, a complex code is used that has a phase angle as well as magnitude. Each symbol has a code length of 8 chips and is transmitted at 11 mchips. This yields the same bandwidth utilization as the 1 and 2 Mbps version, except now each symbol represents 8 data bits. Hence, 11 Mbps is achieved.

The distance of the station from the access point also affects the data rate. The raw data rate of 11 Mbps is valid for distances up to 100 feet with typical indoor installation. As the distance increases, the data rate decreases. Beyond 150 feet, 802.11b has fallback rates of 5.5 Mbps, 2 Mbps, and 1 Mbps to a range of about 350 feet.

Although the raw data rate of transmission is 11 Mbps, the throughput is often less due to several issues. One is the CSMA/CA access control, which requires stations to wait a random amount of time before determining that the medium is free to transmit. The second is the Request to Send/Clear to Send (RTS/ CTS) sequence that is used to overcome the hidden node syndrome. A third impact results from the transmission of a preamble and start of frame delimiter, followed by a PLCP header indicating signal, service, length, and header error check. These are all transmitted at 1 Mbps to ensure compatibility with other stations. This PLCP header and preamble means that for a 1500 octet frame it is at best 85 percent efficient and even less efficient for smaller frames. Options in the standard allow for a shorter header—part of which is transmitted at 2 Mbps—which will improve this efficiency. Some vendors can implement this shorter header. This mode is called high rate direct sequence spread spectrum short (HR/DSSS/short). Yet another factor limiting actual throughput is interference in the 2.4 GHz band, which introduces transmission errors. The result is an effective bandwidth in the range of 6 Mbps or even less when interference is present.

802.11b Example

802.11b Example

In 802.11b, we have all of the radio components—they just happen to be real small. There is the transceiver with its connection to the antenna’s amplifier and then there is the antenna itself. In the 802.11b specification the effective radiated power (ERP) is 4 watts.

Taking the system apart, we see that we have a 1 watt transmitter. The antenna and amplifier are configured to give us an 8 dB antenna gain, but the transmission line has a 2 dB loss. This gives us a net gain of 6 dB, which means that we have a 4 times increase in the transceiver power. This means that the 1 watt from the transceiver will be boosted to 4 watts effective radiated power. As an aside, ERP limitations are the way that the FCC attempts to limit the interference among radio systems operating at the same frequency.

PodSnacks

<mp3>http://podcast.hill-vt.com/podsnacks/2007q3/802.11b.mp3%7Cdownload</mp3> | IEEE 802.11b