inside 802.11n wireless lans - practical insides and analysis

7/31/2019 Inside 802.11n Wireless LANs - Practical Insides and Analysis

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By J. Scott Haugdahl

Founder and CTO, Bitcricket

www.bitcricket.com

A Bitcricket White Paper

Abstract

The IEEE 802.11n Draft Standard specifies anext generation wireless LAN (WLAN)technology promising nearly twice the reach and

far better throughput than 802.11abg legacy devices. The technology is very complex (thedraft is over 470 pages long) and has evolved during the standards process with a history of battle lines between contributors. The dust has

settled and we are finally seeing real deliverabletechnology and interoperability.

This 16-page white paper takes a brief look at the history behind the process, the convergenceto a draft standard, the promise of 802.11n,details on improvements in both transmissionspeed and protocol efficiency, and several major milestones pushing 802.11n into the enterprise.

Unique to this white paper is a look at 802.11nin action by capturing frames from anoperational system using 40 MHz bandwidth,multiple antennas, and multiple streams. Suchcapture and analysis takes us inside 802.11noperation, helping us to better understand how it works, especially new features such as block

ACKs and aggregated frames.

Inside 802.11n Wireless LANsPractical Insights and AnalysisDecember 2007


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Inside 802.11n Wireless LANs: Practical Insights and Analysis

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Copyright 2007 Bitcricket LLC. All rights Reserved.

Contents

A Brief Historical Perspective .............................................................................................. 2 March Madness .................. .................... ................... .................... ............. .................... ..... 4

The Promise .................. .................... ............... .................... .................. ................... ........... 5

Cutting the Overhead .......................................................................................................... 7

802.11n Captured Frame Analysis ...................................................................................... 9

Major Milestones: 802.11n Goes Enterprise ................................ .................... ............. .. 12

An Ominous Cloud? ........................................................................................................... 13

Will 802.11n Replace Ethernet in the Future? .................. .................... ................... ......... 14

Conclusion ......................................................................................................................... 16

A Brief Historical Perspective

Did you know that the original IEEE wireless LAN standard was ratified in 1997 with atop transmission rate of 2 Mbps using frequency-hopping spread spectrum (FHSS)? Itwasnt until the adoption of the IEEE 802.11b standard in 1999 with an option for a fixed

direct sequence spread spectrum (DSSS) operation with data rates up to 11 Mbps thatWiFi really took off, despite a weak security mechanism. Meanwhile, the lower speedfrequency hopping idea gained traction in a different market altogether Bluetooth forlow-power, close-range peripherals.

Interestingly, at the same time as 802.11b, 802.11a for operation in the 5 GHz (vs. 2.4GHz) band at 54 Mbps was also approved. Unfortunately a never really took off,probably due to its shorter range and more costly components for higher frequency andfaster operation.

Limited non-overlapping channels coupled with increasing demand for speed and

mobility escalated the demand for faster wireless. Unlike wired, one cannot simplyincrementally add another 100 Mbps port. In some sense wireless is a throwback to thedays of Ethernet hubs with a shared collision domain and contention for availablebandwidth. Worse, there are numerous other devices in the 802.11b 2.4 GHzunlicensed band that can interfere with operation, from proprietary 2.4 GHz phones andwireless mice to security cameras and even baby monitors. A Logitech 2.4 GHz mouse I


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recently purchased is neither Bluetooth nor 802.11 and actually works up to 100 feetaway! Anything goes when unlicensed. What a mess.

Unlike frequency hopping, a technique popular with military communications to avoidinterference and jamming, 802.11 devices operate on fixed frequencies and cannot

easily get out of the way in a dynamic, congested, and interference-proneenvironment.

Meanwhile 802.11g, approved in 2003 (four years after 802.11b), was a stopgap forincreasing throughput. Despite small improvements in the efficiency of the protocol(such as a tighter slot or interframe time), the real usable data throughput was stilllimited to only slightly better than 1/3 of the raw transmission rate, yielding close to 20Mbps. Worse, 802.11b was so well entrenched that many g performance gains werewiped out by having to coexist with legacy deployments.

