IEEE 802.11b

- an overview or tutorial about the 802.11b the Wi-Fi standard providing data rates of 11 Mbps at 2.4 GHz

IEEE 802.11b was the first wireless LAN standard to be widely adopted and built in to many laptop computers and other forms of equipment. The standard for 802.11b was ratified by the IEEE in July 1999 and the idea for wireless networking quickly caught on with many W-Fi hotspots being set up so that business people could access their emails and surf the Internet as required when they were travelling.

It was only after 802.11 was ratified and products became available that W-Fi took off in a large way. Wi-Fi hotspots were set up in many offices, hotels and airports and the idea of using portable laptop computers while travelling became far easier.

Although the IEEE 802.11a standard was introduced at the same time, it did not catch on in the same way even though it was capable of higher speeds. The main reason for this was that it operated in the 5 GHz ISM band rather than the 2.4 GHz of 802.11b, and this made it more expensive.


802.11b specification

802.11b boasts an impressive performance. It is able to transfer data with raw data rates up to 11 Mbps, and has a good range, although not when operating at its full data rate.

Parameter Value
Date of standard approval July 1999
Maximum data rate (Mbps) 11
Typical data rate (Mbps) 5
Typical range indoors (Metres) ~30
Modulation CCK (DSSS)
RF Band (GHz) 2.4
Channel width (MHz) 20

Summary of 802.11b Wi-Fi Standard Specification

When transmitting data 802.11b uses the CSMA/CA technique that was defined in the original 802.11 base standard and retained for 802.11b. Using this technique, when a node wants to make a transmission it listens for a clear channel and then transmits. It then listens for an acknowledgement and if it does not receive one it backs off a random amount of time, assuming another transmission caused interference, and then listens for a clear channel and then retransmits the data.


RF modulation for 802.11b

The RF signal format used for 802.11b is CCK or complementary Code Keying. This is a slight variation on CDMA (Code Division Multiple Access) that uses the basic DSSS (Direct Sequence Spread Spectrum) as its basis. In view of the fact that the original 802.11 specification use CDMA / DSSS, it was easy to upgrade any existing chipset and other investment to provide the new 802.11b standard. As a result 802.11b chipsets appeared relatively quickly onto the market.


802.11b data rates

Although 802.11b cards are specified to operate at a basic rate of 11 Mbps, the system monitors the signal quality. If the signal falls or interference levels rise, then it is possible for the system to adopt a slower data rate with more error correction that is more resilient. Under these conditions the system will first fall back to a rate of 5.5 Mbps, then 2, and finally 1 Mbps. This scheme is known as Adaptive rate Selection (ARS).

Although the basic raw data rates for transmitting data seem very good, in reality the actual data rates achieved over a real time network are much smaller. Even under reasonably good radio conditions, i.e. good signal and low interference the maximum data rate that might be expected when the system uses TCP is about 5.9 Mbps. This results from a number of factors. One is the use of CSMA/CA where the system has to wait for clear times on a channel to transmit and another is associated with the use of TCP and the additional overhead required. If UDP is used rather than TCP then the data rate can increase to around 7.1 Mbps.

Some 802.11b systems advertise that they support much higher data rates than the basic 802.11b standard specifies. While more recent versions of the 802.11 standard, namely 802.11g, and 802.11n specify much higher speeds, some proprietary improvements were made to 802.11b. These proprietary improvements offered speeds of 22, 33, or 44 Mbps and were sometimes labelled as "802.11b+". These schemes were not endorsed by the IEEE and in any case they have been superseded by later versions of the 802.11 standard.

By Ian Poole


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