IEEE 802.11 n Standard

- an overview or tutorial about the IEEE 802.11 n standard, the new Wi-Fi standard providing increased data rates

This IEEE 802.11 standard for WLANs tutorial is split into several pages each of which addresses different aspects of IEE 802.11 / Wi-Fi operation and technology:

Once Wi-Fi standards including 802.11a, 802.11b, and 802.11g were established, work commenced on looking at how the raw data speeds provided by Wi-Fi, 802.11 networks could be increased still further. The result was that in January 2004, the IEEE announced that it had formed a new committee to develop the new high speed, IEEE 802.11 n standard.

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. The draft is expected to be finalized in November 2008 with its formal publication in July 2009. However many products are already available on the market. Manufacturers are now releasing products based on the early or draft versions of the specifications assuming that the changes will only be minor in their scope.

With the improved performance offered by 802.11n, the standard soon became widespread with many products offered for sale and use. Although initially few Wi-Fi hotspots offered the standard, 802.11n devices were compatible and able to work with the 802.11b and 802.11g based hotspots.

Basic specification for the IEEE 802.11 n standard

The idea behind the IEEE 802.11 n standard was that it would be able to provide much better performance and be able to keep pace with the rapidly growing speeds provided by technologies such as Ethernet. The new 802.11 n standard boasts an impressive performance, the main points of which are summarized below:

To achieve this a number of new features that have been incorporated into the IEEE 802.11n standard to enable the higher performance. The major innovations are summarized below:

• Changes to implementation of OFDM
• Introduction of MIMO
• MIMO power saving
• Wider channel bandwidth
• Antenna technology
• Reduced support for backward compatibility under special circumstances to improve data throughput

Although each of these new innovations adds complexity to the system, much of this can be incorporated into the chipsets, enabling a large amount of the cost increase to be absorbed by the large production runs of the chipsets.

OFDM implementation:     It has been necessary to change the way in which the OFDM modulation scheme is implemented to improve the data throughput of the single signal path. By adapting the way it is set-up, the data rate can be increased from the 54 Mbps data rate achieved for 802.11a and g to 65 Mbps.

Use of MIMO in IEEE 802.11 n:     MIMO or Multiple Input Multiple Output is a technique that exploits multipath propagation. Normally when a signal is transmitted from A to B the signal will reach the receiving antenna via multiple paths, causing interference. MIMO uses this multipath propagation to increase the data rate by using a technique known as spatial division multiplexing. The data is split into a number of what are termed spatial streams and these are transmitted through separate antennas to corresponding antennas at the receiver. Doubling the number of spatial streams doubles the raw data rate, enabling a far greater utilization of the available bandwidth. The current 802.11n standard allows for up to four spatial streams.

IEEE 802.11 n power saving:     One of the problems with using MIMO is that it increases the power of the hardware circuitry. More transmitters and receivers need to be supported and this entails the use of more current. While it is not possible to eliminate the power increase resulting from the use of MIMO in 802.11n, it is possible to make the most efficient use of it. Data is normally transmitted in a "bursty" fashion. This means that there are long periods when the system remains idle or running at a very slow speed. During these periods when MIMO is not required, the circuitry can be held inactive so that it does not consume power.

Increased bandwidth:     An optional mode for the new 802.11 chips is to run using a double sized channel bandwidth. Previous systems used 20 MHz bandwidth, but the new ones have the option of using 40 MHz. The main trade-off for this is that there are less channels that can be used for other devices. There is sufficient room at 2.4 GHz for three 20 MHz channels, but only one 40 MHz channel can be accommodated. Thus the choice of whether to use 20 or 40 MHz has to be made dynamically by the devices in the net.

Antenna technology for 802.11n:     For 802.11n, the antenna associated technologies have been significantly improved by the introduction of beam forming and diversity.

Beam forming focuses the radio signals directly along the path for the receiving antenna to improve the range and overall performance. A higher signal level and better signal to noise ratio will mean that the full use can be made of the channel.

Diversity uses the multiple antennas available and combines or selects the best subset from a larger number of antennas to obtain the optimum signal conditions. This can be achieved because there are often surplus antennas in a MIMO system. As 802.11n supports any number of antennas between one and four, it is possible that one device may have three antennas while another with which it is communicating will only have two. The supposedly surplus antenna can be used to provide diversity reception or transmission as appropriate.

Backward compatibility switching:     While 802.11n provides backward compatibility for devices in a net using earlier versions of 802.11, this adds a significant overhead to any exchanges, thereby reducing the data transfer capacity. To provide the maximum data transfer speeds when all devices in the net at to the 802.11n standard, the backwards compatibility feature can be removed. When earlier devices enter the net, the backward compatibility overhead and features are re-introduced. As with 802.11g, when earlier devices enter a net, the operation of the whole net is considerably slowed. Therefore operating a net in 802.11n only mode offers considerable advantages.

802.11n Access Point operational modes

In view of the features associated with backward compatibility, there are three modes in which an 802.11n access point can operate:

• Greenfield (only 802.11 n) - maximum performance
• Mixed (both 802.11 a, b, g, and n)
• Legacy (only 802.11 a, b, and g)

IEEE 802.11 n summary

The new IEEE 802.11 n standard provides a major improvement in the speed at which data can be transferred over a wireless network. While this may not be needed for many small networks where small files are being transferred, the amount of data being passed over most networks is increasing with many more large files, including photos, video clips (and videos), etc. being transferred. With the levels of data only set to increase, the new 802.11n standard will be able to meet the challenge of providing the required capacity for wireless or Wi-Fi networks.

Further pages from this tutorial
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