This may be hard to comprehend, but think of it this way: An 802.11g device gets a slot

to send a packet and transmits faster (54 Mbps raw best case) than 802.11b. So far sogood. But then b gets a turn, and transmits some 5x slower than g. That robs time awayfrom g when it could have sent five packets in the same amount of time. Thus, the end-to-end throughput of two devices operating at the same time, one 802.11b and one802.11g, is effectively the same!

One of the cool improvements 802.11g (and 802.11a) introduced to achieve higher datarates was orthogonal frequency division multiplexing (OFDM), where a radio channel isdivided into several smaller channels, each with its own subcarrier.

Not long after 802.11g was approved, the IEEE formed a task group designated TGn towork on the next generation of wireless, 802.11n. 802.11n continued to tweak OFDMby making subtle improvements such as adding more subcarriers. The biggestimprovement, however, was the introduction of MIMO (multiple input, multipleoutput). As defined by 802.11n, MIMO is when both the transmitter and receiver usemultiple antennas.

There were originally four competing proposals that were then whittled down to two:TGnSynch (Task Group N Synch) and WWiSE (World-Wide Spectrum Efficiency). Bothapproaches use a technique to send two or more data streams (spatial streams) slightlyout of phase using two or more antennae (one for each stream). This, coupled withnatural reflection of signals in the environment, results in multipath reception at thereceiver as illustrated in the following figure. Receivers then use complex algorithms toseparate the out-of-phase signals and recombine multiple streams back together,resulting in a much higher aggregate data rate when compared to 802.11abg. Per thecurrent 802.11n Draft, up to four streams can be sent simultaneously on a 20 MHz or 40MHz channel. Using a 40 MHz channel, 4 streams, and the optional short guard interval,the maximum transmission rate is 600 Mbps.


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Multiple Signal Transmission with Multipath Reflections

Meanwhile, 2005 turned into a tough year when the standards efforts suffered setbacksdue to heated disagreements between the remaining proposals. Most of it boiled downto technical details in stuff like spatial streams and subcarriers.

A handful of semi-conductor vendors (Atheros, Broadcom, Conexant, Intel and Marvell)decided that enough was enough and formed the Enhanced Wireless Consortium (EWC).When officially announced in October 2005, there were 26 members. Airgo was thenotable holdout, working on a 3 rd generation chipset not compatible with the EWCdirection. Up to now, all currently shipping 2 nd generation Airgo pre-N chips weredesigned around what they called OFDM MIMO, essentially an a MIMO version of 802.11g that operated on a single 20 MHz channel at 108 Mbps. The new chipsetpromised 240 Mbps at best.

The EWC group was pursuing a tweaked OFDM for higher throughput and of course, a40 MHz operation with up to 4 streams to operate up to 600 Mbps. It wasnt longbefore EWC draft specs were published outside the IEEE, and the WiFi Allianceannounced a new certification program for pre-802.11n products.

Trivia: How is MIMO pronounced? According the 802.11n Draft, the correct

pronunciation is mie-mow as approved at an 802.11 interim meeting in 2004 in Berlin.

March Madness

In March of 2006 the IEEE voted 42-0 vote with 2 abstaining to elevate 802.11n to Draft1.0 status, based largely on the consolidated EWC effort and Airgo was sold off toQualcomm in December of that year. Qualcomm has since reengineered the chipset to


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include both 20 MHz and 40 MHz MIMO operation, embracing the 802.11n Draftstandard.

Meanwhile Draft 1.0 needed some additional work to detect legacy devices and back off to single channel transmission to other N devices if necessary. It seems that 802.11n

would play nicely with 802.11gs single OFDM channel while still being able to use 40MHz when needed. The presence of 802.11b, which does not use OFDM and in factuses a slightly wider channel (22 MHz), would confine N to a single channel distancedfrom the 802.11b channel. There are no such restrictions in the 5 GHz band since any802.11a devices already use 20 MHz channels and OFDM.

Nine months later in January of 2007, 802.11n Draft 2.0 was approved, a majormilestone and catalyst for 802.11n acceptance as well see later in this white paper.

Trivia: What was the name of the next generation wireless proposal after TGnSynch and WWiSE? Answer: Mac and MIMO Technologies for More Throughput" or MITMOT for

short. Cute, no? Originally proposed by Motorola and Mitsubishi, MITMOT waseliminated in an IEEE January 05 vote and the two companies eventually jumped ship toEWC.

