| IEEE 802.11 n Standard |
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
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.
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:
| Parameter |
IEEE 802.11 n Standard |
| Date of standard approval |
Anticipated Nov 2008 |
| Maximum data rate (Mbps) |
248 |
| Typical throughput (Mbps) |
74 |
| RF Band (GHz) |
2.4 or 5 |
| Modulation |
CCK, DSSS, or OFDM |
| Number of spatial streams |
1, 2, 3, or 4 |
| Channel width (MHz) |
20, or 40 |
IEEE 802.11n standard salient features
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.
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