802.11n Discussion

802.11n WiFi Technology
Discussion

Understanding The IEEE 802.11n WiFi Standard

What is 802.11n?
802.11n is a set of standards that define a way to transmit and receive wireless LAN data at very high bit rates; significantly higher than 802.11b/g and 802.11a. 802.11n access points implement the 802.11n transmission/reception standards. The first draft of the 802.11n standards was completed by the IEEE in 2006 and the standards were completed and finalized in 2007. In that same year the Wi-Fi Alliance began certification of 802.11n access points and other products. By 2010, 802.11n equipment became the predominant driver of the WiFi market and almost completely displaced the then-pervasive 802.11b/g environment.

How 802.11n Evolved
The IEEE (Institute of Electrical and Electronics Engineers) is an international non-profit professional organization for the advancement of technology related to electricity. Working groups and subcommittees are formed to set standards for affecting a wide range of industries. In February, 1980 a working group was formed to specify standards for networks carrying variable-size data packets. This was the IEEE 802 working group (also associated with the year and month the group was formed!

The 802.3 Committee is well-known for setting standards for Ethernet communication. Power-over-Ethernet (PoE), for carrying power over an Ethernet cable, falls under the 802.3af standards for standard power delivery and the newer 802.3at standards for high-power PoE.

The 802.11 Committee is responsible for setting standards for wireless local area networks (WLANs) in the 2.4 GHz, 3.6 GHz and 5 GHz frequency bands. The original 802.11 standards were released in 1997 and clarified in 1999 and have evolved through the era of 802.11b, 802.11g and 802.11a to the current realm of 802.11n, 802.11ac, and evolving 802.11ax.

802.11n was a major step up in capabilities when it was introduced to replace 802.11b/g in the 2.4 GHz spectrum and 802.11a (which was never widely used) in the 5 GHz spectrum. It included specifications for a number of sophisticated engineering enhancements over its 802.11g and 802.11a predecessors so that data could be transferred 10 to 40 times faster. The final 802.11n standards were ratified in September 2009 after many years of deliberation (the 802.11n Committee was formed in January, 2004). In 2007 the Wi-Fi Alliance (a non-profit association that certifies interoperability between different manufacturer's equipment based on the IEEE 802.11 standards) certified the "Draft 2.0 802.11n Standard" as being suitable for interoperability testing and manufacturers began shipping 802.11n equipment.

The final 802.11n standard expanded on some features and capabilities not included in Draft 2.0 but those features extended (rather than altered) the capabilities of Draft 2.0-compliant equipment. Specifically, while the Draft 2.0 standards defined connectivity up to 300 Mbps (using two "spatial streams" with "multiple-input / multiple-output" - MIMO) the final standards covered connectivity up to 600 Mbps (using four "spatial streams"). The final standards also included specifications for "beam forming" antenna systems which improve connection rates and range of transmission although beamforming wasn't finally made part of the required features until release of the 802.11ac standards.

Early Adopter Questions About 802.11n Compared To 802.11a/b/g
In the early days of 802.11n market penetration there were numerous questions in the WiFi decision maker's market regarding sufficiency and another, separate question of suitability. One initial thing that a decision maker had to consider was whether or not it was suitable to implement 802.11n as a "Greenfield" deployment. The term "Greenfield" when applied to 802.11n means that only 802.11n devices will be supported. No 802.11b, 802.11g or 802.11a. That means that any older notebook computers that use 2.4 GHz 802.11g wouldn't be supported. Portable devices which, at the time, only implemented 2.4 GHz 802.11b/g would not benefit from the 802.11n capabilities. An 802.11n Greenfield deployment means "only 802.11n" and never became a reality. Every 802.11n network provides backwards compatibility with 2.4 GHz 802.11b/g and, with dual-radio technology, full support for 802.11n (and 802.11ac).