The Promise

802.11n promises up to 600 Mbps of raw throughput with greater range and efficiency.By efficiency, I mean less overhead for real data transfer. First, lets look at the increasein speed and the caveats to achieving 600 Mbps beginning with the mandatory

requirements as required by 802.11n.

To obtain Wi-Fi Alliance 802.11n Draft 2.0 Certification, a device must implement aminimum set of mandatory capabilities specified in the IEEE draft. Specifically, a devicemust implement 2 spatial streams in transmit mode, 2 spatial streams in receive mode,the A-MPDU and A-MSDU, and block ACK (more about this later). This simplifies thingsin that only a single 20 MHz channel in the 2.4 GHz band is required. These minimummandatory requirements roughly double the raw data rate over 802.11g in a singlechannel to 130 Mbps (2 x 65 Mbps streams).

To gain the full benefit of 802.11n, a device will also implement the optional 40 MHzoperational mode as well as utilize additional spatial streams to boost throughput.

Another option is the guard interval, an 800 ns gap between symbols whiletransmitting (depending on the data rate, a symbol carries up to 216 bits of data). Ashorter 400 ns interval is optional and boosts a single spatial stream operating at 20MHz from 65 to 72.2 Mbps, and from 135 to 150 Mbps when operating at 40 MHz.


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To take advantage of the short gap option, both the access point and client mustsupport it. The short gap can be used in cases where the difference in reflection pathdistances to the receiver is minimal. This seems like an inexact science with no specificguidelines, so to be safe start with the standard interval then experiment if you wish tooptimize the stream efficiency.

The Wi-Fi Alliance will test various options as well, ensuring interoperability for vendorsthat choose to implement them. In fact, the majority of 2.0-certified devices to datesupport 2 spatial steams. In rough numbers, this gives us 130/144 Mbps for 20 MHzoperation and 270/300 Mbps for 40 MHz.

Yet another interesting option is beam forming. Aside from multiple transmit antennas,beam forming also requires tuning between a transmitter and receiver and, as such,only works when sending to a specific device and only one antenna is used forreception. During beaconing and sending multicast or broadcast traffic, there is nobeam forming. This limits its usefulness as far as coverage area is concerned but is agood way to optimize one-on-one transmission and reception for better throughput andreliability.

To summarize, the promised transmission speed improvements offered by N, WiFicertified 802.11 Draft 2.0 equipment should all interoperate at 130 Mbps. Most willinteroperate at 270 Mbps.

The figure below summarizes the raw transmission rates for various combinations of channels and streams.

Streams 1 2 3 4

20 MHz (Single Channel) 2.4 GHz or 5 GHz Band

Standard Guard Interval 65 130 195 260Short Guard Interval 72.2 144.4 216.7 288.9

40 MHz (Channel Bonding) 5 GHz Band

Standard Guard Interval 135 270 405 540Short Guard Interval 150 300 450 600

Maximum 802.11n Transmission Rates in Mbps. The blue boxed area is mandatory.

Note that 40 MHz operation in the 2.4 GHz band is currently discouraged. It is notrecommended by the WiFi Alliance pending a definitive decision in the 802.11nstandard.

Enterprises will want to gain the maximum from 802.11n which will impact site designand roll-outs. For instance, the old honey-comb coverage rule of 1-6-11 in the 2.4 GHzband need no longer apply. Think about it. With channel bonding for 40 MHz


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operation, there is only one non-interfering, non-overlapping deployment possible! Thebest all 802.11n coverage may be a 1 and 5 and 7 and 11 scenario, but channels 5 and 7are still too close together to be practical.

And of course there are millions of legacy devices to deal with that will be with us for

awhile. How to stagger and optimize coverage for both 802.11n and 802.11bg? Bear inmind that all 802.11n access points are backward compatible to accommodate legacyclients.

Thus the rising interest in 5 GHz originally introduced with 802.11a. The 5 GHz bandcertainly has some attractive attributes including staying out of the way of 802.11bg andits ability to accommodate more N channels (up to 3 non-overlappingchannels). Unfortunately, 5 GHz signals peter out quicker than 2.4 GHz given the sametransmit power (i.e. you get less distance with 5 GHz), so we are back to a similarproblem we had with 802.11bg vs. 802.11a.

