The practical realities of 802.11n in home networking applications - Embedded.com

The practical realities of 802.11n in home networking applications

Many articles have been written about the inner workings of 802.11n;however, few offer practical advice on how to get the most from an802.11n network. The technology is very sophisticated and uses manytricks to deliver blazingly fast performance.

What most of us don't realize, though, is that we have thecapability to unwittingly cripple the performance of our network. Atthe same time, it is also possible to squeeze out an even betterexperience than provided by the base standard. In this short article Iprovide a guide on some simple do's and don'ts.

802.11n isn't a finished IEEE standard yet so the correct thing todo is to add the suffix “Draft.” Most products shipping today arecertified to Draft 2.0 using a Wi-Fi interoperability specification.The final IEEE spec should be completed in early 2009.

Given the growing installed base of products already commerciallyavailable, most people expect that existing products will need just afirmware upgrade to be brought in line with the final specification.

802.11g versus 802.11n
First let's look at the major ways in which the two standards differ(Figure 1 below). There are two big benefits of 802.11n over 802.11g:

* Improved range
* Higher throughput

The first point is often overlooked, but it is very important. Evenif every wireless client in a home network is 802.11g, they will workbetter if the home router or access point supports 802.11n.

So even though 802.11n is still shy of final, it is certainly 100%compatible with 802.11g. It makes a lot of sense to use 802.11n today,especially given the nominal difference in product cost.

Figure 1.A comparison between the two technologies in a homeenvironment. (Data source: www.octoscope.com)

There are some key techniques used to achieve this performance:

Multiple Input ” Multiple Output (MIMO). This is the largestfundamental innovation used in 802.11n. Using multiple radiotransmitters and multiple radio receivers it is possible to takeadvantage of signals received both directly in line of sight andreflections of the same signal.

In 802.11g, reflections created interference and degraded thesignal; simple antenna diversity was used to pick which antenna had thecleanest signal. In a MIMO system the reflections can be received bymultiple antennas simultaneously, improving the signal-to-noise ratioof the received signal and, in doing so, boosting the effective range.

A technique called spatial multiplexing is used to increase thebandwidth of the signal by transmitting different streams of data inthe same band at the same time, but from different antennas. 802.11nallows for up to four spatial streams, and most products shipping todayuse two.

20MHz & 40MHz Channels.
Standard 802.11g used 20MHz channels, and some proprietaryimplementations bonded two channels together to double the effectivebandwidth. In 802.11n both 20MHz and 40MHz channels are standard.

The combination of two spatial streams, a 40MHz channel and someother overhead optimizations within the MAC layer increase the maximumraw data rate on the wireless link by nearly six-fold, from 54Mbps toan impressive 300Mbps.

As with 802.11g, however, it is not possible to transfer data at thenominal maximum data rate. Overhead in the MAC layer and thecharacteristics of any given installation result in the actualthroughput being much less than 300Mbps.

Frame Aggregation
To maintain backwards compatibility with 802.11 a/b/g networks, the MAClayer must surround 802.11n transmissions with information that can beunderstood by legacy clients to avoid collisions and interference.

The order of magnitude difference in the data rates for 802.11b/gand 802.11n rates would make the collision avoidance mechanism a hugeoverhead if it had to be performed prior to each and every 802.11npacket transmission.

In Frame Aggregation, per-packet overhead is reduced by aggregatingmultiple packets into something called an Aggregate MAC Protocol DataUnit (A-MPDU). Each A-MPDU is treated like a single packet that can beas large as 64Kbytes.

The use of A-MPDUs creates some interesting system design challengessince it trades increased throughput for higher latency. The radio musthang on to packets at the transmitter until it creates an A-MPDU of thedesired size or until a timeout value is reached.

Since optimizing this area for particular traffic types is unlikelyto be left to the user I won't go into it here, however products thatare designed for transporting delay-sensitive traffic need to pay closeattention in this area.

2.4GHz versus 5GHz
There is a big push, especially from Microsoft, to separately use 5GHzand 2.4GHz – sometimes referred to as the 'g' and 'a' bands – for videoand data, respectively. The motivation for this approach is that thereare fewer channels available in the 'g' band, while at the same time,the majority of Wi-Fi is deployed in that band so you are more likelyto run into interference.

802.11n allows for operation in both bands. What you will find isthat the least expensive product offerings use the 2.4GHz band only;some products will operate in either band; and others will be equippedwith two radios and can run in both bands concurrently.

