Understanding IEEE 802.11b Wi-Fi

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IEEE 802.11b was the first WiFi standard to be widely adopted. Using 2.4 GHz the technology was much easier and cheaper to develop than the 802.11a which used the higher frequency 5 GHz band.

802.11b was built in to many laptop computers and other form of equipment and this sealed its success.

It was only after 802.11b was ratified in July 1999 and products became available that Wi-Fi and wireless LAN technology 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 and it can now be seen that this was the real start of when wireless LAN technology started to become a central part of everyday connectivity.

802.11b specification

802.11b boasted an impressive performance for its day. It was able to transfer data with raw data rates up to 11 Mbps, and had a good range, although not when operating at its full data rate.

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. This is the same form of anti-collision methodology that is used within Ethernet.

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.

In this way the possibility of more than one node transmitting in the wireless LAN is very significantly reduced. Any collisions that might happen are resolved using error correction.

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 used 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.

In addition to this, the 3G mobile communications systems were starting to be developed using CDMA, so the technology was being introduced in a number of areas and there was a lot of cross fertilisation across the wireless communications industry.

802.11b data rates

Although 802.11b cards were specified to operate at a basic rate of 11 Mbps, the system monitored the signal quality. If the signal fell or interference levels rose, then it was possible for the system to adopt a slower data rate with more error correction that was more resilient.

When the fall-back modes the system first reduced to a rate of 5.5 Mbps, then 2, and finally 1 Mbps. This scheme was known as Adaptive Rate Selection (ARS). It was effectively a forerunner of many adaptive modulation schemes that are used in virtually all modern wireless networks and wireless communications systems these days.

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 advertised that they supported 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.

IEEE 802.11b was the real success story of the IEEE 802.11 standards. Its widespread adoption meant that Wi-Fi became an accepted element of the wireless connectivity arena. Initially it was used in computer equipment as smartphones were Wi-Fi connectivity within mobile phones were not commonplace. Nevertheless IEEE 802.11b was used in many applications and enabled future versions of the standard to build on its success.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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