Meanwhile, there is at least some performance gain even for single channel Ndeployment (as previous mentioned, 130 Mbps using 2 streams on a single 20 MHzchannel). Furthermore, Intel is supporting only single channel operation with Centrino802.11n in the 2.4 GHz band, because of the legacy concerns. Some 802.11n APs willalso be smart enough not to offer the 2nd channel when 802.11bg is present. But again,this speed bump effectively throttles N when in use in the 2.4 GHz band.

I think the dual channel controversy will be put to rest in light of Ciscos late 2007802.11n equipment rollout for dual channel support in the 5 GHz band the first to beWi-Fi Alliance certified for that option.

Trivia: In terms of raw RF energy, 802.11b channels are 22 MHz wide. The efficiency of 802.11b is that, or 11 Mbps. 802.11agn use 20 MHz (and 40 MHz for N wide channels)with a much higher efficiency of bits per Hertz.

Cutting the Overhead

Aside from the RF improvements, 802.11n has also improved the efficiency of the MAC(Media Access Control) protocol using a single ACK mechanism for multiple frames and

the ability to aggregate multiple frames into a single transmission.In 802.11abg, each frame is acknowledged immediately following each transmission.This added robustness comes at a price no one can transmit a frame while the currentsender is waiting for an ACK.

802.11n adds a block ACK (BA) mechanism in which multiple frames can be streamedout and acknowledged by a single frame. This cuts the wait time between frame


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transmission and allows just the missing frames or frames received in error to be resentby checking a compressed bit map. The transmitter need send only one BA request(BAR) to the receiver. If the receiver acknowledges it, the BA mechanism may be usedfor the duration of the physical connection (such as between a client and access point).

802.11n can also send multiple frames per single access to the medium by combiningframes together into one larger frame. There are two forms of frame aggregation:Aggregated Mac Service Data Unit (A-MSDU) and Aggregated Mac Protocol Data Unit (A-MPDU).

A-MSDU increases the maximum frame transmission size from 2,304 bytes to almost 8kbytes (7935 to be exact) while A-MPDU allows up to 64k bytes.

A-MSDU creates the larger frame by combining smaller frames with the same physicalsource and destination end points and traffic class (i.e. QoS) into one large frame with acommon MAC header. One way to visualize this is an access point receiving frames

from the wired side at a rate faster than it can transmit them on the wireless side.Ethernet frames headed for the same wireless client can be queued then combined intoone larger frame for single transmission, cutting down the overhead dramatically.

One caveat, and its a big one, is that like any wireless transmission, the larger the framethe less likely it will be received with no errors. Ive observed that 802.11n senders tendto learn the maximum data rates possible for given frame sizes. Thus for instance, youmay see a frame containing a TCP ACK send at 270 Mbps (or 300 Mbps if a short guardinterval is in use) because its very small (typical 78 bytes including the 802.11overhead). Frames containing FTP data may be sent at 121.5 Mbps simply because

previous attempts at a higher data rate proved futile.

Thus its not clear how much we will gain in efficiency using the A-MSDU. With its framesize up to 7935 bytes, with one MAC header and payload protected by one CRC like anyother single frame, it will most likely be transmitted at a lower data rate for reliablereception.

Contrast this to the A-MPDU which is essentially a chain of individual 802.11 frames sentback-to-back with one access to the medium (i.e. one preamble). The destination muststill be one address and the traffic class (QoS) must be the same for each.

Clearly, there is more overhead with the A-MPDU because we still have individual PDUframe headers vs. one in the A-MSDU. Unlike the A-MSDU however, individual PDUframes also have their own CRC; an error in one PDU will not affect the others in thegroup.


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802.11n Captured Frame Analysis

Now for the fun partlets see how 802.11n really works. The best way to learn is tocapture 802.11n wireless traffic using a protocol analyzer. The first two screen shots arefrom WildPackets OmniPeek using Marvell chipset drivers. As of this writing, the only

two officially supported capture cards are one from Buffalo that handles both the 2.4GHz and 5 GHz bands and is in short supply due to a lawsuit (more on this later), andone from Netgear for the 2.4 GHz band. Others include an 802.11n card from CACE forWireshark, another from AirMagnet supporting their laptop analyzer, and a number of cards containing the Atheros N chipset supported by TamoSoft CommView for WiFi.

