WHITE PAPER The State of 802 - hosteddocs.ittoolbox.comhosteddocs.ittoolbox.com/wp_state_of_11n_0510_v2.pdf · meaning real throughput for TCP applications, not just theoretical data

© Copyright 2010 Meru. All rights reserved.

WHITE PAPER

The State of 802.11n

Date: September 2009


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Copyright © 2010 Meru Networks, Inc. All rights reserved. WP_state_of_11n_0510_v2.doc

TABLE OF CONTENTS

Introduction ..................................................................................................................... 3

1. The State of the Standards................................................................................... 4

2. The State of the Market ...................................................................................... 11

3. The Business Case for 802.11n ......................................................................... 14

4. Issues in 802.11n Network Design ..................................................................... 16

5. Real World Experiences ..................................................................................... 25

Summary....................................................................................................................... 27


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Introduction With the ratification of 802.11n, wireless LANs came of age. Though its status as an official IEEE standard is recent, the technology is mature and proven. The 802.11n specification is built upon the foundation laid more than ten years ago by 802.11b and improved in subsequent years with higher speeds, new network architectures, new radio technology and standards that advance security, quality of service and management. W In September 2009 Meru Networks celebrated the second anniversary of its initial 802.11n product shipments – the first in the enterprise wireless industry. Since that time, most of Meru’s competitors have also shipped 802.11n products, while the number of clients and consumer access points supporting the standard has grown exponentially. About 3000 customers of Meru are already enjoying a wireless experience as simple, predictable and trustworthy as wires.

HIGH PERFORMANCE

But 802.11n is more than just another evolutionary step. As Figure 1 shows, it marks the point at which wireless LANs can truly equal their wired equivalents in terms of raw performance – meaning real throughput for TCP applications, not just theoretical data rate. Many users already treat wireless as their primary means of connectivity, to such an extent that hardware manufacturers have begun to avoid wired Ethernet entirely. And with 802.11n now the most common wireless connectivity option in new client devices, the number of users relying on wireless increases all the time. When combined with Meru’s technology, 802.11n means that almost any organization can become an All-Wireless Enterprise. The move to 802.11n is a big one, but it doesn’t have to be disruptive. For users upgrading from previous wireless technologies, it is designed to be fully backwards-compatible – although care must be taken to ensure smooth coexistence in networks with mixed client types. For those upgrading straight from wired Ethernet, there are technologies that can give 802.11n all the reliability, security and scalability that users, applications and enterprises expect from wired networks.


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Figure 1: TCP throughout in 802.11a/g networks is around 20 Mbps, whereas 802.11n can reach nearly 200 Mbps – twice as fast as Fast Ethernet.

1. The State of the Standards The final 802.11n standard was ratified by the IEEE in September 2009, but products first shipped in 2007. This is because the Wi-Fi Alliance, the group that certifies 802.11 wireless LANs for interoperability, devised a set of tests around a draft of the standard. The initial products were tested to conform to Draft 2.0 of the standard, first published in early 2007. The specification has gone through many revisions since then, with the full standard based on Draft 11.0. But most of these have been very slight, and the final standard is close enough to Draft 2.0 that all products certified as compliant with the earlier draft are also interoperable with products based on the final standard. In fact, no retesting is necessary for existing equipment. As far as the Wi-Fi Alliance is concerned, 802.11n is 802.11n. That isn’t just good news for early adopters who deployed 802.11n access points before September 2009. It’s good news for anyone who bought a Wi-Fi laptop, phone or other device in the last three years, as there’s a good chance that it included a wireless interface based on 802.11n Draft 2.0. With no changes necessary to support the final standard, this means that a large and growing proportion of the installed client base in many organizations is already

11a

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802.11n-capable. All that’s needed to achieve the near-tenfold increase in data rate offered by 802.11n is an upgrade of the wireless infrastructure. “Draft 2.0 products will be allowed to claim to be fully 802.11n certified now.” – Edgar Figueroa, executive director, Wi-Fi Alliance

INNOVATIONS IN 802.11N

IEEE 802.11n improves on previous 802.11 systems in many ways, primarily designed to increase overall throughput but also boosting range and battery life. Because these enhancements affect multiple levels of the networking stack, the real gains in performance over 802.11g and 802.11a are even greater than implied by the increased headline data rate. The move to 802.11n doesn’t just increase the total capacity from 54 Mbps to 300 Mbps; it reduces the proportion wasted in signaling overhead and error-correction so that more is available to real applications. The improvements in 802.11n can be categorized according to which of the three lowest levels of the networking stack they affect as shown in Figure 2.Together, these increase throughput by a factor of about ten. In wireless networks, the lowest level of all is the RF layer, covering radio propagation and other phenomena unique to wireless networks. Above that is the PHY layer; roughly analogous to Layer 1 in wired Ethernet, though the actual technology is very different. Higher still is the MAC layer, corresponding to Layer 2 in wired Ethernet. The similarities between 802.11 and Ethernet are clearer here: the original 802.11 MAC was based loosely on that of Ethernet, though many changes have been made to deal with the unique characteristics of wireless networks, culminating in 802.11n. At Layer 3 and above, wired and wireless are identical. TCP/IP applications can treat an 802.11n network exactly as they would treat wired Ethernet.


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Figure 2: In addition to the MAC and PHY layers, wireless networks include an even-lower RF Layer

802.11n at the RF Layer

At the very lowest layer, beneath the traditional OSI or TCP/IP network stack models, 802.11n uses multiple antennas to improve signal diversity and quality. In an all-802.11n link, multiple antennas can be used to send multiple data streams at once, increasing bandwidth. They also have the useful side effect of improving reception, meaning that a well-designed 802.11n access point will improve performance even when used with legacy clients.

