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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.
How 802.11n Evolved and Where It Is Today
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 to the well-known 802.11b, 802.11g and 802.11a standards.
802.11n is the newest of the WLAN standards. It includes specifications for a number of sophisticated engineering enhancements over its 802.11g and 802.11a predecessors so that data can 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.
Current Draft 2.0 equipment is forwards-compatible with all applicable aspects of the final 802.11n standard. The final standard expanded on some features and capabilities not included in Draft 2.0 but those features extend (rather than alter) 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 cover connectivity up to 600 Mbps (using four "spatial streams"). The final standards also include specifications for "beam forming" antenna systems which improve connection rates and range of transmission.
802.11n Draft 2.0 continues to be a predominant baseline for enterprise-class wireless LAN equipment and is the basis for most WLAN design today. The enhanced features in the final draft will not be prominent in the marketplace for many years.
Should You Deploy 802.11n or Will 802.11g Be Sufficient?
There's a question of sufficiency and another, separate question of suitability. The first thing to consider is whether or not it's 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 won't be supported. Portable devices like the Apple iPhone won't connect to the WLAN. An 802.11n Greenfield deployment means "only 802.11n."
One common approach that Connect802 has used successfully with 802.11n implementations is to utilize dual-radio access points which support only 802.11n in the 5 GHz band and support only 802.11b/g radios in the 2.4 GHz band. This allows for a Greenfield 5 GHz implementation (which avoids the performance degradation associated with mixed-mode designs) but still allows legacy 802.11b/g devices to have network connectivity. It's the best of both worlds!
It may not suite your overall requirements or system design to deploy 802.11n. A fact to consider is that an 802.11n access point can't pass user data traffic any faster than the Ethernet network to which it's connected. Hence, if you have a Fast Ethernet (100 Mbps Ethernet) wired infrastructure, with 100 Mbps switches and routers, you're limited 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.) If you're implementing 802.11n using 100 Mbps Ethernet switches or cable that's not rated for Gigabit Ethernet then the fact that an over-the-air connection rate of 300 Mbps could be achieved is not a factor. You're limited to the speed of your Ethernet cable.
The other aspect to considering throughput is that an 802.11n access point is manufactured using more sophisticated radio circuitry than earlier 802.11a/b/g equipment. 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 the 802.11n radio will provide better performance and better range than an 802.11a/b/g device. The consequence is that 802.11n provides better performance and better range (at a given data rate) than 802.11a/b/g equipment so 802.11n may be an excellent choice even if you can't take advantage of its inherent high-speed connection rates.
If you're deploying an 802.11n 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 T3 45 Mbps Internet connection and you have to support 45 simultaneous users then each user will get 1 Mbps of the T3 line. Providing a 90 Mbps or 200+ Mbps TCP/IP connection through the wireless network may not give the end-user community any advantage - and it's going to cost more than the corresponding 802.11g network would.
These are things to consider when making a decision regarding 802.11n. There's no question that over the lifetime of the network the need to support 802.11n in the 5 GHz U-NII band will rise to the surface even if it's not a requirement today. In any case, 802.11n will provide greater throughput than 802.11g. A best-case 802.11g connection provides just under 25 Mbps of TCP/IP throughput (when connected at 54 Mbps) while a best-case 802.11n connection, offering over 200 Mbps of TCP/IP over-the-air 802.11n throughput, will still have almost four times more capacity even if it's bandwidth limited to 90 Mbps by the Ethernet (i.e. 802.11g provides slightly less than 25 Mbps TCP/IP throughput compared to 90 Mbps with the Ethernet-limited 802.11n access point.)
The Definition of "Faster" Relative to 802.11 Engineering
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 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!
How 802.11n 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 802.11 provides 300 Mbps connectivity (remember, too, that the best-case TCP/IP throughput rate will be roughly 66% of the connection rate):
Modified OFDM
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)
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)
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)
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:
READ THE DETAILED EXPLANATION OF 802.11N CHANNEL CONFIGURATION
Spatial Multiplexing (MIMO)
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.
READ THE DETAILED EXPLANATION OF 802.11N SPATIAL MULTIPLEXING (MIMO)
Web References Relating to 802.11n Technology and Engineering
You'll find numerous Web references and reviews of the IEEE 802.11n standards and 802.11n equipment, 802.11n products and 802.11n services that are available. Below are some targeted discussions to provide you with additional perspective on how an 802.11n solution can be implemented to meet your requirements for a secure, easy-to-manage wireless LAN system.
Here's an excerpt of what Wikipedia says regarding 802.11 and the 802.11n standards:
802.11b and 802.11g use the 2.4 GHz ISM band, operating in the United States under Part 15 of the US Federal Communications Commission Rules and Regulations. Because of this choice of frequency band, 802.11b and g equipment may occasionally suffer interference from microwave ovens, cordless telephones and Bluetooth devices. Both 802.11 and Bluetooth control their interference and susceptibility to interference by using spread spectrum modulation. Bluetooth uses a frequency hopping spread spectrum signaling method (FHSS), while 802.11b and 802.11g use the direct sequence spread spectrum signaling (DSSS) and orthogonal frequency division multiplexing (OFDM) methods, respectively. 802.11a uses the 5 GHz U-NII band, which, for much of the world, offers at least 19 non-overlapping channels rather than the 3 offered in the 2.4 GHz ISM frequency band. Better or worse performance with higher or lower frequencies (channels) may be realized, depending on the environment.
The segment of the radio frequency spectrum used varies between countries. In the US, 802.11a and 802.11g devices may be operated without a license, as allowed in Part 15 of the FCC Rules and Regulations. Frequencies used by channels one through six (802.11b) fall within the 2.4 GHz amateur radio band. Licensed amateur radio operators may operate 802.11b/g devices under Part 97 of the FCC Rules and Regulations, allowing increased power output but not commercial content or encryption.
IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO) and 40 MHz channels to the physical (PHY) layer, and frame aggregation to the MAC layer. MIMO is a technology which uses multiple antennas to coherently resolve more information than possible using a single antenna. Two important benefits it provides to 802.11n are antenna diversity and spatial multiplexing.
Another ability MIMO technology provides is Spatial Division Multiplexing (SDM). SDM 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 MIMO antenna which translates to higher implementation costs compared to non-MIMO systems.
40 MHz channels is another feature incorporated into 802.11n which doubles the channel width from 20 MHz in previous 802.11 PHYs to transmit data. This allows for a doubling of the PHY data rate over a single 20 MHz channel. It can be enabled in the 5 GHz mode, or within the 2.4 GHz if there is knowledge that it will not interfere with any other 802.11 or non-802.11 (such as Bluetooth) system using those same frequencies.
Coupling MIMO architecture with wider bandwidth channels offers increased physical transfer rate over 802.11a (5 GHz) and 802.11g (2.4 GHz).