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802.11n Technology
802.11n dramatically increase the effective throughput of 802.11 devices, and not to simply build a radio capable of higher bit rates. To increase the effective throughput of an 802.11 device requires more than providing a higher bit rate. There are aspects of the 802.11 standard that are "overhead" for the 802.11 protocol.
802.11n is much more than just a new radio for 802.11. In addition to providing higher bit rates (as was done in 802.11a, b, and g), 802.11n makes dramatic changes to the basic frame format that is used by 802.11 devices to communicate with each other.
How is 802.11n really different than current generations of Wi-Fi then?
The 802.11n standard using some new technology and tweaks currenting existing technologies have to give Wi-Fi more speed and range. The more notable new technology is called multiple input, multiple output (MIMO). MIMO uses several antennas to move multiple data streams from one place to another. Instead of sending and receiving a single stream of data, MIMO can simultaneously transmit three streams of data and receive two. This allows more data to be transmitted in the same period of time. This technique can also increase range, or the distance over which data can be transmitted.
A second technology being incorporated into 802.11n is channel bonding, which can use two separate nonoverlapping channels at the same time to transmit data. This technique also increases the amount of data that can be transmitted.
A third technology in 802.11n is called payload optimization or packet aggregation, which, in simple terms, means more data can be stuffed into each transmitted packet.
So, what are the benefits of 802.11n?
Users will notice two things about this new and improved wireless technology: significantly greater speed and range. Both Intel Corp., which has a vested interest in 802.11n because it manufactures wireless chip sets, and independent reviews indicate that the claims of greater speed and range for 802.11n are true.
Specifically, 802.11g products, which have a theoretical maximum throughput speed of 54Mbit/sec., typically provide real-world speeds of 22Mbit/sec. to 24Mbit/sec. In contrast, Intel says it's seeing real-world speeds of 100Mbit/sec. to 140Mbit/sec. for 802.11n equipment.
Range is harder to quantify because it's affected by many variables, such as barriers that could block the signal. However, Intel reports that 802.11n equipment typically delivers more than twice the range of 802.11g equipment, at any given throughput speed.
"At the very end of an open field with no interference, where you could get 1Mbit/sec. with "g" equipment, you'll net 14Mbit/sec. to 16Mbit/sec. with "n" equipment," reports Ashish Gupta, an Intel product manager.
What's in it for business users?
Wireless networking in many companies often fills specific niches, such as providing networking in conference rooms, lunch rooms or in temporary or under-construction office space. That lack of full deployment of wireless is understandable given that Ethernet provides greater reliability and speeds (theoretical maximums of 100Mbit/sec.) and is switched, while wireless LANs offer slower speeds and the bandwidth is shared. The new 802.11n technology will solve the throughput problem for business users, opening the way to far more applications, such as wireless voice over IP and more videoconferencing.
Backward Compatibility
Compatibility with existing 802.11a, b, and g devices is a critical issue addressed in 802.11n. Just as 802.11g provides a protection mode for operation with 802.11b devices, 802.11n has a number of mechanisms to provide backward compatibility with 802.11 a, b, and g devices, allowing these devices to understand the information necessary to allow 802.11n devices to operate in the same area.
For quite a long time, 802.11n will need to operate in the presence of legacy 802.11a, b, and g devices. This mixed-mode operation will continue until all the devices in an area have been upgraded or replaced with 802.11n devices. The mixed-mode protection mechanism for 802.11n is quite similar to the protection mechanism of 802.11g.
Like 802.11g, 802.11n transmits a signal that can't be decoded by devices built to an earlier standard. To avoid descending into absolute chaos in the presence of massive interference and collisions, 802.11n operating in mixed mode transmits a radio preamble and signal field that can be decoded by 802.11a and 802.11g radios. This provides enough information to the legacy radios to allow them to indicate that there is another transmission on the air and how long that transmission will last. Following the legacy preamble and signal field, the 802.11n device sends the remaining information using 802.11n rates and its multiple spatial streams, including an 802.11n preamble and signal field.
Summary of 802.11n Technology
To summarize the benefits of 802.11n technology, it is simplest to say that there are two major areas of improvement over previous 802.11 devices.
The first area of improvement is in the use of MIMO technology to achieve greater SNR on the radio link.
The second area of improvement is in the greater efficiencies in both radio transmissions and the MAC protocol. These improvements translate into benefits in three areas: reliability, predictable coverage, and throughput.
Reliability
Greater SNR on the radio link translates directly to more reliable communication, often at higher data rates. Higher SNR means that more interference is needed to corrupt a transmission. This means greater client densities can be supported.
Predictable Coverage
The use of multiple spatial streams provided by MIMO technology means that there will be fewer dead spots in a coverage area. Areas that previously suffered from destructive multipath interference now make use of that same multipath effect to provide robust communication.
Throughput
The efficiency improvement in 802.11n provides a greater transfer of the high bit rates of the 802.11n radio to effective throughput seen by actual applications, at least in greenfield deployments. Even in mixed-mode deployments with legacy 802.11 devices, 802.11n will provide greater effective throughput, although significantly less than the greenfield mode.
Migration to 802.11n
The migration to 802.11n, at least 802.11n draft 2.0, has already begun. 802.11n client devices, beginning with laptops, are already available. New client devices will begin to appear over the next several quarters, until 802.11n draft 2.0 will be the default WLAN adapter in any portable or mobile device. These client devices are completely compatible with existing 802.11a, b, and g access points and will operate just as existing devices do today. Planning to migrate the infrastructure portion of a network to support 802.11n on these new client devices is straightforward.
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