IEEE 802.11b
- an overview or tutorial about the 802.11b the new 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.
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.
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