Spatial Multiplexing and MIMO

The most well-known innovation in 802.11n is MIMO (multiple-input, multiple-output), which uses parallel radio streams: sending multiple signals that each encode a different set of data and each travel via a different path, shown in Figure 2. Though the standard supports up to four streams, only two are needed to reach a data rate of 300 Mbps. Most equipment supports two, so Wi-Fi Alliance interoperability tests initially covered one or two streams. A system using two separate streams can send up to twice as much data as a system that uses only one. The receiver needs to recombine the two streams into one, similar to how someone listening to music can hear a different instrument in each ear and then recombine them into a tune. Just as the music listener needs two ears to do this, the receiver needs to have at least as many antennas as there are spatial streams so that it can tell them apart. Each stream needs to be transmitted from a separate antenna too, so the number of streams is limited by the number of antennas. To support two streams, both the transmitter and receiver need to have two antennas.

Layer 0: Radio Frequency Multiple antennas improve signal fidelity and add path diversity for increased reliability. Transmitters send multiple streams of data via different spatial routes to increase capacity.

Layer 1: Physical: Bonds two 20 MHz channels into one 40 MHz channel to double bandwidth Improved modulation and shorter gaps between signals boost the number of bits per Hertz.

Layer 2: Medium Access Control More efficient bursting, acknowledgement and retransmission algorithms make more capacity available to applications. Power saving techniques increase client battery life.


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Figure 2: The signal travels via two different paths from the transmitter (left) to the receiver (right.) This was traditionally a problem because of interference between different paths at the receiver. However, MIMO turns it into an advantage by sending a different signal along each path.

Antenna Diversity

Adding more antennas doesn’t necessarily add more spatial streams. Many access points available today include three antennas even though they only support two spatial streams. The extra antenna adds diversity, essentially improving reception for better coverage. Because all 802.11 systems drop down to lower data rates when signal strength is poor, this also helps improve data rate at most distances from an access point. A receiver with more than one antenna can also perform maximum ratio combining, which is essentially echo cancellation. This is needed because a signal can take different routes from transmitter to receiver, each copy arriving at a slightly different time. The receiver listens to the signal on two or three different antennas and calculates the time difference between these echoes, then reassemble the signal so that bits are in the correct order. Antenna diversity works better for receivers than transmitters, as the transmitter has no easy way to know how multipath interference will affect the signal it is sending. And while multiple transmitting antennas could theoretically increase the strength of a signal, this is prohibited by FCC regulations (and similar rules from regulators in other countries) that limit the transmission power allowed from unlicensed radios. Because antenna diversity is usually implemented at the access point, the speed boost it offers is asymmetric, improving upstream but not downstream performance.

High Performance 802.11n at the PHY Layer

At the physical layer, 802.11n uses several innovations to squeeze more data through the airwaves while improving reliability.


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Channel Bonding

The 802.11a/g standards used channels that were 20 MHz wide. The 802.11n standard offers the option of combining two together into a single 40 MHz channel without needing an additional radio. By itself, this doubles the available capacity from each radio in an access point. Channel bonding is perhaps the simplest technique used in 802.11n: twice as much spectrum leads to twice the performance. However, using larger channels means that fewer channels are available, an important consideration in network design. There can be particular problems when building out networks based on a “micro cell” architecture in which adjacent access points must use non-overlapping channels, especially in the narrow 2.4 GHz band.

More OFDM Subcarriers

The main innovation in 802.11a and 802.11g was the introduction of OFDM (orthogonal frequency division multiplexing), a technique that divides the available channel into many narrowband subcarriers. The advantage is that if many subcarriers are used, each one only has to support a relatively low data rate, improving reliability and predictability by making the system more resilient to interference and multipath effects. For example, 802.11a/g has a maximum data rate of 54 Mbps, achieved by slicing a 20 MHz. channel into 52 subcarriers. Each of these subcarriers only has to carry data at just over 1 Mbps. In 802.11n, the same size channel is divided into 56 subcarriers but the data rate of each subcarrier remains the same, increasing the overall throughput by about 8%. Because the 40 MHz. channel is twice as wide, it can accommodate 114 subcarriers, an increase of 119% as shown in Figure 4.

Figure 4: The number of subcarriers is increased in 802.11n, especially when Channel Bonding is used.

802.11a/g

52 subcarriers in 20-MHz Channel

56 subcarriers in 20-MHz Channel

802.11n

114 subcarriers in 40-MHz Mode Channel


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Short Guard Interval

To avoid confusion between multiple signals sent in a series, the 802.11 standards require that a short gap be left between them. In 802.11n, there is the option of shortening this gap (guard interval) from the 800 ns used in previous versions of the standard to only 400 ns. Because the radio spends less time waiting and more time transmitting, more data can be sent.

High Performance 802.11n at the MAC Layer

Although 802.11a/g offered physical layer data rates of 54 Mbps, real throughput in many networks is less than half that number thanks to protocol overhead. The 802.11n MAC layer is designed to be more efficient, leading to potential TCP throughput of around 200 Mbps.

Frame Aggregation and Selective Retransmission

Previous versions of 802.11need to send a header and a beacon before every data frame, then check for an acknowledgement (ACK) that the frame has been received. In 802.11n, up to 64 frames (or 64 Kbytes) can be sent together in a block called an AMPDU (Aggregated MAC Protocol Data Unit.)

Each AMPDU is acknowledged with a single ACK (a block ACK), avoiding the need for up to 63 headers, beacons and ACKs. Because frame loss is common in all forms of 802.11, the block ACK can specify exactly which frames it received, enabling retransmission of only those that were lost. Once all frames have been received loud and clear, software in the 802.11n device reassembles them into the correct order before passing them off to an application. The benefits of frame aggregation depend on the applications running. It is most useful for applications in which large amounts of data need to be sent at once such as file transfers. It isn’t useful in voice, as VoIP requires that a packet be sent every few milliseconds to avoid a noticeable gap in conversation. With the relatively low bandwidth requirement of voice, there are frequent small packets and no opportunity for aggregation.

