IEEE 802.11n-2009

Wireless networking standard in the 802.11 family

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. As the first Wi Fi standard that introduced MIMO (multiple input and multiple output) support, sometimes devices/systems that support 802.11n standard (or draft version of the standard) are being referred to as MIMO (Wi Fi products), especially before the introduction of the next generation standard. The use of MIMO OFDM (orthogonal frequency division multiplexing) 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 (slightly higher gross bit rate including for example error correction codes, and slightly lower maximum throughput) 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.

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 (MIMO) system and 40 MHz channels to the PHY (physical layer) 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 (SDM), 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 (such as Bluetooth) system using the same frequencies. The MIMO architecture, together with the wider channels, offers increased physical transfer rate over standard 802.11a (5 GHz) and 802.11g (2.4 GHz).

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.

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 (a) is the maximum number of transmit antennas or transmitting TF chains that can be used by the radio. The second number (b) is the maximum number of receive antennas or receiving RF chains that can be used by the radio. The third number (c) 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 2 × 3 : 2. The 802.11n draft allows up to 4 × 4 : 4. Common configurations of 11n devices are 2 × 2 : 2, 2 × 3 : 2, and 3 × 2 : 2. 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, 3 × 3 : 3 is becoming common, which has a higher throughput, due to the additional data stream.

Assuming equal operating parameters to an 802.11g network achieving 54 megabits per second (on a single 20 MHz channel with one antenna), an 802.11n network can achieve 72 megabits per second (on a single 20 MHz channel with one antenna and 400 ns guard interval); 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 (one, two, three or four as specified by the 802.11n standard) 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 (Guard Interval): 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 (20 MHz/64) (3.2 μs). 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 (also known as short guard interval (SGI)) or 0.8 microseconds. +Modulation and coding schemesMCSindexSpatialstreamsModulationtypeCodingrateData rate (Mbit/s)20 MHz channel 40 MHz channel800 ns GI 400 ns GI 800 ns GI 400 ns GI 0 1 BPSK 1/2 6.5 7.2 13.5 15 1 1 QPSK 1/2 13 14.4 27 30 2 1 QPSK 3/4 19.5 21.7 40.5 45 3 1 16 QAM 1/2 26 28.9 54 60 4 1 16 QAM 3/4 39 43.3 81 90 5 1 64 QAM 2/3 52 57.8 108 120 6 1 64 QAM 3/4 58.5 65 121.5 135 7 1 64 QAM 5/6 65 72.2 135 150 8 2 BPSK 1/2 13 14.4 27 30 9 2 QPSK 1/2 26 28.9 54 60 10 2 QPSK 3/4 39 43.3 81 90 11 2 16 QAM 1/2 52 57.8 108 120 12 2 16 QAM 3/4 78 86.7 162 180 13 2 64 QAM 2/3 104 115.6 216 240 14 2 64 QAM 3/4 117 130 243 270 15 2 64 QAM 5/6 130 144.4 270 300 16 3 BPSK 1/2 19.5 21.7 40.5 45 17 3 QPSK 1/2 39 43.3 81 90 18 3 QPSK 3/4 58.5 65 121.5 135 19 3 16 QAM 1/2 78 86.7 162 180 20 3 16 QAM 3/4 117 130 243 270 21 3 64 QAM 2/3 156 173.3 324 360 22 3 64 QAM 3/4 175.5 195 364.5 405 23 3 64 QAM 5/6 195 216.7 405 450 24 4 BPSK 1/2 26 28.8 54 60 25 4 QPSK 1/2 52 57.6 108 120 26 4 QPSK 3/4 78 86.8 162 180 27 4 16 QAM 1/2 104 115.6 216 240 28 4 16 QAM 3/4 156 173.2 324 360 29 4 64 QAM 2/3 208 231.2 432 480 30 4 64 QAM 3/4 234 260 486 540 31 4 64 QAM 5/6 260 288.8 540 600 32 1 BPSK 1/4 6.0 6.7 33 – 38 2 Asymmetric mod. 39 – 52 3 Asymmetric mod. 53 – 76 4 Asymmetric mod. 77 – 127

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:

PHY level protection: Mixed Mode Format protection (also known as L SIG TXOP 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. 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. MAC level protection: An RTS/CTS frame exchange or CTS frame transmission at legacy rates can be used to protect subsequent 11n transmission.

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 are only support the 2.4 GHz and there is no practical way to upgrade them to support 5 GHz. 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.