A fact to consider is that an access point, whether it's implementing 802.11n or 802.11ac, can't pass user data traffic any faster than the Ethernet network to which it's connected. Hence, if a site only has a Fast Ethernet (100 Mbps Ethernet) wired infrastructure, with 100 Mbps switches and routers, there would be a limitation to roughly 90 Mbps of aggregate TCP/IP throughput, asymmetrically (half-duplex) to and from any individual access point. (A 100 Mbps Ethernet cable provides roughly a maximum 90 Mbps TCP/IP throughput rate after protocol overhead is taken into consideration.) You're limited to the speed of your Ethernet cable and wired infrastructure. The slowest part of the data path sets the upper limit on data transfer through that path. This fact becomes even more significant when the WLAN supports Gigabit speeds (as is the case for 802.11ac and 802.11ax) where one starts to think about the fiber interconnect capacity between wiring closets.

The other aspect to considering throughput is that new access point equipment is manufactured using more sophisticated radio circuitry than earlier  equipment. An 802.11n access point has better, more sophisticated RF in chipset components than it's 802.11b/g predecessors and, too, 802.11ac equipment surpasses 802.11n. This trend is the natural result of technology evolution. This means that even when a client device is at the edge of a signal coverage area or in an environment with noise or interference, newer technology will generally provide better performance and better range as compared to older technology, all other things being equal. 

If you're deploying an 802.11n (or 802.11ac) wireless network to provide connectivity to the Internet then you'll consider the number of users, the density of users in any one area, the types of Internet activities being performed (email, Web, streaming video, HD television, Voice-over-IP, etc.) and how many simultaneous users will be active. In the end, if you have a 45 Mbps Internet connection and you have to support 45 simultaneous users then each user will get 1 Mbps maximum throughput. Even through your 802.11n network may provide 150 Mbps connectivity and an 802.11ac or 802.11ax network may provide 230 Mbps or more (perhaps MUCH more) client connectivity, the limitation will always be the size of the Internet pipe; an issue that is dealt with through your ISP and your Internet connection contract.

When WiFi decision makers and managers were contemplating the move to 802.11n they recognized that over the lifetime of the network they would need to support 802.11n in the 5 GHz U-NII band and that 5 GHz capability would rise to the surface even if wasn't an immediate requirement at the time. That is exactly how the market evolved and, today, the 5 GHz 802.11n, 802.11ac, and 802.11ax spectrum is the default standard for optimal network performance. 
 
What Makes 802.11n Faster Than 802.11g and 802.11a?
There are two fundamental ways to specify the "speed" of an 802.11 connection. The first way, which is used in all spec sheets and marketing literature and in every standard Wi-Fi driver and management interface, is to specify a connection rate. When 802.11b is said to offer speeds up to 11 Mbps or 802.11g and 802.11a offering speeds up to 54 Mbps the "speed" being referred to is the connection rate (also called the modulation rate). This "speed" refers to the rate at which the 802.11 transmitter is able to send a constant stream of bits. When considering the useful "speed" of TCP/IP data transmission (as would be relevant to measuring the speed of a web page or email message download or an FTP file transfer) the raw burst rate of bits is only part of the equation.

A TCP/IP data transfer involves 802.11 acknowledgements, TCP acknowledgements and all of the individual packet overhead (which includes source and destination addressing, for example). In addition, there are mandatory gaps between each successive 802.11 packet. Ultimately, too, not all 802.11 packets arrive without corruption from environmental factors. The overhead associated with packet transfer makes the actual TCP/IP data throughput rate less than the specified 802.11 connection rate
 
802.11b, 802.11g and 802.11a provides slightly less than 50% of the connection rate as the best-case TCP/IP throughput rate
A best-case 54 Mbps 802.11g or 802.11a connection will be measured at slightly less than 27 Mbps TCP/IP throughput
The 50% overhead encountered relative to TCP/IP transmission over an 802.11b/g or 802.11a network includes the 802.11 overhead and the TCP/IP overhead. TCP/IP itself accounts for roughly 8% of the overhead (whereas the connectionless UDP protocol introduces closer to 5% overhead). Even Ethernet itself, on the wired side of the network, introduces roughly 3% overhead. This is why the best-case TCP/IP throughput on a 100 Mbps Fast Ethernet line is measured at closer to 90 Mbps rather than ever seeing a full 100 Mbps TCP/IP throughput.