Reduced Interframe Spacing

Frame aggregation requires that all frames are being sent to the same client. When an access point needs to transmit multiple frames to different clients, it usually leaves a small gap in between during which it checks that it still has access to the airwaves (i.e. that no client is about to transmit.) In 802.11n, access points are given preferential treatment so that they can avoid this check, transmitting multiple frames with a shorter space between them. Because Reduced Interframe Spacing requires that clients respect the access point’s right to send multiple frames in quick succession, it only works with 802.11n clients and cannot be used in mixed-mode networks that include legacy clients.


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DIFFERENCES BETWEEN DRAFT 2.0 AND DRAFT 11.0

The final IEEE 802.11n standard (Draft 11.0) is fully interoperable with equipment based on the earlier Draft 2.0 and products certified for Draft 2.0 are able to claim compliance with the standard. However, the two are not entirely identical, as shown in Table 1. The major new features in Draft 11.0 are:

Aggregation

Although many 802.11n devices have supported frame aggregation from the start, it was not included in the Wi-Fi Alliance’s tests covering Draft 2.0. Some early 802.11n client chipsets only include a 32 Kbyte buffer, which limits the maximum AMPDU size to this rather than the 64 Kbytes of most equipment. Because this is a hardware limitation, it is not something that can be changed without replacing the network interface card. Such clients can still use all 802.11n’s other performance-enhancing features.

Three Spatial Streams

The 802.11n standard supports up to four spatial streams, though this is optional and has not been implemented. With approval of the final standard, the Wi-Fi Alliance will start testing equipment that uses three. However, this too will be optional, while all equipment (except phones) must be able to support two. Although many access points include three antennas, most support two streams and use the extra antenna for increased diversity (better reception), which leads to improved reliability. Supporting three streams will require three antennas on clients, which adds to their cost and physical size. Transmitting from extra antennas can also increase power consumption, though this is mitigated by sending data faster and so needing to transmit for less time.

Coexistence Features

The final 802.11n standard also includes two extra features aimed at smoother interoperability between different forms of 802.11n. At the RF layer, space-time block coding lets single-stream devices (usually phones) join networks without adversely affecting performance for clients able to support two or more streams. At the PHY layer, a bonded 40 MHz channel can drop down to a single 20 MHz channel if one part of the channel is blocked by interference.

Table 1: Technologies Used in 802.11n Drafts Draft 2.0 Draft 11.0

Maximum Data Rate 300 Mbps 300 Mbps Channel Size 20 MHz, 40 MHz 20 MHz, 40 MHz Band 2.4 GHz, 5 GHz 2.4 GHz, 5 GHz OFDM Subcarriers 52, 56 or 114 52, 56 or 114 Guard Interval 400 or 800 ns 400 ns or 800 ns Number of streams 1 or 2; 3 or 4 optional 1 or 2; 3 or 4 optional Wi-Fi Certification 2 streams 2 streams (3 optional) Number of antennas 2 or more (1 for phones) 2 or more (1 for phones)


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Frame Aggregation Not mandatory Yes Backward compatibility 802.11a/b/g 802.11a/b/g

2. The State of the Market With products shipping for more than two years, many wireless devices already support 802.11n. The Wi-Fi Alliance has already certified more than 600 products as compliant with Draft 2.0 of the standard, none of which will need to be recertified. In addition, the final standard means that many more products are expected to ship over the next year. These are not just laptops or other clients where wireless replaces Ethernet. They include devices such as cell phones and tablets that previously have not offered (wired or wireless) LAN connectivity.

802.11N CLIENTS

Many networks that have not yet deployed 802.11n infrastructure already have a significant proportion of 802.11n clients among their installed base, thanks to decisions by laptop makers to include 802.11n as standard in new computers. This proportion is growing all the time and will be spurred by the ratification of 802.11n

Market Size

As Figure 5 shows, the 802.11n client market overtook the legacy 802.11 client market in mid-2009 even before the standard was officially ratified. Research firm Dell’Oro Group predicts that legacy 802.11 shipments will continue to fall rapidly, with all new wireless network interface cards supporting 802.11n by 2012. One important factor to note is that client adaptors only include mini-PCI, USB and PC cards that are typically included in devices such as laptops or bought separately for upgrades. They do not include devices such as cell phones that include 802.11 radios built-in. This is why the total size of the market is projected to fall: 802.11n capability will be standard on so many devices that separate wireless network interface cards become unnecessary.


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Figure 5: Increasing share of 802.11n clients. Source: Dell’Oro Group

Wireless Only Devices

An increasing number of devices now lack wired Ethernet entirely. The Apple MacBook is the best example among laptops, but many netbooks are also relying exclusively on wireless network connections. Wireless links are practical for classes of client that have never included Ethernet connectivity, from cell phones to locator badges. The wireless LAN is extending the network out to make it truly ubiquitous, a trend that will be accelerated by 802.11n.

INFRASTRUCTURE SIDE

Meru Networks shipped the first enterprise 802.11n infrastructure products in early 2007. Since then, many other enterprise vendors have shipped access points supporting Draft 2.0 of the standard. Take-up has been even more rapid in the home market, which has helped to drive client adoption and user expectations of high-performance wireless connectivity.

Market Size

As figure 6 shows, the enterprise 802.11n access point market continued to grow despite the recession. Although the overall market for wireless LAN hardware shrank in 2009, a result of the same economic forces that have affected the rest of the networking industry, the growing proportion of access points featuring 802.11n has more than compensated for this.


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Figure 6: Growth in 802.11n enterprise access point market. Source: Dell’Oro Group

As with the figures for client devices, the dollar numbers on the access point market do not tell the whole story. This is because much of the intelligence in most enterprise wireless LAN systems resides in controllers and management software rather than access points. The market share of controller-based systems is projected to increase as wireless access becomes increasingly critical, as independent access points are generally not suitable for large scale networks. The overall size of the enterprise wireless LAN market is about twice that for access points alone. Most controllers are not 802.11n-specific, so 802.11n controllers do not make up a separate category. However, the move to 802.11n is a significant boost to the controller market, as both the higher data rates of 8022.11n and the increased demand for wireless access will necessitate faster (or more) controllers.

