802.11n
Background
The 802.11n standard is designed to provide improvements on existing Wi-Fi technologies, for data, voice and video
applications. 802.11n reached Draft 2.0 status in June 2007 and became a published standard in October 2009.
802.11 divides each of the 2.4 and 5GHz bands into channels. For example the 2.4000-2.4835 GHz band is divided into
13 channels each of width 22 MHz but spaced only 5 MHz apart, with channel 1 centered on 2412 MHz and 13 on 2472 MHz.
Regarding security, 802.11n devices are required to support WPA2 and AES encryption. There is a
requirement to backwardly support TKIP due to legacy 802.11a/b/g clients.
Improvements on existing Wi-Fi radio technologies include:
- Throughput - as much as 5 times current levels, data throughput typically 74Mbps, may be up to 248Mbps
- Reliability - requiring fewer packet retries and therefore taking up bandwidth
- Predictability - consistent coverage (up to 70m indoors) and throughput
- Compatibility - backwards support for 802.11a/b/g, 802.11n uses both 2.4 and 5GHz frequency bands
In order to achieve these improvements there are three main components that have been introduced in 802.11n:
- Multiple Input Multiple Output (MIMO) - multiple radios
- Packet Aggregation - at the MAC layer multiple packets may be aggregated in a single stream
- 40MHz Channels - the normal 20MHz channels may be combined to make 40MHz if they are next to each other
in the frequency spectrum
Multiple Input Multiple Output (MIMO)
There are three technologies that work within MIMO these are described below:
Maximal Ratio Combining
Most environments create multiple paths that the radio signal traverses as it bounces around reflective surfaces.
As a result the actual true signal becomes compromised somewhat by the weaker signals that suffer from propagation
delay. The radio receiver on the MIMO AP uses Maximal Ratio Combining on its 3 antennae to take advantage of the multiple signals
that each carry an identical copy of the data,
by combining the received signals and performing algorithms that increase the sensitivity to the received signal.
A non-802.11n client is able to benefit from this as well as an 802.11n client.
Transmit Beam Forming
The transmitter on a MIMO AP is able to adjust the transmitted signal by modifying the transmitted beam
from each of its antenna according to the reflective environment. This ensures an improved in-phase signal
at the client antennae. This improves receive sensitivity for 802.11n and non-802.11n clients.
Spatial Multiplexing
Multiple antennae combine to transmit the same data across all of them, this therefore increases the bandwidth
available, however it requires the client to have multiple antennae and be 802.11n compliant.
40MHz channels
802.11n supports both the traditional 20MHz and 40MHz channels. This applies to the AP and doubles
the bandwidth for transmission, however it has no bearing on available bandwidth in the spectrum as a whole
as this remains constant.
Packet Aggregation
Rather than have a header for each data unit, it is possible now to aggregate data units under one header
which in turn releases a little more bandwidth.
For a 1500 Byte frame being transmitted at 300Mbps on 802.11n
this would take 220�s which is a vast improvement on 360�s when running at 54Mbps. There is a problem however
in that small packets e.g. 64 Byte frames take 181�s at 300Mbps rather than 145�s at 54Mbps.
This is due to the increase header size present in 802.11n and will have a bearing on Voice over Wi-Fi
deployments.
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