802.11n Technology Delivers Throughput, Range, and Reliability
802.11n is a major step in the evolution of Wi-Fi technology because, unlike its predecessors 802.11a and 802.11g, 802.11n is more than a new physical layer protocol (PHY). Wi-Fi CERTIFIED n incorporates several significant enhancements that can deliver five times or more throughput and more robust connections at up to twice the range over legacy technology. In fact, 802.11n technology can easily cover a typical house with sufficient bandwidth to support video, gaming, data, and voice applications. The leap in performance and network robustness is easily experienced and measured when users upgrade from legacy networks to 802.11n.
The technology enhancements in the 802.11n standard are sophisticated, and the benefits of individual features are often expressed in more than one way. The Wi-Fi CERTIFIED n program incorporates the most important of these features to deliver significant improvements to throughput and range.
Throughput is the real measure of performance; it is what the user perceives as a device’s performance as he uses it in everyday ways. Throughput is the transmission capability that is available to applications after the overhead required to address the needs of upper layer protocols has been addressed. By definition, throughput is a subset of a device’s physical layer data rate (data rate). Data rate is an expression of a device’s raw transmission capability at the lowest, physical layer. It is an essential contributor to device performance, but an end user will not experience performance equivalent to a device’s data rate.
Range is the distance at which a device can operate effectively. Range incorporates elements of both throughput and robustness — that is the user’s perception of a device’s range combines distance with expectations about the speed and quality of the connection. The value of 802.11n’s improved range is most clearly seen in applications like voice and video.
The features contributing to the improved throughput and range are discussed below and depicted in Figure 1:
Multiple Antennas or MIMO
MIMO (multiple-input/multiple-output) leverages multipath. Multipath is a phenomenon in wireless transmissions in which the signal reflects from walls and objects, such as furniture. Reflections can combine, distorting the signal at the receiver. While legacy 802.11a/b/g radios use techniques to overcome the negative effects of multipath, the IEEE 802.11n standard incorporates MIMO, which instead uses multipath to enhance communication.
A MIMO system has some number of transmitters (N) and receivers (M). Signals from each of the N transmitters can reach each of the M receivers via a different path in the channel. A MIMO device with multiple antennas is capable of sending multiple spatial streams — spatially distinct data streams within the same channel. A MIMO device with multiple antennas is capable of receiving multiple spatial streams. Multipath helps decorrelate the received signals enabling transmission of multiple data streams through the same MIMO channel — a technique called spatial multiplexing. MIMO can multiply data rate through spatial multiplexing – dividing a data stream into several branches and sending it as multiple parallel data streams simultaneously in the same channel.
MIMO can also be used to improve the robustness and range of 802.11n communications through a technique called spatial diversity. When the same data stream is transmitted across multiple spatial streams error rate can be reduced. An additional technique improving range and reliability called Space Time Block Coding (STBC) is also incorporated into Wi-Fi CERTIFIED n.
STBC improves reception by coding the data stream in blocks which are distributed for transmission across multiple transmitting antennas and across time. At the receiving antenna, the data is recombined in an optimal way making use of the coding. STBC requires multiple transmit antennas and delivers benefits to devices with the ability to receive one or more data streams.
Channelization
In addition to the benefits reaped from spatial multiplexing in MIMO, 802.11n technology employs several other techniques to support faster data rates by enabling the use of wider channels.
While 802.11 a/b/g networks operate in a 20 MHz channel, 802.11n defines the use of 20 and 40 MHz channels. 40 MHz channels allow doubling of the data rate to 150 Mbps. All 802.11 devices send a packet over the air as a sequence of symbols. Devices using 40 MHz channels can encode and transmit more data in each symbol. Depending on the degree of complexity that the environment can support, 802.11 devices choose an appropriate data rate for use over the air.
For example, the IEEE 802.11b standard supports data rates of 1, 2, 5.5, and 11 Mbps.
Figure 2 shows the range of data rates for different 802.11 technologies.
Efficiency Measures
The improved throughput of 802.11n comes through two paths. Several features increase data rate in the physical layer, with some proportion of that effect visible in throughput increases. 802.11n also includes innovations that reduce overhead and improve efficiency of transmissions — directly contributing to improved throughput.
As discussed previously, MIMO and channelization both directly affect a device’s data rate. In addition, an 802.11n technique called Short Guard Interval (SGI) can also improve data rate by reducing the size of the gap between symbols.
Much of the throughput improvement in 802.11n comes from aggregation techniques. Frame aggregation improves the efficiency of 802.11n systems by reducing the protocol overhead required for transmitting protocol frames. Video traffic can benefit from being transported using maximum size aggregate frames since video traffic sends many frames to the same destination.
The Aggregated Medium Access Control Service Data Unit (A-MSDU) mechanism increases the frame size used in transmitting Medium Access Control (MAC) protocol frames. The Aggregated MAC Protocol Data Unit (A-MPDU) mechanism increases the maximum size of the 802.11 frames transported on the air from the legacy 2304 bytes to 64k bytes.
Throughput Improvement
Industry performance numbers cited for throughput calculations, measurements and comparisons are typically based on transmission of the TCP/IP protocol suite. Figure 3 shows how optional features may contribute to improvements in throughput of Wi-Fi CERTIFIED n devices.