The All-Wireless Edge

Dell’Oro group projects that unlike the market for network interface cards, the market for enterprise access points will grow for the foreseeable future. Much of this growth will come at the expense of wired networks, as 802.11n allows an increasing number of people to use wireless as their primary means of network connectivity. Few organizations will actually rip out their wires. In many cases, upgrading to 802.11n will be an alternative to upgrading Ethernet switches and cables, as wireless is the mode of access seeing an increase in traffic while wired traffic remains constant or even declines. Other organizations will forego cabling in new buildings, choosing instead to rely on wireless. The latter strategy is already popular in some industries such as higher education and healthcare, and likely to extend elsewhere as the benefits of 802.11n become clearer.


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3. The Business Case for 802.11n With IT budgets under constant pressure, many organizations are resistant to the idea of upgrading or installing a new wireless network. However, the 802.11n market is still growing, as even more organizations recognize that it can actually save money. The reason is that when properly designed and built, an 802.11n wireless network makes upgrades to edge Ethernet unnecessary.

INCREASED CAPACITY AND PERFORMANCE

The most obvious reason to upgrade to 802.11n is its increased data rate. For the first time, wireless networks are faster than their wired counterparts, opening up the possibility of replacing wires in almost all applications. Many users had already switched away from Ethernet to wireless links based on older versions of 802.11, but this often meant compromising performance. With 802.11n, such compromises are no longer necessary. In tests conducted by independent research firm Novarum1, a single 802.11n radio link using a Meru access point consistently offered real TCP throughput of more than 190 Mbps – about twice that of switched Ethernet. In addition to support for more bandwidth-hungry applications like video, this high performance enables greater user density and increased reliability.

Runs All Applications

Many applications require more bandwidth than 802.11a/g can comfortably provide. While the maximum data rate from an 802.11a/g network is typically about 20 Mbps, applications requiring lower data rates can still be impacted by insufficient bandwidth. This is because all 802.11 systems drop down to lower data rates whenever the signal to noise ratio is low, for example when the client is far from an access point or in the presence of interference. With 802.11n’s maximum of nearly 200 Mbps, there is much more bandwidth to spare, increasing reliability. In addition, antenna diversity means that the system is less likely to need to drop to lower data rates in a well-architected network. This opens up many new applications such as:

• Electronic medical record systems that need at least 24 Mbps • High-definition video streaming, important in schools as well as in the home market • Two-way video communication, used by some hospitals for real-time interpreting

between spoken language and sign language

Higher User Densities

Even applications that never need to consume more than 10 Mbps can benefit from 802.11n, as the greater overall capacity enables more users to share the same access point. The most important such example is voice, whose bandwidth requirements rarely exceed a few kbps yet is very demanding in terms of latency and quality-of-service.

1 * Enterprise 802.11n Wireless LAN Access Point Benchmark, available at: http://www.novarum.com/publications.php


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Because each voice client consumes less airtime using 802.11n, networks using 802.11n can support more users per access point. They can also offer each packet a high quality of service while ensuring minimal impact on data applications. In tests by Novarum, a single 802.11n radio on a Meru access point comfortably handled ten legacy voice clients with each call consistently sounding better than toll quality, all while maintaining high speeds for data users. When other vendors’ access points were similarly loaded, voice was unintelligible or silent altogether.

Extended Coverage

Equally important for many applications, an 802.11n network improves range and coverage compared to legacy 802.11g and 802.11a. This is due mostly to the multiple antenna systems used at both ends of the wireless link, and means that an 802.11n network can offer higher performance even when used with legacy equipment. Because the data rate in 802.11 systems depends on the client’s distance from the access point, improving range can also improve capacity. The area over which the maximum data rate is available will generally be greater in an 802.11n system than in a legacy system.

LOWER TOTAL COST OF OWNERSHIP

According to independent consultancy Network Strategy Partners2, an 802.11n wireless LAN based on Meru technology generally costs less than one third of a wired Ethernet system that provides similar capacity. For example, in an office measuring 250,000 square feet, wireless LAN network infrastructure would cost 57 cents per square foot compared to $2.07 per square foot for wired Ethernet. These savings have always been possible. However, previous wireless technologies could not always match the performance and predictability of wired Ethernet, meaning that a direct wired vs. wireless comparison was not possible for organizations with very demanding applications. Wireless was effectively a drain on resources – an extra edge network that had to be maintained and supported in addition to wired Ethernet. The increased performance of 802.11n changes things. Wireless becomes a cost saver, able to rival and then replace Ethernet. Enterprises can move to wireless confident that all their existing applications will run as well as on a wired network, saving money while users gain the added benefits of mobility.

BATTERY LIFE AND POWER SAVINGS

The extended range of an 802.11n network cuts power consumption compared to a legacy network, reducing cost and environmental impact at the same time as increasing the uptime offered from a UPS system of a given capacity. But the most significant energy-saving benefits are on the client side. For a device to be truly wireless, it needs to be able to run without a power cord for an extended period of time so battery life is critically important.

2 A Total Cost of Ownership Analysis of the Meru Networks Virtual Cell Wireless LAN Architecture, available at: http://www.nspllc.com/NewPages/meru.pdf


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Faster Transmissions to Clients

The faster a client can transmit data, the less time it needs to spend transmitting and thus the longer its radio can spend switched off. Higher data rates for downstream (access point to client) traffic also help, as actively receiving data uses more than passively waiting for it.

Better Reception at the Access Point

Many access points include extra antennas for increased gain through maximal ratio combining. This allows the client to transmit at a lower power, further reducing power consumption.

MAC Layer Energy Savings

In addition to speed improvements, the 802.11n MAC includes two mechanisms directly designed to reduce power consumption, increasing client battery life and reliability.