The following figure shows a detailed view of a beacon frame from an 802.11n-capable


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802.11n HT Capability Information Element inside a Beacon Frame

access point. Beacon frames are sent out on a standard 22 MHz channel using an low-rate, non-OFDM signaling to be compatible all the way back to 802.11b (thus the 22MHz vs. 20 MHz wide channel). Because of the additional 802.11n capability

information carried in the beacon frame, notice how much larger the beacon frame sizeis compared with a beacon from a non-802.11n access point in the packet just above it.

The beacon frame contains a new element ID for HT (High Throughput), which includesall the mandatory and optional components of 802.11n as supported by the accesspoint. Notable in this case, we see that the access point (a Linksys business classaccess point) supports 40 MHz operation and the short guard interval. Two spatialstreams are supported. The maximum A-MSDU size is 3839 bytes, less than half of the7935 byte maximum allowed by the standard. The maximum frame burst (A-MPDU) isthe full 64k bytes.

Other details in the HT element ID include encoding information about each spatialstream, beam forming capabilities, and antenna selection capability.

A second element ID (extended HT info) includes even more information such aswhether the 2 nd channel offset is above or below the primary channel (an AP will beaconon the primary channel although dual beacon is an option), transmit burst limit, theoperating mode (pure HT, i.e. pure N or with protection), MCS (Modulation and CodingScheme) information for each spatial stream, and so on.

The biggest difference youll notice during data transfer is the high transmission rate

(compared to 802.11bg). You will also see how the A-MSDU, A-MPDU, and Block ACKsinteract. The screen shot below illustrates this. In order to make the packet list morereadable, Ive reduced the number of columns shown and used logical names in place of MAC and IP addresses to minimize the overall width.


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802.11n Data Transfer Showing A-MSDU, A-MDPU, and Block ACK

N Client is the physical address of the 802.11n client, N AP is the physical address(also the BSSID) of the 802.11n access point, Client IP is used in place of the Clients IPaddress, and likewise for the server.

Notice how each large A-MSDU frame is acknowledged in the conventional mannerbya single 802.11 ACK. We also see that packet 10161 was not ACKed and thus resent in10162. Again, larger frames can be problematic, especially as we increase our distancefrom the access point. A good sign in this trace is that the majority of the larger frameswere successfully delivered at 270 Mbps, the maximum transmission rate for this clientand AP configuration.

The CTS you see in frame 10164 is intended to be a protection mechanism sent out bythe N AP alerting non-802.11n devices in the area of a pending transmission. Dependingon the 802.11n access point implementation and options, some APs will use the full

RTS/CTS mechanism.

The next burst of frames you see, 10165 through 10173, is a block of A-MPDU frames.The protocol column is somewhat misleading, a characteristic of OmniPeek. Theprotocol is TCP there is no CIFS information whatsoever in the frame. These are pureTCP ACKs at the transport layer having nothing to do with upper layer protocols.OmniPeek is simply sticking the logical name of the TCP port number in that column. Sowhy this burst of TCP ACKs?

The A-MSDU frames you see in 10159, 10161, and 10162 contain multiple embedded

TCP packets. Carefully note in the screen shot that these frames are sent to thephysical address of the access point. A-MSDU are then broken apart by the AP and sentas one or more packets on the wired side. Cool, no?

Contrast this to frame 1065 forward. In this case we have a burst of frames (which werethe TCP ACKs we talked about) that comprise an A-MDPU that are acknowledged at the802.11n layer by the Block ACK we see in frame 10174.

Finally, lets look inside an A-MSDU broken out by subframes. Wireshark does this verywell, as illustrated at the top of the next page. The A-MSDU, highlighted by the blueline, is broken out into two subframes, each of which contains two complete packets

from the MAC address to the IP addresses to the TCP 1460 byte payload.


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802.11n A-MSDU Expanded Out into Subframes

Major Milestones: 802.11n Goes Enterprise

The year 2007 started out with a bang beginning in January with the approval by theIEEE of 802.11n Draft 2.0. I was fortunate to be able to test the first batch of commercially available gear based on the latest draft and confirm, for the first time, real802.11n interoperability between access points and clients from different vendors. Waycool! Draft 2.0 was looking good.