While MIMO increases over-the-air data rates in well-defined ways, the impact of aggregation techniques on throughput is dependent on traffic pattern. The impact of all features is dependent on the environment in which the equipment is used, equipment configuration and proximity.
Identifying Wi-Fi CERTIFIED n Products
The Wi-Fi CERTIFIED logo provides a simple indication of products which work together: the a, b, g, and n components of the Wi-Fi CERTIFIED logo indicate physical layer compatibility.
Because 802.11n technology has become quite complex, and most vendors offer a variety of products with differing feature sets, the Wi-Fi Alliance has developed some additional branding elements for Wi-Fi CERTIFIED n products to help consumer users differentiate products and understand their capabilities. (See Figure 4.)
An up-to-date listing of all Wi-Fi CERTIFIED products can be found at the Wi-Fi Alliance website: www.wi-fi.org. Users can search via product category or criteria including manufacturer, certification date, etc., and can view the interoperability certificate for certified products. For detailed information for consumer and enterprise users, including video tutorials, shopping guides, and a glossary of terms, see www.11nbasics.org.
Wi-Fi CERTIFIED n: Home, Enterprise, and Campus
ABI Research forecasts that shipments of 802.11n semiconductors will represent 45 percent of the total market in 2009 and grow to nearly 60 percent in 2012.[4] This considerable rate of growth reflects the fact that 802.11n and the Wi-Fi CERTIFIED n program promote expansion of Wi-Fi into new market segments and applications in the home and in the enterprise.
Just what does it do for the home? Just a few are listed below:
• With its increased coverage and throughput, Wi-Fi CERTIFIED n enables HD video and audio-visual (AV) multimedia applications in the home environment.
• The increased throughput and coverage provides sufficient bandwidth to transport multiple video streams to Wi-Fi enabled set-top boxes or TV sets around the house.
• The high bandwidth and QoS of Wi-Fi CERTIFIED n systems helps ensure that an Internet connection can be reliably shared by the increasing number and type of Wi-Fi enabled devices in the home without degradation of service.
• The increased range of Wi-Fi CERTIFIED n provides coverage to the entire house, reaching farther than the legacy technology and reducing dead spots or low-rate areas in the home.
Most network transactions, including voice and data services, will benefit significantly from frame aggregation technology. Printing files from PCs to printers, transferring files between PCs and network drives and sharing files between PCs, laptops and other devices on the network becomes more efficient thanks to frame aggregation.
But it doesn’t stop there. Wi-Fi CERTIFIED n is also enterprise-grade technology that provides IT managers with the reliable, secure service they have come to expect. Mission-critical enterprise applications all benefit from the increased throughput and range while relying on the next generation security protections.
Campus and municipal networks typically operate in challenging environments where range is the biggest issue. Wi-Fi CERTIFIED n is well-equipped to improve the operating range even for single-antenna handheld devices used in such outdoors networks. Increased range of handheld devices is achieved through access point transmit and receive diversity mechanisms. Transmit diversity of access points, including STBC, improves the downlink range performance. Receive diversity of access points reciprocate the transmit diversity and thus maintain the range for both the uplink and the downlink directions.
Wi-Fi at the Next Level: Wi-Fi CERTIFIED n
The Wi-Fi CERTIFIED n interoperability test program extends the improved throughput and range of 802.11n to a wide range of devices. Today’s Wi-Fi CERTIFIED n devices can deliver five times or more throughput and more robust connections at up to twice the range of legacy 802.11 technology.
The strong adoption of 802.11n in the wake of the Wi-Fi Alliance’s draft-n program in 2007 is a strong indication of the potential of the technology. The updated Wi-Fi CERTIFIED n program including security protections and quality-of-service testing will catalyze broader adoption of 802.11n by expanding the optional feature set enabling significant performance gains for a broad and growing range of devices.
About the Wi-Fi Alliance
The Wi-Fi Alliance is a global non-profit industry association of hundreds of leading companies devoted to the proliferation of Wi-Fi technology across devices and market segments. With technology development, market building, and regulatory programs, the Wi-Fi Alliance has enabled widespread adoption of Wi-Fi worldwide. For more information, visit www.wi-fi.org.
The Wi-Fi CERTIFIED™ program was launched in March 2000. It provides a widely-recognized designation of interoperability and quality, and it helps to ensure that Wi-Fi enabled products deliver the best user experience. The Wi-Fi Alliance has completed more than 6,000 product certifications to date, encouraging the expanded use of Wi-Fi products and services in new and established markets. For detailed information for consumer and enterprise users, including video tutorials, shopping guides, and a glossary of terms, see www.11nbasics.org.
Wi-Fi®, Wi-Fi Alliance®, WMM®, Wi-Fi Protected Access® (WPA), the Wi-Fi CERTIFIED logo, the Wi-Fi logo, and the Wi-Fi ZONE logo are registered trademarks of the Wi-Fi Alliance; Wi-Fi CERTIFIED™, Wi-Fi Protected Setup™, Wi-Fi Multimedia™, and the Wi-Fi Alliance logo are trademarks of the Wi-Fi Alliance.