Spatial Multiplexing Power Save

Using multiple antennas can increase the power drain compared to using just one, even if they are set to listen only. Spatial Multiplexing Power Save allows the system to turn off all but one antenna so that the client can still listen for transmissions without wasting energy. The antennas that are powered down can rapidly switch back on when needed.

Power Save Multi Poll

If a client knows that it will not be receiving a transmission for a given amount of time, it can switch off all its radios and antennas until that time for dramatic power savings. Previous 802.11 standards were based on random access to the airwaves, with one device having no way of knowing when another would try to contact it. In 802.11n, clients can reserve a transmission at a specific time, making the transmission process more predictable and allowing them to switch off.

4. Issues in 802.11n Network Design Because 802.11n is so different from previous wireless technologies, network architecture principles that worked for 802.11a/b/g cannot necessarily be applied to 802.11n. Multipath effects and MIMO change signal propagation dramatically, making planning more difficult. The need for backward compatibility complicates things, as many networks will need to ensure that older clients are supported without slowing down 802.11n clients. The increased demands on 802.11n networks also entail new considerations in design and planning. With wireless a critical, primary network connection, it needs to match Ethernet in all ways, not just performance. While 802.11n itself provides increased wirelike speed, other techniques are needed to ensure that the network offers wirelike stability, security and scalability.


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DESIGNING THE NETWORK FOR MULTIPATH

Multipath effects are a result of the different routes that a signal can take between a transmitter and receiver. They have traditionally been regarded as a problem because signals that take different routes can interfere with each other. MIMO turns them from a bug into a feature by using the multiple paths to carry different streams of data. However, multipath effects can still cause issues. Because paths depend on obstacles between the transmitter, they are difficult to predict and fluctuate from moment to moment as people or objects move. This makes network design much more complicated.

Access Point Coverage is Unpredictable

In 802.11a/b/g networks, maps showing the radio footprint of each access point are relatively simple. An access point’s coverage area is a contiguous blob, with stronger signals available closer to the access point. Coverage can be represented as a series of concentric circles emanating from each of the APs, with higher data rate circles closer to the AP – not an entirely accurate representation, but one close enough for rough planning purposes. The main factor affecting coverage is distance from access point. Some types of obstacles can partially block radio waves, but these simply reduce the coverage area. In 802.11n, coverage is much less predictable. It depends on the way in which radio waves reflect off, refract through or diffract around obstacles such as walls, cube dividers and even people. Figure 7 shows the actual coverage of an 802.11n AP in an office building, higher data rates indicated by darker shades. Some areas of high performance coverage are very far away from the AP and not contiguous with those closer to it.

Figure 7: Coverage area of an 802.11n access point

Coverage Planning is Complicated

To ensure that a wireless network is available over a large area, the radio footprints of access points need to overlap so that clients can move from one to another without any interruption.


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However, overlapping coverage areas usually cause interference, regardless of which version of 802.11 the network is using. There are two different ways to solve this problem:

Microcell

Most 802.11a/b/g networks still use a microcell architecture, so-called because it is essentially a scaled-down version of the design pattern used in early cellular systems. Each access point is tuned to a different channel from its neighbors to avoid interference. The advantage of this approach is that it allows a network to be deployed with little coordination between access points, an important consideration when they were standalone devices that had to be managed independently. The traditional disadvantages are that it requires a lot of radio spectrum – at least three non-overlapping channels – and that it forces clients to retune to a new channel as they move between cells, guessing for themselves which access point they should connect to.

Figure 8: Microcell 20 MHz channel plan at 2.4 GHz in 802.11n (left) vs. 802.11g (right)

The move to 802.11n highlights another problem of the microcell architecture: the difficulty in planning the coverage pattern. As Figure 8 illustrates, three channels are sufficient to provide wide area coverage without interference in a network where all access points have a roughly circular coverage area such as one based on 802.11a/b/g. If the same AP placement is tried with an 802.11n network, it cannot operate at full power because the extended range results in increased interference. Turning down the power introduces dead zones – areas with no signal – because the spiky coverage areas are harder to fit together. Trying to fill these dead zones with new access points causes more interference, forcing a new channel plan to be created in which some access points’ power output is further reduced, perhaps causing new dead zones. Even introducing more channels is not always helpful, as shown in Figure 9.


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Figure 9: Attempt to provide pervasive 802.11n coverage using microcell at 5 GHz.

Virtual Cell

A Virtual Cell tightly coordinates the transmissions of adjacent access points so that all can use the same channel (or the same channels, as most modern access points have more than one radio.) The advantages of this approach are that less radio spectrum is needed for each later of coverage, as in the single-channel architecture of 3G and CDMA cellular networks, and that client devices are not responsible for connectivity decisions. As with 3G and CMA networks, this architecture lets multiple channels be used in the same physical space so that multiple Virtual Cells can coexist. The difference is that whereas 3G and CDMA use the multiple channels to support multiple competing operators, the Virtual Cell uses them to add capacity and redundancy to the network for increased performance and reliability, an architecture known as Channel Layering. The disadvantage is that it requires a lot of intelligence within the network. It is currently only available from Meru Networks. Although initially used with 802.11a/b/g, the Virtual Cell’s advantages are greatest in an 802.11n network. This is because it avoids the problems caused by co-channel interference automatically, allowing all access points to operate at full power. A dead zone can be filled by a new access point without causing problems for others. Rather than causing problems for each other, adjacent access points augment each other like light bulbs of the same color.

MIXED MODE NETWORKS

One important feature of 802.11n is full backward compatibility with previous wireless standards: legacy clients can connect to an 802.11n network, and the improved radio reception enabled by antenna diversity means that they should perform at least as well (if not better) than when connected to a legacy network. However, supporting legacy clients has its drawbacks. The diversity of wireless devices and drivers already causes issues with existing 802.11g networks, which can sometimes be held back by 802.11b clients. If the network is not well-designed, a single legacy client can slow down the network for all users. This issue is likely to get worse with 802.11n. The standard offers so many options that the difference between the fastest and slowest client is much more dramatic than in 802.11g. Data rates range all the way from 300 Mbps down to 1 Mbps.


