| IEEE 802.11 Wi-Fi tutorial |
IEEE 802.11 Wi-Fi tutorial
- an overview or tutorial about the 802.11 series of standards for Wi-Fi and
WLAN applications
Mobile connectivity for computers is a rapidly growing
requirement. Of the schemes that are available the IEEE 802.11 standard, often
termed Wi-Fi has become the de-facto standard. With peak operating speeds of
around 54 Mbps, and soon to be more, Wi-Fi is able to compete with many wired
systems. As a result of the flexibility and performance of the system, many
Wi-Fi "hotpots" have been set up and more are following. These enable people to
use their laptop computers as they wait in hotels, airport lounges, cafes, and
many other places using a wire-less link rather than needing to use a cable. In
view of its convenience, 802.11, Wi-Fi is now incorporated into virtually all
new laptop computers.
Standards
There is a plethora of standards under the IEEE 802 LMSC (LAN / MAN Standards
Committee). Of these even 802.11 has a variety of standards, each with a letter
suffix. These cover everything from the wireless standards themselves, to
standards for security aspects, quality of service and the like:
- 802.11a - Wireless network bearer operating in the 5 GHz ISM band with
data rate up to 54 Mbps
- 802.11b - Wireless network bearer operating in the 2.4 GHz ISM band with
data rates up to 11 Mbps
- 802.11e - Quality of service and prioritisation
- 802.11f - Handover
- 802.11g - Wireless network bearer operating in 2.4 GHz ISM band with
data rates up to 54 Mbps
- 802.11h - Power control
- 802.11i - Authentication and encryption
- 802.11j - Interworking
- 802.11k - Measurement reporting
- 802.11n - Wireless network bearer operating in the 2.4 and 5 GHz ISM
bands with data rates up to 600 Mbps
- 802.11s - Mesh networking
Of these the standards that are most widely known are the network bearer
standards, 802.11a, 802.11b, and 802.11g.
Network bearer standards
All the 802.11 Wi-Fi standards operate within the ISM (Industrial, Scientific
and Medical) frequency bands. These are shared by a variety of other users, but
no license is required for operation within these frequencies. This makes them
ideal for a general system for widespread use.
There are a number of bearer standards that are in common
use. These are the 802.11a, 802.11b, and 802.11g standards. Shortly the new
802.11n standard will be ratified and it is expected products will quickly
become available for this.
Each of the different standards has different features and
they were launched at different times. The first accepted 802.11 WLAN standard
was 802.11b. This used frequencies in the 2.4 GHz Industrial Scientific and
Medial (ISM) frequency band, this offered raw, over the air data rates of 11
Mbps using a modulation scheme known as Complementary Code Keying (CCK) as well
as supporting Direct-Sequence Spread Spectrum, or DSSS, from the original 802.11
specification. Almost in parallel with this a second standard was defined. This
was 802.11a which used a different modulation technique, Orthogonal Frequency
Division Multiplexing (OFDM) and used the 5 GHz ISM band. Of the two standards
it was the 802.11b variant that caught on. This was primarily because the chips
for the lower 2.4 GHz band were easier and cheaper to manufacture.
The 802.11b standard became the main Wi-Fi standard. Looking
to increase the speeds, another standard, 802.11g was introduced and ratified in
June 2003. Using the more popular 2.4 GHz band and OFDM, it offered raw data
rates of 54 Mbps, the same as 802.11b. In addition to this, it offered backward
compatibility to 802.11b. Even before the standard was ratified, many vendors
were offering chipsets for the new standard, and today the vast majority of
computer networking that is shipped uses 802.11g.
Then in January 2004, the IEEE announced it had formed a new
committee to develop an even higher speed standard. With much of the work now
complete, 802.11n is beginning to establish itself in the same way as 802.11g.
The industry came to a substantive agreement about the features for 802.11n in
early 2006. This gave many chip manufacturers sufficient information to get
their developments under way. As a result it is anticipated that before long,
with ratification of 802.11n expected in 2007, that some cards and routers will
find their way into the stores.
| |
802.11a |
802.11b |
802.11g |
802.11n |
| Date of standard approval |
July 1999 |
July 1999 |
June 2003 |
Not yet ratified |
| Maximum data rate (Mbps) |
54 |
11 |
54 |
~600 |
| Modulation |
OFDM |
CCK or DSSS |
CCK, DSSS, or OFDM |
CCK, DSSS, or OFDM |
| RF Band (GHz) |
5 |
2.4 |
2.4 |
2.4 or 5 |
| Number of spatial streams |
1 |
1 |
1 |
1, 2, 3, or 4 |
| Channel width (MHz) |
20 |
20 |
20 |
20, or 40 |
Summary of 802.11 Wi-Fi Standards
Networks
There are two types of network that can be formed: infrastructure networks; and
ad-hoc networks.
The infrastructure application is aimed at office areas or to
provide a "hotspot". It can be installed instead of a wired system, and can
provide considerable cost savings, especially when used in established offices.
A backbone wired network is still required and is connected to a server. The
wireless network is then split up into a number of cells, each serviced by a
base station or Access Point (AP) which acts as a controller for the cell. Each
Access Point may have a range of between 30 and 300 metres dependent upon the
environment and the location of the Access Point.
The other type of network that may be used is termed an
Ad-Hoc network. These are formed when a number of computers and peripherals are
brought together. They may be needed when several people come together and need
to share data or if they need to access a printer without the need for having to
use wire connections. In this situation the users only communicate with each
other and not with a larger wired network. As a result there is no Access Point
and special algorithms within the protocols are used to enable one of the
peripherals to take over the role of master to control the network with the
others acting as slaves.
Summary
Wi-Fi has established itself in a number areas for networking laptop computers.
Although it is not possible to consistently achieve the maximum data rates, and
802.11 is affected by interference, it is nevertheless a particularly useful
technology. This is being proved by its rapidly increasing popularity.
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