06-11-2012, 01:42 PM
802.11n: The Next Generation of Wireless Performance
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Executive Summary
The IEEE 802.11 Working Group has now completed 802.11n, the multiyear effort to standardize an upgrade to
the 802.11 radio specification. 802.11n provides a new set of capabilities dramatically improving the reliability of
communications, the predictability of coverage, and the overall throughput of devices.
There were no additional mandatory features introduced between the widely adopted draft 2.0 version of 802.11n
and the final version. Therefore, customers who already have chosen the draft 2.0 devices can continue to operate
these devices with confidence and without any hardware or software changes. Also, those who have been holding
back waiting for the standards to be finally ratified before moving over to the 802.11n can start migration now.
The 802.11n protocol has several enhancements in the physical layer and the MAC sublayer that provide
exceptional benefits to wireless deployments. The four critical features are:
● Multiple-input multiple-output (MIMO). MIMO uses the diversity and duplication of signals using the multiple
transmit and receive antennas.
● 40-MHz operation bonds adjacent channels, combined with some of the reserved channel space between
the two, to more than double the data rate.
● Frame aggregation reduces the overhead of 802.11 by coalescing multiple packets.
● Backward compatibility, which makes it possible for a/b/g and draft 2.0 802.11n devices to coexist, thereby
allowing customers to phase in their access point and/or client migrations over time.
This white paper describes these new capabilities in detail, explains how the benefits provided by 802.11n are
achieved, and examines the compatibility of this new standard with existing deployments. This white paper also
describes the issues to address when planning migration of an existing 802.11a/g deployment to 802.11n and the
results that can be expected from such a migration. But first, it is important to understand just what 802.11n is, and
what it is not.
The Role of the IEEE and the Wi-Fi Alliance
802.11n was a seven-year endeavor at the IEEE, with three major phases: Study Group, Task Group, and
Sponsor Ballot. The High Throughput Study Group first met on September 11, 2002, and this led to the 802.11n
Task Group a year later. Considerable work ensued to explore the core set of features that provided the maximum
benefit for the broadest set of devices. These features were finalized in draft 2.0. Cisco provided valuable
leadership toward robust backward compatibility and the security of the enhanced protocols.
The Wi-Fi Alliance, an industry organization that provides interoperability certification for 802.11 devices, first
began certifying the interoperability of draft 2.0 802.11n devices in June 2007. The certification tested the core
features of 802.11n. The program was broadly adopted by the Wi-Fi vendors and their equipment enjoyed strong
market penetration. To date, several hundred draft 2.0 802.11 products have been certified by the Wi-Fi Alliance,
and several tens of millions of devices have been deployed worldwide.
802.11n Technology
The goal of the work on 802.11n is to dramatically increase the effective throughput of 802.11 devices available to
end-user applications, not to simply build a radio capable of higher bit rates.
The difference between these goals is like the difference between replacing commuter cars with buses and
redesigning commuter cars to do away with the back seat. Although the car is shorter and so more cars can be
packed more densely in the lane than buses, the benefit of the redesign is dwarfed by the continuing need for a
safe following distance, the engine bay, and the trunk. In the same way, increasing the effective throughput of an
802.11 device requires more than providing a higher bit rate: every aspect of 802.11 that introduces overhead
needs to be minimized as far as possible.
MIMO
Multiple-input multiple-output (MIMO) is the heart of 802.11n. This technical discussion of MIMO provides a basis
for understanding how 802.11n can raise reliability and reach data rates many times higher than 802.11a/b/g in
the same radio spectrum. These benefits improve the experience of all wireless users, and are especially valuable
for customers with difficult RF environments, such as thick walls or small rooms, and for customers with high
throughput or quality-of-service (QoS) requirements, such as voice or video users.
Radio Operation Basics
To understand the improvement brought by MIMO technology, it is important to understand some of the basics
that determine how well a traditional radio operates. In a traditional, single-input single-output radio, the amount of
information that can be carried by a received radio signal depends on the amount by which the received signal
strength exceeds the noise at the receiver, called the signal-to-noise ratio, or SNR. SNR is typically expressed in
decibels (dB). Higher SNR enables more information to be carried on the signal and recovered by the receiver.
To understand the improvements that MIMO technology brings to 802.11, imagine trying to drive quickly down a
road. The bigger the car engine, the better its tires and suspension, and the lower the wind resistance, the faster
the car can go. At the same time, a road that is full of pot-holes or wind slows the car down. In this analogy, the
capabilities of the car are the signal, and the poor road quality is the noise. And better SNR means a faster
experience.
Spatial Propagation for MIMO
In typical indoor WLAN deployments - for example, enterprise offices, classrooms, hospitals, and warehouses - it
is rare for the radio signal to take only the direct, shortest path from the transmitter to the receiver. Often there is
no "line of sight" between the transmitter and the receiver: there is a cube wall, door, or other structure that
obscures the line of sight. All of these obstructions reduce the strength of the radio signal as it passes through
them, perhaps to an unusable level.
Luckily, there are other ways for the radio signal to travel from transmitter to receiver. Most environments are full
of surfaces that reflect a radio signal as well as a mirror reflects light. Imagine that all of the metallic surfaces,
large and small, that are in an environment were actually mirrors. Nails and screws, door frames, ceiling
suspension grids, pipes and structural beams are all reflectors of radio signals. It would be possible to see the
same WLAN access point in many of these mirrors simultaneously. Some of the images of the access point would
be a direct reflection through a single mirror. Some images would be a reflection of a reflection. Still others would
involve an even greater number of reflections.