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Because slower clients take longer than faster clients to send the same amount of data, they tend to dominate networks that allow random access. If all clients are allowed an equal chance to send a packet, they send equal numbers of packets on average and so an 802.11n network could spend nearly all its time listening to slow 802.11b transmissions. The worst client dominates the airwaves and the performance of the entire network suffers due to the slower clients. There are two effective ways of dealing with this: Airtime Fairness and Channel Stacking.

Airtime Fairness

Airtime fairness is based on bit fairness, a concept originally proposed in the 1980s for wired networks. Now implemented extensively in switched Ethernet, bit fairness ensures that all stations connected to a network receive an equal share of the network’s capacity. Wireless networks are more complex because different devices have different data rates and different loss rates. This is true even in single-mode networks: One 802.11n client connected to a network might see no packet loss and experience a full 200 Mbps, while another might be in a more noisy environment and suffer a lot of packet loss. The unicast MAC layer of 802.11n automatically retransmits lost packets so that applications don’t notice them, but these retransmissions take time, lowering the data rate. For example, if half the packets are lost, the data rate would fall by about a third So that slow clients do not hog the airwaves, fairness in wireless networks is better measured by time than data quantity. Instead of letting each client at a particular QoS level transmit the same amount of data, each is given access to the airwaves for the same amount of time. Thus, a fast 802.11n client can transmit about ten times as much data as a slower 802.11g client, just as each would in an all-802.11n or all-802.11g network. Meru Networks first introduced Airtime Fairness into production wireless LANs in 2003, used to prevent a legacy 802.11b device from taking over a channel in 802.11g networks. The introduction of 802.11n makes it even more critical. Although its benefits are most important in mixed-mode networks that combine 802.11n with legacy clients, Airtime Fairness is needed in every wireless network. It enables reliable service in two other ways:

Fairness Among Clients

If multiple 802.11n clients need to send or receive traffic, all should receive the same amount of airtime. This ensures predictable data rates: There is no use having a 200 Mbps network if one client takes all the bandwidth and leaves others with nothing. The important metric here is the minimum data rate of an 802.11n client. Ideally, this should be as close as possible to the average (and to the maximum.) For example, Novarum tests measured a total data rate of 180 Mbps when ten clients were connected to a Meru access point, making the average throughput 18 Mbps. The minimum was 15 Mbps so the network was fair and predictable. With another vendor’s access point, the total throughput was 140 Mbps but this was distributed very unfairly. Instead of each client getting roughly 14 Mbps, some got a lot more and one was denied any bandwidth at all.


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Uplink vs. Downlink fairness

As the number of devices connected to a network increases, the number of clients contending for uplink capacity far exceeds the number of access points contending for downlink capacity. WLAN infrastructure must be sophisticated enough to manage this ratio, ensuring that an access point is given enough airtime to transmit packets to all clients. A system that gives the access point the same airtime as an individual client will result in much lower downlink than uplink data rates.

POWER OVER ETHERNET

Most 802.11n access points incorporate multiple radios, each of which transmits on more than one spatial stream. Because of the multiple transmissions, an 802.11n access point requires more power than an 802.11a/b/g access point for the same number of radios, resulting in potential power supply issues. Traditionally, wireless access points have been powered through Power Over Ethernet (PoE) to avoid the need for a separate cable. The industry standard for PoE is IEEE 802.3af, in which DC power is injected by a switch or a separate inline device and carried over a twisted-pair cable. The nominal limit for 802.3af is 12.95W, too low for many 802.11n APs. Many vendors will claim that their 802.11n access points can be powered by standard 802.3af. However, it’s important to check the detail of these claims. Some may not operate at full power because the maker is assuming that they will usually be part of a microcell network in which most access points’ power needs to be turned down anyway. Others may not be able to use all of their antennas or support the maximum number of spatial streams when powered by 802.3af. For access points that require more power than 802.3af such as those with more than two radios, the other options are typically:

• A local DC power supply. This will generally provide the exact amount of power necessary, but it requires an AC outlet and means that remote power management is not possible.

• The new 802.3at standard for PoE. This aims to deliver at least 30W, but is not finalized

yet and requires new LAN switches or injectors.

• Multiplexed 802.3af. This requires an access point that can support more than one cable uplink, often for a redundant backend data connection as well as additional power.

CLIENT DESIGN ISSUES

The greatest barrier to 802.11n clients is the need for multiple antennas. As well as increasing the BoM (bill of materials) for building each client, they also increase the physical size: the antennas used to support MIMO must be separated by at least half a wavelength – 3 to 6 cm. at the frequencies used by 802.11. This is simply too large for many cell phones.


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As a result, the Wi-Fi Alliance is certifying phones for 802.11n support even if they include only one antenna. These do not benefit from the performance gains of MIMO but still support other performance improvements such as channel bonding and short guard intervals. Though they don’t run at 300 Mbps, they offer data rates at least twice as high as those of 802.11a/g devices without any increase in size or cost. Without MIMO, the maximum data rate is about 150 Mbps. This increase over 802.11/b/g is obviously useful for smart phones with data capability, but also brings benefits in networks with many devices used only for voice. Though the actual throughput requirement of most codecs is measured in kbps, the high data rate helps voice networks by allowing a greater user density per access point, extending battery life on the client and reducing the impact of voice on data users.

BAND AND CHANNEL SELECTION

Previous 802.11n standards were limited to either 2.4 GHz or 5 GHz, giving users no choice of band. Either can be used in 802.11n, and not all clients support both. Combined with the need for legacy client support, this means many deployments will need to support both.

