IEEE 802.11n-2009


IEEE 802.11n-2009, or 802.11n, is a wireless-networking standard that uses multiple antennas to increase data rates. The Wi-Fi Alliance has also retroactively labelled the technology for the standard as Wi-Fi 4. It standardized support for multiple-input multiple-output, frame aggregation, and security improvements, among other features, and can be used in the 2.4 GHz or 5 GHz frequency bands.
Being the first Wi-Fi standard to introduce MIMO support, devices and systems which supported the 802.11n standard were sometimes referred to as MIMO Wi-Fi products, especially prior to the introduction of the next generation standard. The use of MIMO-OFDM to increase the data rate while maintaining the same spectrum as 802.11a was first demonstrated by Airgo Networks.
The purpose of the standard is to improve network throughput over the two previous standards—802.11a and 802.11g—with a significant increase in the maximum net data rate from 54 Mbit/s to 72 Mbit/s with a single spatial stream in a 20 MHz channel, and 600 Mbit/s with the use of four spatial streams at a channel width of 40 MHz.
IEEE 802.11n-2009 is an amendment to the IEEE 802.11-2007 wireless-networking standard. 802.11 is a set of IEEE standards that govern wireless networking transmission methods. They are commonly used today in their 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac and 802.11ax versions to provide wireless connectivity in homes and businesses. Development of 802.11n began in 2002, seven years before publication. The 802.11n protocol is now Clause 20 of the published IEEE 802.11-2012 standard and subsequently renamed to clause 19 of the published IEEE 802.11-2020 standard.

Description

IEEE 802.11n is an amendment to IEEE 802.11-2007 as amended by IEEE 802.11k-2008, IEEE 802.11r-2008, IEEE 802.11y-2008, and IEEE 802.11w-2009, and builds on previous 802.11 standards by adding a multiple-input multiple-output system and 40 MHz channels to the PHY and frame aggregation to the MAC layer. There were older proprietary implementations of MIMO and 40MHz channels such as Xpress, Super G and Nitro which were based upon 802.11g and 802.11a technology, but this was the first time it was standardized across all radio manufacturers.
MIMO is a technology that uses multiple antennas to coherently resolve more information than possible using a single antenna. One way it provides this is through spatial division multiplexing, which spatially multiplexes multiple independent data streams, transferred simultaneously within one spectral channel of bandwidth. MIMO SDM can significantly increase data throughput as the number of resolved spatial data streams is increased. Each spatial stream requires a discrete antenna at both the transmitter and the receiver. In addition, MIMO technology requires a separate radio-frequency chain and analog-to-digital converter for each antenna, making it more expensive to implement than non-MIMO systems.
Channels operating with a width of 40 MHz are another feature incorporated into 802.11n; this doubles the channel width from 20 MHz in previous 802.11 PHYs to transmit data, and provides twice the PHY data rate available over a single 20 MHz channel. It can be enabled in the 5 GHz mode, or within the 2.4 GHz mode if there is knowledge that it will not interfere with any other 802.11 or non-802.11 system using the same frequencies. The MIMO architecture, together with the wider channels, offers increased physical transfer rate over standard 802.11a and 802.11g.

Data encoding

The transmitter and receiver use precoding and postcoding techniques, respectively, to achieve the capacity of a MIMO link. Precoding includes spatial beamforming and spatial coding, where spatial beamforming improves the received signal quality at the decoding stage. Spatial coding can increase data throughput via spatial multiplexing and increase range by exploiting the spatial diversity, through techniques such as Alamouti coding.

Numbers of antennas and data streams

The number of simultaneous data streams is limited by the minimum number of antennas in use on both sides of the link. However, the individual radios often further limit the number of spatial streams that may carry unique data. The notation helps identify what a given radio is capable of. The first number is the maximum number of transmit antennas or transmitting TF chains that can be used by the radio. The second number is the maximum number of receive antennas or receiving RF chains that can be used by the radio. The third number is the maximum number of data spatial streams the radio can use. For example, a radio that can transmit on two antennas and receive on three, but can only send or receive two data streams, would be
The 802.11n draft allows up to Common configurations of 11n devices are,, and. All three configurations have the same maximum throughputs and features, and differ only in the amount of diversity the antenna systems provide. In addition, a fourth configuration, is becoming common, which has a higher throughput, due to the additional data stream.

