802 11n Next Generation Wireless

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Whitepaper

802.11n The Next Generation

in Wireless Technology


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I n t r o d u c t i o nWireless technology continues to evolve and add value with its inherent characteristics. First came 802.11, then a & b, followed by g, and now802.11n. It has taken more than seven years to arrive to the final ratified proposal bringing with it several enhancements.

This document describes the new capabilities and summarizes the benefits from the improvements made by 802.11n.

8 0 2 . 1 1 n Te c h n o l o g yFor the past seven years, the IEEE standards body has been working on a new standard that would standardize an upgrade to the 802.11 radio

while providing a new set of capabilities, dramatically improving reliability of 802.11 communications, the predictability of wireless coverage, andincreasing the overall throughput of 802.11 devices, all the while keeping backwards compatibility with legacy 802.11 environments.

The initial 802.11n work has largely been based on the 802.11n Wi-Fi Alliance draft version 2.0 which includes the following key devicerequirements which the final version now includes:

MIMO Describes the use of multiple-input multiple-output (MIMO) technology

Radio Enhancements Increased channel size, higher modulation rates, and lower overhead

MAC Enhancements Modifies the frame formats used by 802.11n devices from those of existing 802.11 devices

M I M OMultiple-input multiple-output (MIMO) is the heart of 802.11n. This technical discussion of MIMO provides a basis for understanding how802.11n can reach data rates of 600 Mbps.

The nature of wireless communications is vulnerable to all sorts of interference, and distortions or noise. Similar to wired technology, signal tonoise ratio (SNR) efficiency is crucial to the ability to provide efficient data transmission. The higher the SNR number the more information thatcan be carried on the signal and be recovered by the receiver.

802.11n employs two interesting techniques to improve SNR and multipath environments: Beamforming and multipath or spatial diversity. Thefollowing sections describe their functionalities and benefits.

Transmit BeamformingBeamforming is a technique used when there is more than one transmit antenna and a single receiver in an open or limited obstruction location.

When there are more than one transmit antenna, each radio signal being transmitted will have a different angular phase. These differences affect

the overall signal to noise ratio. By adjusting these phases so that they match at the receiver, the signal to noise ratio is dramatically increased,and thus adding to the total amount of information that can be carried by the signals and recovered by the receiver.

By now, we can see that this particular methodology depends closely on a feedback mechanism between the transmitter and receiver. Informationfrom the received signal is sent back to the transmitter allowing the transmitter to adjust its radio signals.

There are several caveats that should be mentioned when implementing beamforming.

1. This particular technique is only available with 802.11n transmitters and receivers.

2. It can only be implemented when transmitting to a single receiver.

3. The feedback mechanism between receiver and transmitter is not immediate and short. If either transmitter or receiver moves, therelationship would have to be re-established.

Multipath or Spatial DiversityMIMO technology makes use of multiple radio signals where each signal has its own spatial stream sent from its own antenna. In a typicalwireless environment, any signal transmitted will without a doubt encounter some type of interference or reflection. As the number of transmittersincrease, the number of signals being transmitted increase. As a result, signals are being received from different paths and at different times. Thiscondition is referred as multipath.

Since each antenna functions independent from the other, different data streams will source from each. At the receiving end, each data streamfrom each radio is combined, and after some complex processing, a cleaner or stronger signal is arrived resulting in a higher signal to noiseratio.

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R a d i o E n h a n c e m e n t sBesides the introduction of a more efficient use of antennas, 802.11n has made additional changes to the radio to increase the effective throughputof the WLAN. The most important of these changes are increased channel size, higher modulation rates, and reduced overhead. The followingsection will describe each of these changes and the effect they have on WLAN throughput.

20- and 40-MHz ChannelsA brief background on spectral efficiency and channel bonding is needed in order to understand how the enhancements on radio frequencycontribute to the overall increased performance.

In the original 802.11 standard as well as with 802.11b, their spectral efficiency or channel bandwidth is one-half the bits per hertz, with 802.11aand 802.11g, the spectral efficiency is 2.7 bits per hertz @ 54Mbps. The higher the spectral efficiency the more efficient the usage of a limitedfrequency space can be attained.

In addition to spectral efficiency, there are some proprietary WLAN systems that use a clever technique that bonds each 802.11g channel(54Mbps) into two channels known as Super G or channel bonding providing up to 108Mbps.

With channel bonding, the spectral efficiency is the same as 802.11a and 802.11g, but the channel bandwidth is twice as great. This provides asimple way of doubling the data rate.

One of the key enhancements found in the new 802.11n standard is its efficient usage of both 20-MHz and 40-MHz channels. Similar to otherproprietary products, the 40-MHz channels in 802.11n consists of two 20-MHz channels, bonded together. When using the 40-MHz bondedchannel, 802.11n takes advantage of the fact that each 20-MHz channel has a small amount of the channel that is reserved at the top and

bottom, to reduce interference in those adjacent channels. When using 40-MHz channels, the top of the lower channel and the bottom of theupper channel don't have to be reserved to avoid interference. These small parts of the channel can now be used to carry information. By usingthe two 20-MHz channels more efficiently in this way, 802.11n achieves slightly more than doubling the data rate when moving from 20-MHz to40-MHz channels (see figure 1).

Increased Modulation Rates802.11n uses a well known modulation technique known as orthogonal frequency division multiplexing (OFDM) which divides a radio channelinto a large number of smaller channels, each with its own subcarrier signal (see Figure 1 above). Each of these carrier signals conveysinformation independent of all the other carrier signals. Think of a group of independent radio frequencies bunched together.