2.4 GHz: Limited Channel Availability

The 2.4 GHz band is overwhelmingly the most popular, used in 802.11g and 802.11b. It traditionally had two huge benefits: lower cost client radios and longer range. The cost difference in radios is now disappearing, but the range difference results from the laws of physics and so remains. Though 802.11n increases the range and data rate at both frequencies, a 2.4 GHz signal will always go further than a 5 GHz signal. The big disadvantage of the 2.4 GHz band is that it is narrow and crowded, with room for only three non-overlapping 20 MHz. channels. With channel bonding in 802.11n, that number is reduced to one – a big problem for microcell architectures that require non-overlapping channels to avoid interference. As a result, most microcell vendors recommend that 802.11n only be deployed at 5 GHz. The Virtual Cell can support a 40 MHz. channel at 2.4 GHz, allowing full data rate deployments of 802.11n without interference. Because this only requires two of the three available non-overlapping channels, an 802.11b/g network can also be deployed in the remaining 20 MHz band.

5 GHz: Dynamic Frequency Selection

When the 802.11 standards were originally defined, they were restricted to a few channels at 2.4 GHz. and 5 GHz. Since then, the FCC and other national regulators have opened up more channels around 5 GHz, expanding the spectrum available to 802.11 networks. However, many of these channels are also shared with other users, notably radar systems. Radar takes priority, so wireless LANs need to move away from a channel when they detect a radar system using it, a process called Dynamic Frequency Selection.


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In a July 2008 research note3, analyst firm Gartner found several problems with most vendors’ implementations of DFS. Many access points ship with it switched on by default on all channels, even those where it is not required. By using it too much, networks risk becoming overwhelmed by frequent changes that can cascade across access points.

Switched on By Default

Though the FCC only requires DFS in certain 5 GHz channels, some access points apply it everywhere – even at 2.4 GHz. Because DFS can degrade performance, customers need to ensure that it is switched off except where really needed.

Ripple Effect

In networks based on a microcell architecture, retuning one access point can force all of its neighbors to retune too because adjacent APs must use non-overlapping frequencies. In turn, all its neighbors’ neighbors will also need to retune, ultimately causing a cascade of changes throughout the network. To make these changes, the network management system often attempts to calculate a new channel plan in real time, a process that tends to result in coverage holes and increased interference. These problems are more likely to occur in 802.11n than legacy networks because of the unpredictable coverage caused by multipath effects. Another potential problem with the newly-available channels at 5 GHz is that not all equipment supports every channel. This applies to both clients and access points, so it is critical to check specific channel support when choosing equipment. The newer channels may not be a good choice when guest access or other clients not owned by the IT department need to supported. As in 2.4 GHz, the Virtual Cell architecture avoids most of the problems with DFS because it requires only one channel for each layer of coverage. Multiple channels can be used to provide multiple layers, either network-wide or only in areas where increased coverage is needed as shown in Figure 10.

3 Wireless Dynamic Frequency Selection for WLANs Can Be a Losing Proposition, at: http://www.gartner.com/DisplayDocument?ref=g_search&id=721709


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Figure 10: Two additional channels are in use in the area on the right, while a third provides network wide coverage.

SECURITY

The 802.11n specification itself does not define any new security technologies as it is intended mainly to boost data rates. However, previous standards such as 802.11i have largely solved the earlier security issues found in wireless networks. With the right approach, wireless can actually be more trustworthy than wired. To ensure that no holes are left open in the network, 802.1n makes the advanced security of 802.11i mandatory. In addition, all Wi-Fi certified 802.11n products – Draft 2.0 and Draft 11.0 alike – must support WPA2, the set of interoperability tests covering 802.11i. The former assures users that their links are secure; the latter assures buyers that equipment can be secured easily. However, this can cause issues when supporting legacy applications.

WPA2 Mandatory

All Wi-Fi Certified 802.11n equipment supports Wi-Fi Protected Access 2 and 802.11i, with mandatory AES encryption for all secured links. In addition, most equipment aimed at business users supports WPA2 Enterprise, with authentication and key exchange via 802.1x. The enterprise version is more secure than the home version for two reasons: strong per-user authentication and new keys generated on the fly. Without WPA2 Enterprise, a permanent key is generated and shared between many users, making the key more likely to be compromised. Because 802.11n deliberately drops support for the older RC4 encryption algorithm, if offers only two encryption modes:

CLEAR

Traffic is not encrypted. This is useful in guest access and open networks. Organizations and end users can still encrypt data using software-based techniques such as TLS or IPsec VPNs, but encryption is not provided by the wireless network itself.

AES Encrypted

AES is the algorithm specified in 802.11i. Encryption is completely transparent to the user and to the IT department, while authentication can normally use the same client- and server- side software as on the wired network. Supporting Legacy Clients Most newer clients support AES, as do all Wi-FI certified 802.11n devices. However, many legacy applications and devices do not. If a device does not support AES, it will be unable to connect to an 802.11n network using its built-in encryption. This is true even if a device is physically capable of supporting 802.11n data speeds, and it doesn’t just exclude the notoriously insecure WEP. It also excludes TKIP, the form of RC4 specified by the original WPA certification.


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Non-AES devices connecting to an 802.11n network thus have two options: Don’t use encryption at all, or drop down to 802.1a/g. Because backward-compatibility is built into the standard, an 802.11n access point will still support all the encryption options offered by the older standards, including RC4-based WEP and TKIP. However, it will only support them at 802.11a/g data rates. A bigger problem for most enterprises is supporting legacy 802.11a/b/g devices that lack the capability to handle 802.1x and thus cannot use the enterprise version of WPA2. Because a network is only as secure as its weakest link, allowing these devices to connect using pre-shared keys (or TKIP) and without enterprise-grade authentication may open up a security hole that could be exploited by an attacker. To prevent such exploits, enterprises need additional lines of defense beyond that provided by encryption. These include:

Per-User Firewalls

A per-user firewall enables fine-grained control over the access rights of each device on a network, limiting it to specific authorized activities. For example, many older 802.11 phones lack 802.1x capability. An application firewall can ensure that devices which access the network as a legacy phone without data capability are only able to send VoIP packets, not access other systems. For maximum security, a per-user firewall needs to use flow signatures in addition to the more widespread technique of deep packet inspection, as DPI cannot understand the contents of encrypted packets.