You can never transfer data at the full connection rate. There's always overhead at the 802.11 protocol level and at the TCP/IP higher-layer protocol levels. 802.11 marketing literature and technical specifications always present connection rates. When you see a reference to the Mbps rate as 1, 2, 5.5, 6, 11, 12, 18, 24, 36, 48, 54 and, for 802.11n 65, 72.5, 150, 300, 450, 600... you know you're seeing a reference to the connection rate (also called the "modulation rate").

The TCP/IP throughput rate is roughly half the connection rate for 802.11b/g and 802.11a. The TCP/IP throughput rate is closer to 66% or even 70% of the connection rate for 802.11n! 802.11n provides slightly less than 66% of the connection rate as the best-case TCP/IP throughput rate A best-case 300 Mbps 802.11n connection will be measured at slightly more than 200 Mbps TCP/IP throughput
 

How 802.11n Was Able To Provide 300 Mbps Connectivity
There are some fundamental engineering enhancements that are part of 802.11n bit and packet transmission. Some of these enhancements are dependent on the environment and will either be active or not depending on device configuration and environmental characteristics. The most talked-about enhancement, "MIMO" (Multiple Input / Multiple Output), where more than one data stream is transmitted at a time (multiple "spatial streams") is a statistical probability and will vary within any given environment. Here's the breakdown of how 802.11n provides 300 Mbps connectivity (remember, too, that the best-case TCP/IP throughput rate will be roughly 66% of the connection rate):
Modified OFDM
Sub-carriers increased from 48 to 52
Maximum connection rate increased from 54 Mbps to 58.5 Mbps
Orthogonal Frequency Division Multiplexing (OFDM) is the technique used for representing a short burst of individual data bits using a specific pattern of electromagnetic signals. The data stream is broken down into "sub-carriers" and multiple sub-carriers are transmitted at closely-spaced adjacent frequencies. 802.11g and 802.11a used 48 sub-carriers. Better hardware circuitry and more sophisticated engineering in 802.11n equipment allowed the number of sub-carriers to be increased to 52.

Forward Error Correction (FEC)
Sender adds redundant data to allow the receiver to detect and correct errors
Maximum connection rate increased from 58.5 Mbps to 65 Mbps
The structure of transmitted bits is mathematically modified to help the receiver make sense out of bit streams which may have been corrupted by noise or interference. To do this the transmitter adds additional bits in a pre-defined way. The result is referred to as a "coding scheme." 802.11g and 802.11a add one extra bit for every three bits transmitted. This referred to as a 3/4 coding scheme. 802.11n introduces technology that allows a 5/6 coding scheme where one extra bit is needed for each 5 data bits. The enhanced error correction scheme reduces the number of redundant bits needed to compensate for noise and interference.

Shorter Guard Interval (GI)
OFDM inter-symbol guard interval minimum reduced from 800ns to 400ns
Maximum connection rate increased from 65 Mbps to 72.2 Mbps
Transmitted electromagnetic signals are reflected off various surfaces as they travel through space to a receiver's antenna. This means that some signal will take a longer path, and hence a longer amount of time, to reach the receiver's antenna. After a transmitter has sent an OFDM burst of bits it must wait long enough to allow the majority of reflected signals to reach any potential receiver within the intended range. In essence, it has to wait for the air to get quiet again before sending another OFDM symbol. In 802.11g and 802.11a this required 800ns but the improved engineering and circuitry in an 802.11n radio allows proper operation after only 400ns.

It's very important to note that use of the shorter guard interval is dependent on the environment. In some spaces a large number of signal reflections may arrive over a long period of time (i.e. greater than 400ns). In this case the 802.11n chipset automatically backs off to the 800ns GI even if the radio has been configured to use the 400ns GI. In almost all real-world environments it will be found that use of the 400ns GI is precluded. Most real-world situations just don't allow for use of the shorter guard interval. Consequently, the data rate can't be increased from 65 Mbps to 72.2 Mbps and this single enhancement is lost.