The first hint that 802.11n was going enterprise came from Meru when theydemonstrated new gear based on Draft 2.0 at Interop. Their new access point wasdesigned to carry up to two 802.11n radios (operating as two separate access points) orone 802.11n and one 802.11abg radio.

Not to be outdone, another major milestone was from none other than Cisco when theyannounced their first line of enterprise-level wireless gear meeting the 802.11n Draft2.0 specification (their consumer division, Linksys, has been shipping draft n gear for

some time). Their Draft-certified access point has been dubbed the Aironet 1250Series. This AP can run in unified (LWAP) or autonomous (stand alone) mode. It lookslike unified operation requires the new Cisco Wireless Service Module (WiSM), whichpops into a Catalyst 6500, as well as Cisco Unified Wireless Network 4.2 software.Another point of interest is that running the power hungry 1250 AP on Power overEthernet (PoE) will require a blade upgrade in existing 3750, 4500 and 6500 Catalystswitches.


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Wisely, Cisco is not immediately offering its own client card. Lets face it there aresimply too many notebook options out there (PC Card, ExpressCard, USB, built-in, and soon). Instead, they have been working with Intel to pre-test, certify, and offer new Client5.0 software that will work with the Centrino 4965agn chipset.

Network Computing Online mentioned some early performance reports: "Sites have seen rates of 120, 130, and even has high as 138 Mbps per radio. While thisis likely in a greenfield environment without the debilitating effects of legacy clients,these claims did exceed by a large margin those privately demonstrated by both Meruand Trapeze at Interop Las Vegas."

While technically Meru beat Cisco to the punch, the Cisco announcement was a hugeboost in resurrecting interest in the almost forgotten 802.11a 5 GHz band. In fact, Intelhas previously stated that the aforementioned Centrino chip will support 40 MHz (two20 MHz channels) operation only in the 5 GHz band.

Shortly after the Cisco announcement, The Wi-Fi Alliance announced that 95 productshave been certified for 802.11n Draft 2.0. The product mix is from some 28 vendors,which is pretty impressive considering the testing program began just three monthsprior. Big name chip vendors including Atheros, Intel, and Marvell are in the fray, as wellas heavyweights Apple, Cisco, HP, Sony, and Toshiba to name a few. As of this writing,the number is up to 140 products and climbing.

In November of 2007, Draft 3.0 was approved by the IEEE TGn working group. Draft 3.0mostly clarifies hundreds of editorial comments (many of them redundant) since Draft2.0 with only minor technical changes. It is anticipated that the remaining work will becomplete and a an official 802.11n standard issued by October 2008.

An Ominous Cloud?

The Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO)holds patents related to multi-stream transmission. In the U.S., the patent is 5,487,069.You can search it online for some light reading. CSIRO has failed to provide a letter of release to the IEEE that would basically promise not to sue any vendors.

As with any IEEE standard, if any technology incorporated into the standard has anypatents, they must be waived by the respective patent holders. Naturally, the variousengineers and companies that contribute to the IEEE try to avoid such conflicts in thefirst place or issue the letter.

CISRO is certainly not alone in patents pertaining to technologies employed by802.11n. In fact, the IEEE has listed some 23 other companies and patents related to


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802.11n. Furthermore, there are some 600+ U.S. patent applications and over 250already granted pertaining to MIMO.

Meanwhile, Buffalo is the first vendor to be seriously impacted by CISRO. A U.S. courtupheld a lawsuit preventing Buffalo from shipping gear in the U.S. Not just 802.11n

Draft gear, but any Buffalo product that uses multiple streams. Unfortunately, virtuallyall Buffalo 802.11 products offer a proprietary turbo mode, so those product shipmentshave been halted as well.

According to Buffalo, CSIRO's lawsuits are against the entire wireless LAN industry andcould affect the supply of wireless LAN products by any manufacturer, not just Buffalo.The entire industry is resisting CSIRO's attempts to enjoin the sale of wireless LANproducts. Recently, Microsoft, 3COM Corporation, SMC Networks, Accton TechnologyCorporation, Intel, Atheros Communications, Belkin International, Dell, Hewlett-Packard,Nortel Networks, Nvidia Corporation, Oracle Corporation, SAP AG, Yahoo, Nokia, andthe Consumer Electronics Association filed briefs in support of Buffalo's position thatinjunctive relief is inappropriate in this case.