Physical Security

A physical barrier can prevent radio waves from leaking outside a perimeter, making a wireless network completely undetectable. This was traditionally accomplished using radio jamming systems or Faraday cages – large metallic walls – but is now possible using selective signal blocking technology built into wireless networks themselves.

5. Real World Experiences Products based on IEEE 802.11n Draft 2.0 draft have been shipping since September 2007, giving enterprises across all industries nearly three years to prove the technology in real deployments. It is already in use by thousands of users worldwide, with more adopting it every day

MORRISVILLE STATE COLLEGE: PERVASIVE COVERAGE, HIGH PERFORMANCE

Morrisville State College in New York deployed the world’s first all-802.11n network two years ago. It is still one of the world’s largest, providing coverage in 43 buildings as well as outdoor areas like courtyards and the football field. It is the primary network connection for more than 3000 students, part of a generation for whom phones have always been mobile and computers have always been laptops.


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The network now handles more than 1500 simultaneous users, carrying a mix of data, voice and video traffic. It sees speeds right up to the maximum 300 Mbps, averaging download times nine times faster than with 802.11g. Whereas uploading a 50MB file from a laptop used to take 3 min 51 seconds, it now takes only 26 seconds. Table 2: Morrisville 802.11n Timeline

Clients Access Points Coverage August 2007 Laptops (Windows

only) 350 Raylink FHSS (2 Mbps)

Residence halls, some classrooms

October 2007 Laptops (Windows, Mac, Linux)

720 Meru AP150 (802.11a/b/g)

July 2009 Laptops, phones (iPhone, iPod Touch), Gaming devices (Nintendo DSS, Xbox 360)

750 Meru AP320 (802.11n)

September 2009 More than 400 different client types

800 Meru AP320 (802.11n)

All residence halls, classrooms and student hangout areas as well as the parking lot, equine stables and other outdoor areas

HALIFAX HEALTH: 100% RELIABILITY, PREDICTABLE AND TRUSTWORTHY

Halifax Health has been using wireless LANs since the early 2000s, successfully covering its flagship 764-bed hospital in Datona Beach with an 802.11a/b/g network used for voice and data applications such as barcode scanning for drug dose verification. But the legacy network proved insufficient for a move to full electronic medical records which Halifax was implementing in a new ten-storey tower. Though the legacy technology could theoretically reach the 24 Mbps that the EMR applications need, real bandwidth was often much less due to microcell interference or contention for access from multiple users. Halifax upgraded to an 802.11n network based on a Virtual Cell architecture, blanketing the 500,000 square feet of the facility with pervasive coverage. The old network is still in use in older parts of the building, the two coexisting with no problems. The new 802.11n network supplies enough bandwidth for the EMR application as well as Lifelinks remote interpreting, a video system that connects a hearing-impaired patient to a live interpreter who translates a healthcare worker’s speech into sign language in real-time. Both the EMR application and the Lifelinks system are hosted on “Workstations on Wheels”, carts that staff move around to wherever needed. The hospital is also expanding its use of wireless telephony, using both Siemens VoIP phones and Vocera badges, as well as exploring new applications such as a semi-autonomous janitorial robot. But the major benefit of 802.11n in a Virtual Cell is reliability: If the wireless EMR system went down, doctors and nurses would have to resort to a time-consuming manual process. With it, they can devote themselves fully to patient care.


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THE WASHINGTON NATIONALS: VERY HIGH USER DENSITY

Opened in March 2008, Nationals Stadium is more than just the first Major League Baseball park to offer 802.11n coverage in all areas. It is a showcase for innovation, relying on its wireless network both to make its own operations more efficient and to improve the experience for all fans of the Washington Nationals– whether they are on the bleachers, in the park's hospitality suites or at home watching games through the media. The stadium’s network is available throughout the 41,88-seater park, offering Internet access to guests as well as multiple services to the stadium’s own staff who use Avaya phones and Symbol handheld ticket readers that can be carried to the most crowded entry points. The readers verify each ticket's authenticity with a database at Tickets.com to eliminate forgeries and scalping, so even a fraction of a second's delay in checking each ticket could add up to long lines. The network is also made available to the media and to the stadium’s more than 200 food concessions who need always-on on connectivity when verifying customer payment information. Future plans call for "room service"-style ordering, allowing fans to buy food or drink over the network and have it delivered to their seats so that they don't risk missing a crucial part of the game.

Summary With thousands of customers already using it for critical applications, 802.11n is a mature and reliable technology. Its high-performance is proven in industries including education, healthcare, manufacturing, retail and hospitality, running data, voice and video simultaneously over networks spanning hundreds of access points. The technology is used both indoors and outdoors, serving laptops, phones, locator badges and client devices of all kinds. When combined with the appropriate architectural choices, it gives wireless the speed, security and scalability of wired Ethernet – all while realizing cost savings over legacy wired or wireless systems. With official ratification, 802.11n is already taking over the wireless networking market on both the infrastructure and the client side. But its true implications are broader, with continued growth likely at the expense of wired Ethernet. As wireless now offers all the benefits of wires but with added mobility and reduced cost, an increasing number of enterprises move to an all-wireless edge. Deploying 802.11n is more than just a matter of replacing legacy radios. To maximize its benefits, organizations need to design networks for 802.11n from the ground up. This means taking account of multipath effects, bonded channels and the need to support legacy clients, as well as the standard’s full security implications and the increased demands likely on the network. Doing so will make the network simple and trustworthy, assuring predictable service levels for all users. By using an architecture purpose-built for 802.11n, IT departments can ensure that users and applications receive the same performance and reliability that they expect from wired Ethernet.

Documents

WHITE PAPER The State of 802 - hosteddocs.ittoolbox.comhosteddocs.ittoolbox.com/wp_state_of_11n_0510_v2.pdf · meaning real throughput for TCP applications, not just theoretical data