Channel Bonding (Use of 40 MHz, "Double-Wide" Channels)
Channel bandwidth is increased from 20 MHz to 40 MHz
Maximum connection rate is increased from 72.2 Mbps to 150 Mbps
If the 400ns GI is unavailable then maximum connection rate increased from 65 Mbps to 130 Mbps
802.11g and 802.11a transmit data in a 20 MHz wide channel. At the upper and lower boundaries of a channel's frequency span the transmitted signal's power must drop off to avoid adjacent-channel interference. When two channels are "bonded" to create an 802.11n 40 MHz channel the result is that more than twice the original available bandwidth is created. This is because the space between the original two channels, where the signal originally had to drop off, is eliminated so you not only get double the original bandwidth but you don't loose the adjacent-channel gap that was between the two original channels. That's why the connection rate more than doubles when using 40 MHz channels.

It's very important to note that 40 MHz channels can't always be used. There are severe limitations to the use of 40 Mhz channels in the 2.4 GHz spectrum used by 802.11b and 802.11g. There can be some limitations in the 5 GHz spectrum used by 802.11a. A complete, detailed discussion of 802.11n channel configuration is available here:

Spatial Multiplexing (MIMO)
802.11n Draft 2.0 supports two spatial streams
The final 802.11n standard (September 2009) supports up to four spatial streams
Each additional spatial stream increases the maximum connection rate by 150 Mbps
Two spatial streams creates a connection rate of 300 Mbps
If the 400ns GI is unavailable then two spatial streams creates a connection rate of 260 Mbps
A 600 Mbps connection rate is achieved with four spatial streams
A stream of bits is converted into electromagnetic signals. An 802.11b/g or 802.11a transmitter sends these electromagnetic signals out a single antenna. If two or more antennas are present then one is selected at any given instant as the "best choice" for transmission. This is referred to as a "single input" transmission - one antenna sends signals into the air. On the receiving end, a single antenna is used to receive the transmitted bit stream. If two or more antennas are present then they're sampled and compared and the one that is receiving the higher quality signal is selected for use at that instant. This is referred to as "single output" since only one antenna takes signal out of the air at any given moment. "Antenna diversity" is the term for the technique used with most 802.11b/g and 802.11a radios where the receiver has two antennas and selects the best one at each successive moment. Nonetheless, only one antenna is used for reception and the technique remains "single output."
802.11n has sophisticated hardware circuitry and implements software that uses advanced mathematical algorithms to allow a transmitters bit stream to be split and simultaneously transmitted using two (or more) antennas at the same time. Both antennas transmit at exactly the same frequency and on the same channel but the mathematics allows the receiver to differentiate between the two transmitted streams.

Each transmitted stream is called a "spatial stream" because they're differentiated based on which antenna transmitted it and the two antennas are separated in space by a few inches - they're spatially separated. Consequently, multiple spatial streams are being input into the air ("multiple input") and they're then being taken out of the air ("multiple output") by the receiver. This the definition of "Multiple Input / Multiple Output" or "MIMO" (pronounced "my-moe").

For two spatial streams to be separated by the receiver demands that the arrival time of their respective reflected signals be sufficiently unique to allow the mathematical algorithms to work their magic. There's no guarantee that any particular environment will have a reflective nature that is suitable. Moreover, there's no guarantee that a client device will be positioned in a spot where signal reflections will allow differentiation. In fact, sometimes moving a client device only one or two inches can take it from a place where signal reflections do allow differentiation between spatial streams and one where multiple spatial streams can't be separated.

MIMO is a dynamic capability based on the statistical probability that a suitably reflective environment will exist in the very small place where the receiver's antennas are located. In a particular room some users may be able to connect with multiple spatial streams while others may only be able to use a single spatial stream. The situation is totally dynamic, changes from moment to moment, and can't be absolutely predicted.

An optimal 802.11n design attempts to maximize the probability that multiple spatial streams can be used by paying attention to the signal scattering effects in the environment. With a correct 802.11n design it's possible for almost everyone's device in a room to capture multiple spatial streams and realize the associated doubling (or more) of their throughput.

802.11ac

Definition
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