Practically speaking, because the 'g' band is only 80MHz wide, it isusually fairly difficult for an 802.11n AP to actually find and use a40MHz channel. In the 'a' band it is usually easier to operate 40MHzchannels.

For most applications I have found the 'g' band to be sufficient; Ihave seen a significant improvement in range; and while I don't see thetheoretical peak performance, a D-Link DIR-655 is more than adequatewhen streaming video between PCs or to an XBOX360 or Apple TV.

If you really want to optimize for the maximum possible performanceof the network to support multiple simultaneous video streams, youshould consider 5GHz. If you already have an 802.11g router you couldrepurpose that as an access point and run 802.11n in 5GHz or you couldopt for a dual-band router.

My approach to getting the most out of 802.11n has not been tosimply upgrade everything in the network to 5GHz, but rather to makethe most of the bandwidth I can install cost-effectively, and byavoiding a range of things that can cripple performance.

Legacy clients in an 11n network
This is one area that is really misunderstood and perhaps the easiestway to inadvertently bring down the total available bandwidthconsiderably. When you connect an 802.11g or even an 802.11b client toan 802.11n network it can steal many times its own bandwidth.

Consider an 802.11n network which can achieve at least 80Mbps ofthroughput to any 802.11n client in that home. Connect an 802.11gclient and stream a movie at 10Mbps. How much throughput is left on thenetwork? Most people would say 70Mbps. Nope.

The actual answer is more like 40Mbps. Since the peak performance ofthe 802.11g client is 20Mbps, it transmits at one quarter the speed of802.11n. So 10Mbps of throughput to an 11g client is essentiallyequivalent to 40Mbps to an 11n client. An 802.11b client is even worse.An 11b client carrying just 2Mbps reduces the remaining bandwidth byhalf!

When wired Ethernet is better
Wireless is a shared medium, so if you're moving data from one wirelessclient to another, you'll use twice as much wireless bandwidth. Thiswill sound like heresy coming from someone in the wireless business,but when convenient to do so, use an Ethernet cable to connect to therouter. Use wireless for mobility and for those hard-to-reach places..

Sorting out traffic
Once the wireless infrastructure is optimized for an installation, thenext thing to consider is how to optimize the use of the availablebandwidth. This is where quality of service comes in. In Wi-Fi, thecore certification to look for is WMM (Wireless Multi-Media).

This requires that certified devices be able to manage fourdifferent traffic types: Voice, Video, Best Effort, and Background. Thebiggest drawback with WMM is that it takes QoS techniques developed forthe enterprise and uses them in a home environment.

To work properly, traffic has to be accurately tagged as theappropriate traffic class and must appear in the right proportions. Inan enterprise, an IT department ensures this by designing andconfiguring the network and the applications that run on it. But athome, every network differs dramatically in its equipment,applications, and traffic patterns. Oh, and there's no IT staff.

So in a home environment many things can go wrong with the WMMscheme. The traffic may simply not get a tag in the first place. Or itmay be tagged by an application, but then lose that tag due to a poorlyimplemented network driver, or by passing through some older networkgear.

Add to that the fact that for traffic arriving into the home, theservice provider treats it all simply as best effort, and you'll beginto see why voice, video and gaming have such a hard time in homenetworks.

Fortunately, some products on the market do have the ability to dointelligent analysis of the traffic itself and then treat itappropriately even when it doesn't have a tag. I strongly recommendequipment that can do this automatically since life is too short to bespent tinkering to try to make it work reliably for each and every oneof your use cases.

Certifications
At this point I think it is worth mentioning a couple ofcertifications: IEEE 802.11n draft 2.0 and Wi-Fi Protected Set-Up.

The former ensures that you will see good interoperability betweenall of network devices, and the latter provides push-buttonconfiguration of the wireless network. As more and more devices shipwith Wi-Fi, this capability really simplifies adding gadgets to thenetwork.

When Microsoft brought out Vista, they spent a lot of effort on thenetworking infrastructure and on certifications for network devices. Itis in their interest that media works well on the network so the Workswith Vista and Certified for Windows Vista logos have some realtechnology behind them.

They need to make sure that everything handles QoS tags properly andeven allowing applications to draw a map of your network and calculatethe available bandwidth between two points. Network vendors had to do alot of work to improve their products and meet these logo requirementsso it isn't just a marketing exercise.

Keith Morris, vice president ofmarketing at  Ubicom has 20 years of experience incommunications and networking and has held key roles at Plessey Crypto,Fujitsu Microelectronics, MMC Networks.

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