Naturally all affected parties (other than Buffalo that already has the injunction) aremum on the issue, probably for legal reasons. It will be interesting to follow the newsand how the combined heavyweights fight, license, or buy CSIRO technology.

Will 802.11n Replace Ethernet in the Future?

Controversial reports are designed to generate lots of media attention. A bomb

dropped by the Burton Group was no exception; it was a report entitled 802.11n: TheEnd of Ethernet? I figured with the buzz generated by the report, why not providesome counterpoints?

The report claims that 802.11n marks the beginning a rapid shift away from LANdeployments. It does not state when this beginning is or was. 802.11n is still in draftstage. If anything, the aforementioned Cisco 802.11n product announcement marks thebeginning, but the report was released back in June. So wheres the mark?

The report mentioned that 802.11n will put pervasive mobility on the fast track. Waita second. I thought WiFi period put pervasive mobility on the fast track. Rememberwhen laptops required PCMCIA cards to connect to Ethernet and now every laptop hasEthernet built-in? The same thing has already happened with wireless. Virtually everylaptop built in the past year or so has built-in 802.11bg. Thus, they are already enjoyingpervasive mobility today.


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Dont get me wrong. You can probably tell from reading this white paper thus far thatIm bullish on 802.11n. So what is it about 802.11n that spells doom for Ethernet,according to the report?

According to the press release for the report, 802.11n is an appropriate LAN access

substitute for wired Ethernet when:

The number of laptop users is growing The enterprise uses mobile applications Fast Ethernet (100 Mbps) throughput is good enough The enterprise deploys Voice over Internet Protocol (VoIP) Moves/adds/changes are frequently made The risk of deliberate denial of service attack is low to moderate Ethernet cable installation is difficult

After looking at this list, Im thinking to myself: Which of these are existing enterprise802.11 wireless installs failing to satisfy? It seems to boil down to the promise of a fasterdata rate offered by 802.11n enabling support for a larger number of mobile and VoIPusers.

Sure, Fast Ethernet is adequate for the majority of workstation and laptop users,keeping in mind that weve migrated to a dedicated 100 Mbps switch port for everyuser, as I mentioned earlier in this white paper. With single channel 802.11n MIMOoperation were looking at a raw bandwidth of 150 Mbps. In single user lab tests, an802.11n file transfer can crack 100 Mbps. Single user lab tests are not real-world.

The hole in the Burton Groups argument is that if we move away from our establishedone-port per user switched model, were going back to the days of shared Ethernet, onlyworse. With 802.11n, not only is contention still a factor, but other issues affectperformance as well:

Slower data rates at distances farther away from the access point Environmental interference (not necessarily other RF sources, but physical

obstructions and attenuators) Having to manage QoS policies to support VoIP over wireless Being subject to RF jamming, and so on In other words, many of the same issues weve faced with any WiFi technology

to date.

Are we ready to go back to shared Ethernet? I dont believe so.


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Conclusion

802.11n meets the need for faster throughput over greater distances, but not withoutcaveats. This white paper illustrated one example in that taking advantage of featuresdesigned to cut the 802.11n protocol overhead at the MAC layer comes at the expenseof transmission speed at the PHY layer.

There will need to be more complex tuning in both single and multivendorenvironments. Tuning performance by lowering the guard interval to achieve a full 300Mbps, for instance, may have other hidden implications such as higher error rates,especially in a dynamically changing mobile office or warehouse environment. Otherfactors include how well 802.11n will work in a complicated multi-cell, multi-user setup.

We are just beginning to understand this complex technology and getting some of theanswers. There will be many more issues and hopefully answers - as we roll it out.

J. Scott Haugdahl is Founder and CTO of Bitcricket, a company that specializes in on-sitenetwork analysis consulting, hands-on analysis training, network troubleshooting, and software tools for the network engineer. For more information, please peruse the

Bitcricket web site at www.bitcricket.com.

inside 802.11n wireless lans - practical insides and analysis

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