802.11n increases the number of subcarriers in each 20-MHz channel from 48 to 52. This marginally increases the data rate to a maximum of65 Mbps, for a single-transmit radio. 802.11n provides a selection of eight data rates for a transmitter to use and also increases the number oftransmitters allowable to four. For two transmitters, the maximum data rate is 130 Mbps. Three transmitters provide a maximum data rate of 195Mbps. The maximum four transmitters can deliver 260 Mbps. In total, 802.11n provides up to 32 data rates for use in a 20-MHz channel.

When using 40-MHz channels, 802.11n increases the number of subcarriers available to 108. This provides a maximum data rate of 135 Mbps,270 Mbps, 405 Mbps, and 540 Mbps for one through four transmitters, respectively. Similarly, there are eight data rates provided for eachtransmitter, 32 in total, for the 40-MHz channel.

Figure 1. 20 MHz and 40 MHz Channels


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Data

Lowered Overhead: Guard IntervalOFDM incorporates a safety mechanism that prevents any intersymbol interference found in multipath environments. The intersymbol interferencecondition takes place when the beginning of a new symbol arrives at the receiver before the end of the last symbol is done (see figure 2).

802.11n uses 800 nanoseconds as the guard interval allowing for multipath difference of 800 feet. However, this guard interval can be reducedprovided the multipath environment is not too rigid about the 800 feet difference between the receiver and trasnmitter. 802.11n can configure theguard interval to 400 nanoseconds or 4 microseconds. This small change provides an increase in data rates. For 20-MHz channels, maximumdata rates for one to four transmitters with the reduced guard interval are 72, 144, 216, and 288 Mbps, and 150, 300, 450, and 600 Mbps fora 40-MHz channel.

M A C E n h a n c e m e n t sIn the previous sections we can see how improvements in radio data rates have resulted in increased performance. However, these improvementscan only do so much.

Every packet or frame transmitted has a certain amount of overhead. To be more precise, MAC layer protocol overhead. In addition to thisoverhead, interframe spaces and acknowledgements contribute in the reduction of the effective maximum throughput. 802.11n has introducedseveral changes in the way MAC layer protocol overhead is handled.

To reduce this overhead, 802.11n introduces frame aggregation. Frame aggregation is essentially putting two or more frames together into asingle transmission. 802.11n introduces two methods for frame aggregation: Mac Service Data Units (MSDU) aggregation and Message ProtocolData Unit (MPDU) aggregation. Both aggregation methods reduce the overhead to only a single radio preamble for each frame transmission (seeFigure 3).

MAC Protocol Data Unit AggregationMPDU aggregation works differently than MSDU aggregation. Rather than collecting Ethernet frames, MPDU aggregation translates each Etherneframe to 802.11 format and then collects the 802.11 frames for a common destination. The collection doesn't require wrapping of another802.11 frame, since the collected frames already begin with an 802.11 MAC header (see Figure 4).

RP = Radio Preamble, RH = Rapid Header, MH = MAC Header, MSDU = Ethernet Frame

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Figure 2. Guard Interval

Figure 3. Aggregation

Figure 4. MPDU Aggregation

RadioPreamble

RadioHeader

MACHeader

FCS

RP RH MH MH 1 Data 1 MH 2 Data 2 MH 3 Data 3 MH N Data N FCS


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MPDU is less efficient because of the extra overhead that is part of the aggregated frame. Figure 4 shows the frame format with a headerfollowed by the data payload. In addition, this efficiency is further reduced when encryption is included.

Another interesting feature that is part of MPDU aggregation is the use of block acknowledgement. With block acknowledgement, a singleacknowledgement frame is produced by the recipient to the sender as a result of any frames that are not acknowledged. In environments wherehigh number of errors are inevitable, reducing the number of acknowledgment frames is critical.

MAC Service Data Unit AggregationMSDU aggregation is the more efficient of the two types of aggregation. MSDU works by aggregating Ethernet frames with a common destination

wraps them into a single 802.11 frame and then transmits that 802.11-wrapped collection of Ethernet frames (see Figure 5).

MSDU= Ethernet Frame

Unlike MPDU where each frame had a header as part of the aggregated frame, MSDU has only one header consisting of the radio preamble,radio header, and MAC header. Furthermore, the aggregated frame is encrypted only once, whereas, with MPDU, each individual frame isencrypted.

Similar to MPDU, there are some restrictions in that all of the constituent frames must be of the same quality-of -service (QoS) level. It is notpermitted to mix voice frames with best-effort frames.

S u m m a r yWith the ratification of 802.11n, wireless technology continues to mature and deliver on key features that translate into increased benefits fortodays and tomorrows businesses.

The enhancements brought up by 802.11n deliver the following key benefits:

Increased area coverage The introduction of MIMO technology and its multipath effect reduces dramatically the chances of deadspots. The same locations that suffered from multipath effects now make use of this condition to enhance area coverage.

Increased data transfer reliability Another advantage with MIMO technology is its SNR efficiency increase. This increase translates intogreater data transmission and more robust to interference laden environments.

Increased throughput The efficiencies made in the 20-Mhz and 40-MHz channel spectrum, channel bonding, and guard intervaltiming reduction have contributed to an overall increase in throughput.

With the benefits that come with 802.11n, there is no reason why businesses considering deploying wireless technology should not be lookinginto 802.11n. This new standard has the potential to increase the WLAN capacity and throughput of every client while keeping cost low,turn-up-time short, return on investment fast, and seamless transition

MSDU 3

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Figure 5. MSDU Aggregation

RadioPreamble

RadioHeader

MACHeader

MSDU 1 MSDU 2 MSDU N FCS


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802 11n Next Generation Wireless