The 2.4 GHz ISM band is fairly congested. With 802.11n, there is the option to double the bandwidth per channel to 40 MHz (fat channel) which results in slightly more than double the data rate. However, in North America, when in 2.4 GHz, enabling this option takes up to 82% of the unlicensed band. For example, channel 3 SCA (secondary channel above), also known as 3+7, reserves the first 9 out of the 11 channels available. In Europe and other places where channels 1–13 are available, allocating 1+5 uses slightly more than 50% of the channels, but the overlap with 9+13 is not usually significant as it lies at the edges of the bands, and so two 40 MHz bands typically work unless the transmitters are physically very closely spaced. The specification calls for requiring one primary 20 MHz channel as well as a secondary adjacent channel spaced ±20 MHz away. The primary channel is used for communications with clients incapable of 40 MHz mode. When in 40 MHz mode, the center frequency is actually the mean of the primary and secondary channels. Primarychannel 20 MHz 40 MHz above 40 MHz below Blocks 2nd ch. Center Blocks 2nd ch. Center Blocks 1 1–3 5 3 1–7 colspan=3 2 1–4 6 4 1–8 colspan=3 3 1–5 7 5 1–9 colspan=3 4 2–6 8 6 2–10 colspan=3 5 3–7 9 7 3–11 1 3 1–7 6 4–8 10 8 4–12 2 4 1–8 7 5–9 11 9 5–13 3 5 1–9 8 6–10 12 10 6–13 4 6 2–10 9 7–11 13 11 7–13 5 7 3–11 10 8–12 colspan=3 6 8 4–12 11 9–13 colspan=3 7 9 5–13 12 10–13 colspan=3 8 10 6–13 13 11–13 colspan=3 9 11 7–13

Local regulations may restrict certain channels from operation. For example, Channels 12 and 13 are normally unavailable for use as either a primary or secondary channel in North America. For further information, see List of WLAN channels.

The Wi Fi Alliance has upgraded its suite of compatibility tests for some enhancements that were finalized after a 2.0. Furthermore, it has affirmed that all draft n certified products remain compatible with the products conforming to the final standards.

After the first draft of the IEEE 802.11n standard was published in 2006, many manufacturers began producing so called "draft n" products that claimed to comply with the standard draft, even before standard finalization which mean they might not be inter operational with products produced according to IEEE 802.11 standard after the standard publication, nor even among themselves. The Wi Fi Alliance began certifying products based on IEEE 802.11n draft 2.0 mid 2007. This certification program established a set of features and a level of interoperability across vendors supporting those features, thus providing one definition of "draft n" to ensure compatibility and interoperability. The baseline certification covers both 20 MHz and 40 MHz wide channels, and up to two spatial streams, for maximum throughputs of 144.4 Mbit/s for 20 MHz and 300 Mbit/s for 40 MHz (with short guard interval). A number of vendors in both the consumer and enterprise spaces have built products that have achieved this certification.

The following are milestones in the development of 802.11n:

September 11, 2002 The first meeting of the High Throughput Study Group (HTSG) was held. Earlier in the year, in the Wireless Next Generation standing committee (WNG SC), presentations were heard on why they need change and what the target throughput would be required to justify the amendments. Compromise was reached in May 2002 to delay the start of the Study Group until September to allow 11g to complete major work during the July 2002 session. September 11, 2003 The IEEE SA New Standards Committee (NesCom) approved the Project Authorization Request (PAR) for the purpose of amending the 802.11 2007 standard. The new 802.11 Task Group (TGn) is to develop a new amendment. The TGn amendment is based on IEEE Std 802.11 2007, as amended by IEEE Std 802.11k 2008, IEEE Std 802.11r 2008, IEEE Std 802.11y 2008 and IEEE P802.11w. TGn will be the 5th amendment to the 802.11 2007 standard. The scope of this project is to define an amendment that shall define standardized modifications to both the 802.11 physical layers (PHY) and the 802.11 Medium Access Control Layer (MAC) so that modes of operation can be enabled that are capable of much higher throughputs, with a maximum throughput of at least 100 Mbit/s, as measured at the MAC data service access point (SAP). September 15, 2003 The first meeting of the new 802.11 Task Group (TGn). May 17, 2004 Call for Proposals was issued. September 13, 2004 32 first round of proposals were heard. March 2005 Proposals were downselected to a single proposal, but there is not a 75% consensus on the one proposal. Further efforts were expended over the next 3 sessions without being able to agree on one proposal. July 2005 Previous competitors TGn Sync, WWiSE, and a third group, MITMOT, said that they would merge their respective proposals as a draft. The standardization process was expected to be completed by the second quarter of 2009. January 19, 2006 The IEEE 802.11n Task Group approved the Joint Proposal's specification, enhanced by EWC's draft specification. Unanimous approval to send Final WG draft 11.0 to RevCom. September 11, 2009 RevCom/Standards Board approval. October 29, 2009 Published.

IEEE 802.11n 2009