[attachment=38298]
Executive Summary
The IEEE 802.11 Working Group has now completed 802.11n, the multiyear effort to standardize an upgrade to
the 802.11 radio specification. 802.11n provides a new set of capabilities dramatically improving the reliability of
communications, the predictability of coverage, and the overall throughput of devices.
There were no additional mandatory features introduced between the widely adopted draft 2.0 version of 802.11n
and the final version. Therefore, customers who already have chosen the draft 2.0 devices can continue to operate
these devices with confidence and without any hardware or software changes. Also, those who have been holding
back waiting for the standards to be finally ratified before moving over to the 802.11n can start migration now.
The 802.11n protocol has several enhancements in the physical layer and the MAC sublayer that provide
exceptional benefits to wireless deployments. The four critical features are:
● Multiple-input multiple-output (MIMO). MIMO uses the diversity and duplication of signals using the multiple
transmit and receive antennas.
● 40-MHz operation bonds adjacent channels, combined with some of the reserved channel space between
the two, to more than double the data rate.
● Frame aggregation reduces the overhead of 802.11 by coalescing multiple packets.
● Backward compatibility, which makes it possible for a/b/g and draft 2.0 802.11n devices to coexist, thereby
allowing customers to phase in their access point and/or client migrations over time.
This white paper describes these new capabilities in detail, explains how the benefits provided by 802.11n are
achieved, and examines the compatibility of this new standard with existing deployments. This white paper also
describes the issues to address when planning migration of an existing 802.11a/g deployment to 802.11n and the
results that can be expected from such a migration. But first, it is important to understand just what 802.11n is, and
what it is not.
The Role of the IEEE and the Wi-Fi Alliance
802.11n was a seven-year endeavor at the IEEE, with three major phases: Study Group, Task Group, and
Sponsor Ballot. The High Throughput Study Group first met on September 11, 2002, and this led to the 802.11n
Task Group a year later. Considerable work ensued to explore the core set of features that provided the maximum
benefit for the broadest set of devices. These features were finalized in draft 2.0. Cisco provided valuable
leadership toward robust backward compatibility and the security of the enhanced protocols.
The Wi-Fi Alliance, an industry organization that provides interoperability certification for 802.11 devices, first
began certifying the interoperability of draft 2.0 802.11n devices in June 2007. The certification tested the core
features of 802.11n. The program was broadly adopted by the Wi-Fi vendors and their equipment enjoyed strong
market penetration. To date, several hundred draft 2.0 802.11 products have been certified by the Wi-Fi Alliance,
and several tens of millions of devices have been deployed worldwide.
802.11n Technology
The goal of the work on 802.11n is to dramatically increase the effective throughput of 802.11 devices available to
end-user applications, not to simply build a radio capable of higher bit rates.
The difference between these goals is like the difference between replacing commuter cars with buses and
redesigning commuter cars to do away with the back seat. Although the car is shorter and so more cars can be
packed more densely in the lane than buses, the benefit of the redesign is dwarfed by the continuing need for a
safe following distance, the engine bay, and the trunk. In the same way, increasing the effective throughput of an
802.11 device requires more than providing a higher bit rate: every aspect of 802.11 that introduces overhead
needs to be minimized as far as possible.
MIMO
Multiple-input multiple-output (MIMO) is the heart of 802.11n. This technical discussion of MIMO provides a basis
for understanding how 802.11n can raise reliability and reach data rates many times higher than 802.11a/b/g in
the same radio spectrum. These benefits improve the experience of all wireless users, and are especially valuable
for customers with difficult RF environments, such as thick walls or small rooms, and for customers with high
throughput or quality-of-service (QoS) requirements, such as voice or video users.
Radio Operation Basics
To understand the improvement brought by MIMO technology, it is important to understand some of the basics
that determine how well a traditional radio operates. In a traditional, single-input single-output radio, the amount of
information that can be carried by a received radio signal depends on the amount by which the received signal
strength exceeds the noise at the receiver, called the signal-to-noise ratio, or SNR. SNR is typically expressed in
decibels (dB). Higher SNR enables more information to be carried on the signal and recovered by the receiver.
To understand the improvements that MIMO technology brings to 802.11, imagine trying to drive quickly down a
road. The bigger the car engine, the better its tires and suspension, and the lower the wind resistance, the faster
the car can go. At the same time, a road that is full of pot-holes or wind slows the car down. In this analogy, the
capabilities of the car are the signal, and the poor road quality is the noise. And better SNR means a faster
experience.
Spatial Propagation for MIMO
In typical indoor WLAN deployments - for example, enterprise offices, classrooms, hospitals, and warehouses - it
is rare for the radio signal to take only the direct, shortest path from the transmitter to the receiver. Often there is
no "line of sight" between the transmitter and the receiver: there is a cube wall, door, or other structure that
obscures the line of sight. All of these obstructions reduce the strength of the radio signal as it passes through
them, perhaps to an unusable level.
Luckily, there are other ways for the radio signal to travel from transmitter to receiver. Most environments are full
of surfaces that reflect a radio signal as well as a mirror reflects light. Imagine that all of the metallic surfaces,
large and small, that are in an environment were actually mirrors. Nails and screws, door frames, ceiling
suspension grids, pipes and structural beams are all reflectors of radio signals. It would be possible to see the
same WLAN access point in many of these mirrors simultaneously. Some of the images of the access point would
be a direct reflection through a single mirror. Some images would be a reflection of a reflection. Still others would
involve an even greater number of reflections.