802.11N Features

IEEE 802.11n is an amendment to the IEEE 802.11-2007 wireless networking standard to improve network throughput over the two previous standards — 802.11a and 802.11g — with a significant increase in the maximum raw data rate from 54 Mbit/s to 600 Mbit/s with the use of four spatial streams at a channel width of 40 MHz.

There are various factors that make 802.11n more effective as compared to older 802.11a/g network deployments.

MIMO – Multiple Input and Multiple Output

Unlike traditional 802.11a/b/g radios, which use single-input and single-output (SISO), 802.11n radios use MIMO technology to increase throughput by increasing the number of radio transmit and receive chains.

An AP or client may have up to four transmit and four receive chains, and it is possible to have a different number of transmit vs. receive chains.
For e.g.- What does a 4×4:3 for a wireless station represents,
This basically means, the device has 4 Transmitters, 4 Receivers and is capable of sending/receiving 3 individual streams of data (3 spacial Streams)

Many 802.11a/b/g APs have two antennas, they are not capable of using both antennas at the same time. Instead, the two antennas provide diversity. Each antenna receives a different receive signal strength and the AP selects the strongest one to use for each reception. To send a signal, typically the AP uses the antenna that was last used to receive a signal.

What are Spatial streams ?

Spatial Stream is a unique/independent steam of data. It’s the ability to transmit and receive single or multiple streams of data, by efficiently utilizing two or more transmitter and receiver radios (like adding additional lanes to a road).

Multiple spatial streams will allow the wireless AP to transmit more data simultaneously.

The advantage of MIMO and spatial streams is that the APs can use multipath transmissions to its advantage.
SISO systems see performance degradation due to multipath transmissions because the multipath may add to signal degradation.
However, 11n APs use multipath transmission to reach their full speeds. The delay in the propagation of paths at different rates allows MIMO and spatial streams to be received correctly at the other end of the transmission link.
In a SISO system, that delay can cause interference.

Multiple antennas are needed to transmit and receive multiple spatial streams. Depending on hardware, an AP or client can transmit or receive spatial streams equal to the number of antennas it has or the AP may have more antennas than the spatial streams used.

40 MHz Channels

802.11a/b/g deployments used to work on 20 MHz channels, with each AP set to a single, non-overlapping channel.
With 802.11n, two channels can be bonded, which actually more than doubles the bandwidth because the guard channels in between also are used.

Below figure shows the difference in width for a 40 MHz spectral mask as opposed to the 20 MHz mask originally specified for 802.11 transmissions.

Image from – CWAP Study Guide

In the 5 GHz band, multiple 40 MHz channels are available, and depending on the regulatory domain, additional channels are available with dynamic frequency selection (DFS) enabled.

The limited number of channels in the 2.4 GHz band makes 40 MHz channels unsuitable for use. The 2.4 GHz band has only three 20 MHz non-overlapping channels available in most regulatory domains. If a single 40 MHz channel is deployed in the 2.4 GHz band, the channel covers two of the three usable channels.

This is where an AP advertises its 40 MHz capabilities in a beacon frame.

Improved OFDM Subcarriers

Orthogonal frequency-division multiplexing (OFDM) is the encoding scheme that is used in Wi-Fi transmissions.
OFDM splits a single channel into very small subcarriers that can transport independent pieces of data as symbols.
Each symbol represents some amount of data, which depends on the encoding scheme.
The data subcarrier count has increased from the original 48 to 52 subcarriers in 20 MHz channels and 108 subcarriers in 40 MHz channels.
This increase means that each additional subcarrier can carry more data over the channel, which increases throughput.

Here you can see the difference in sub-carriers that 802.11n brings to 20 MHz channels, as well as the number of carriers available with 40 MHz channels.

Short Guard Interval (SGI)

What is delay spread ?

A signal when transmitted from transmission antenna, may take different paths.
Each path may lead to different travel distance if the signal get reflected by one or more obstacles on the way to the receiver antenna. This is called multipath.
Different paths may have different physical properties of propagation media, due to which the signal traveling through different path would arrive at the receiver antenna at different times.
So, if you send a signal from a transmitter antenna and measure the arrival time at the receiver antenna which is a certain distance away from the transmitter antenna, you would get multiple different arrival timing.
If these time stamps are calculated on time axis, the variations of those values are referred as Delay Spread.

The guard interval is the spacing between OFDM transmissions from a client. This interval prevents frames that are taking a longer path from colliding with subsequent transmissions that are taking a shorter path.

A shorter OFDM guard interval between frames, from 800 ns to 400 ns, means that transmissions can begin sooner in environments where the delay between frames is low and this can increase the data rate by 11 percent.

SGI and LGI usage,
Using SGI is ideal for smaller enclosed environment, but LGI (Long Guard Interval) is recommended in larger environments (like warehouses) due to presence of highly reflective surrounding like metals. So, by increasing the delay in consecutive symbols, it reduces the chance of the signals getting destroyed at the receiving end.

Aggregate MAC Service Data Unit (A-MSDU)

A-MSDU allows stations that have multiple packets to send to a single destination address and application by combining those frames into a single MAC frame. When these frames are combined, less overhead is created and less airtime is spent on transmissions and acknowledgements.

A-MSDU was first introduced in 802.11n networks and is build by combining multiple MSDU’s known as A-MSDU subframes.

A-MSDU has a maximum packet size of 7935 bytes.

Image taken from Aruba 802.11n Validated Reference Guide


With A-MSDU, the maximum frame body size is determined by the maximum A-MSDU size of 3,839 or 7,935 octets, depending upon the STA’s capability, plus any overhead from encryption.

Aruba MPDU configuration:

Aggregate MAC Protocol Data Unit (A-MPDU)

A-MPDU combines multiple packets that are destined for the same address but different applications into a single wireless transmission.
A-MPDU is not as efficient as A-MSDU, but the airtime and overhead is reduced. The maximum packet size is 65535 bytes.

Figure 10 shows the operation of A-MPDU operation

Image taken from Aruba 802.11n Validated Reference Guide

Block Acknowledgement

As we know, the wireless transmissions need acknowledgement to confirm the arrival of wireless frames and since these requirements for ACK are mandated for every frame, it involves in a lot of unwanted airtime and throughput reduction.

To improve this functionality, Block acknowledgement mechanism was introduced, with which the recipient can reply with a single acknowledgement after receiving a set of transmissions (such as aggregated MPDU’s),

Block acknowledgement (BA) was initially defined in IEEE 802.11e as an optional scheme to improve the MAC efficiency. 
802.11n amendment ratified in 2009 enhances this BA mechanism then made it as mandatory to support by all 802.11n-capable devices

Block acknowledgements also can be used to acknowledge a number of frames from the same client that are not aggregated. One acknowledgement for a set of frames consumes less airtime.

Detailed information on Block Acknowledgment here

This is an overall view of how 802.11n with combining multiple technologies improves the throughput and wireless user experience.

Image from – Aruba 802.11n VRD

References:

Aruba 802.11n Validated Reference Guide

CWAP Study Guide

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