Data rates

Assuming equal operating parameters to an 802.11g network achieving 54 megabits per second, an 802.11n network can achieve 72 megabits per second ; 802.11n's speed may go up to 150 megabits per second if there are not other Bluetooth, microwave or Wi-Fi emissions in the neighborhood by using two 20 MHz channels in 40 MHz mode. If more antennas are used, then 802.11n can go up to 288 megabits per second in 20 MHz mode with four antennas, or 600 megabits per second in 40 MHz mode with four antennas and 400 ns guard interval. Because the 2.4 GHz band is seriously congested in most urban areas, 802.11n networks usually have more success in increasing data rate by utilizing more antennas in 20 MHz mode rather than by operating in the 40 MHz mode, as the 40 MHz mode requires a relatively free radio spectrum which is only available in rural areas away from cities. Thus, network engineers installing an 802.11n network should strive to select routers and wireless clients with the most antennas possible and try to make sure that the network's bandwidth will be satisfactory even on the 20 MHz mode.
Data rates up to 600 Mbit/s are achieved only with the maximum of four spatial streams using one 40 MHz-wide channel. Various modulation schemes and coding rates are defined by the standard, which also assigns an arbitrary number to each; this number is the modulation and coding scheme index, or MCS index. The table below shows the relationships between the variables that allow for the maximum data rate. GI : Timing between symbols.
20 MHz channel uses an FFT of 64, of which: 56 OFDM subcarriers, 52 are for data and 4 are pilot tones with a carrier separation of 0.3125 MHz . Each of these subcarriers can be a BPSK, QPSK, 16-QAM or 64-QAM. The total bandwidth is 20 MHz with an occupied bandwidth of 17.8 MHz. Total symbol duration is 3.6 or 4 microseconds, which includes a guard interval of 0.4 or 0.8 microseconds.

Frame aggregation

PHY level data rate does not match user level throughput because of 802.11 protocol overheads, like the contention process, interframe spacing, PHY level headers and acknowledgment frames. The main media access control feature that provides a performance improvement is aggregation. Two types of aggregation are defined:
  1. Aggregation of MAC service data units at the top of the MAC
  2. Aggregation of MAC protocol data units at the bottom of the MAC
Frame aggregation is a process of packing multiple MSDUs or MPDUs together to reduce the overheads and average them over multiple frames, thereby increasing the user level data rate. A-MPDU aggregation requires the use of block acknowledgement or BlockAck, which was introduced in 802.11e and has been optimized in 802.11n.

Backward compatibility

When 802.11g was released to share the band with existing 802.11b devices, it provided ways of ensuring backward compatibility between legacy and successor devices. 802.11n extends the coexistence management to protect its transmissions from legacy devices, which include 802.11g, 802.11b and 802.11a. There are MAC and PHY level protection mechanisms as listed below:
  1. PHY level protection: Mixed Mode Format protection : In mixed mode, each 802.11n transmission is always embedded in an 802.11a or 802.11g transmission. For 20 MHz transmissions, this embedding takes care of the protection with 802.11a and 802.11g. However, 802.11b devices still need CTS protection.
  2. PHY level protection: Transmissions using a 40 MHz channel in the presence of 802.11a or 802.11g clients require using CTS protection on both 20 MHz halves of the 40 MHz channel, to prevent interference with legacy devices.
  3. MAC level protection: An RTS/CTS frame exchange or CTS frame transmission at legacy rates can be used to protect subsequent 11n transmission.

    Deployment strategies

To achieve maximum output, a pure 802.11n 5 GHz network is recommended. The 5 GHz band has substantial capacity due to many non-overlapping radio channels and less radio interference as compared to the 2.4 GHz band. An 802.11n-only network may be impractical for many users because they need to support legacy equipment that still is 802.11b/g only. In a mixed-mode system, an optimal solution would be to use a dual-radio access point and place the 802.11b/g traffic on the 2.4 GHz radio and the 802.11n traffic on the 5 GHz radio. This setup assumes that all the 802.11n clients are 5 GHz capable, which is not a requirement of the standard. 5 GHz is optional on Wi-Fi 4; quite some Wi-Fi 4 capable devices only support 2.4 GHz and there is no practical way to upgrade them to support 5 GHz.

Band steering

Some enterprise-grade APs use band steering to send 802.11n clients to the 5 GHz band, leaving the 2.4 GHz band for legacy clients. Band steering works by responding only to 5 GHz association requests and not the 2.4 GHz requests from dual-band clients.