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Retrofitting Networks

Wireless LANs Shift into High Gear

A home network can be an important communications backbone, a vital resource that many different applicationsranging from home automation and security to distributed audio and videocan leverage. That is provided, of course, that the home has such a resource built in, or that it can be retrofitted.

Needless to say, when building a new home, the installation a good network cabling infrastructure can be one of the smartest moves anyone can make. But integrators find lots of potential clients with existing homes that lack network cabling. Fortunately, wireless networking technologies and other good retrofit options are becoming more widely available.

For our purposes, network technologies can be classified into two broad categories: control networks, which use low-rate signaling for lighting and other home control applications; and high-speed media networks, which are capable of carrying video and audio. This article will focus on IEEE 802.11n, a newly emerging high-speed wireless technology that meets the throughput requirements for a converged (audio/video/data) media network.

High-rate Media Networks
When it comes to delivering video and high-fidelity music over a network, high data-transfer rates are critical. So is low latency, which is a delay in packet (a block of formatted data) arrival and low jitter (packet-to-packet variations in arrival times).

Most wireless local area networking (WLAN) equipment being offered today is built to the IEEEs 802.11g standard, which offers a maximum data rate of 54 Mbps. But this is its nominal or advertised data rate. With WLANs, radio/processing latencies, radio turnaround times, packet overhead, inter-packet gaps, background retransmissions of lost packets, and other general operational overhead means that real-world transfer of user data ends up being about half the nominal published rate, or in the case of .11g, about 25 Mbps.

Work has been under way on developing a new, high-rate standard, which will be known as IEEE 802.11n. Actual user data rates of 100 Mbps or more are expected. Though the final standard likely wont be finished until late 2008 or early 2009, a first draft of it was issued in March 2006, and Draft 2.0 was approved in March of this year.

But now that the draft of the standard has reached 2.0, there is a feeling by some that the technical outlines for 802.11n have solidified enough that firmware updates could be used to make Draft 2.0-compliant products conform to any further changes that get worked into the final, approved standard. Indeed, manufacturers are now offering Draft 2.0 products with just these claims of upgradeability (though, to my knowledge, no one is willing to make a guarantee, in writing).

The Austin-based Wi-Fi Alliance has begun an interoperability testing program for products designed to Draft 2.0 of the specification, and certifies those that pass those tests. At this writing, the Alliance has certified its first crop of candidate products, from vendors D-Link, Linksys, and Netgear.

Equipment built to the 802.11n standard must be able to inter-operate with products built to earlier WLAN standards. This means that if you have a notebook computer outfitted with a legacy 802.11b WLAN adapter, it can communicate with an 802.11n access point, for example, with the highest data transfer rate to be the 11 Mbps as that is the maximum supported by 802.11b.

The IEEE 802.11n standard will utilize a technique known as multiple input/multiple output (MIMO) as its chief means of improving the signal-to-noise ratio and achieving higher data rates. MIMO involves the use of multiple antennae (see Sidebar 2).

The standard permits operation in the 2.4-GHz radio band utilized by 802.11b/g devices as well as the 5-GHz band used by 802.11a-based products. Whether or not a given product operates in both bands will be a matter of vendor implementation, so when considering .11n and Draft-.11n products, youll need to decide which bands you want or need to use, and select products that use that band. (Some Draft-.11n products, such as D-Links DIR-855 Wireless Router, operate dual-band [2.4-GHz and 5-GHz], effectively doubling aggregate data rates). To take advantage of the ability to work with 802.11b/g devices, 802.11n devices will need to use the 2.4-GHz band.

Another way that 802.11n devices can gain a speed advantage over products built to the previous standards is that while 802.11a/b/g devices operate over a 20-MHz-wide radio channel, .11n devices can optionally use a 40-MHz-wide channel, which can double the potential data rate. Of course, this also increases the potential for interference between the .11n WLAN and any other WLANs that may be within range, such as your neighbors network. Because of this, .11n devices that sense another network are supposed to automatically fall back to using a 20-MHz channel.

The 5-GHz band is much wider than the 2.4-GHz band, and the additional spectrum makes it easier to support wide-channel (40-MHz) operation in this band. Theres also currently far fewer users in this band, so theres much less potential for interference.

Your Mileage May Vary
Theres no simple answer to the question of what data rates the 802.11n specification will deliver, as there are so many options. The specification requires MIMO, but with a minimum of two antennas and two streams. There are throughput advantages to going with more antennas and more streams. Will you be operating with double-wide, 40-MHz channels, or 20-MHz? Will you be using the 2.4-GHz band, the 5-GHz band, or both, simultaneously?

In general, we can expect to see at least twice the throughput of 802.11g gear, as .11n specifies at least two streams of MIMO, and using a 40-MHz channel can double that again. Adding simultaneous 2.4-GHz/5-GHz operation can yet again double a dual-band access points aggregate throughput, for an eight-fold advantage over 802.1g.

Safe and Secure
In the past, security has been a weak point for WLANs. The wired equivalent privacy (WEP) security scheme, developed at the time the 802.11b specification rolled out, has proven to be extremely easy to crack. However, the newest security specificationsWiFi Protected Access (WPA) and WPA2are solid, with no known vulnerabilities. IEEE 802.11n does not bring any new security scheme, but utilizes the proven WPA/WPA2.

802.11n: Ready for Prime Time?
The key question for integrators is whether to recommend Draft 2.0-compliant (and WiFi-certified) 802.11n products to their customers, or whether to counsel waiting for the final standard.

This is a tough call to make, and depends largely on each customers appetite for risk. As mentioned earlier, Gartner Inc. has been recommending that its clients waitat least with regard to access points. But Ken Dulaney, vice president and distinguished analyst for Gartner, says that his firm does recognize that laptops and other computers outfitted with Draft 2.0-compliant network adapters are coming to market, and that if faced with going for the older .11g models or opting for .11n, theres a case to be made for choosing those with the Draft 2.0 .11n product. Dulaney says that if you put an 802.11n network client into a legacy (802.11a/b/g) network, because of the MIMO technology, it will perform slightly better [than a legacy client would]. For that reason, Dulaney said, if youre buying a new laptop today, and you can get it with Draft-n capability, its worth doing that. On the infrastructure or access point side, Dulaney said that theyre still advising waiting for the final standard.

Note, however, that Gartners clients are mostly large enterprises, where the consequences of making a purchasing/deployment mistake or miscalculation can be huge. Many large organizations have dozens or scores of access points, and hundreds or thousands of client computers. Risk that might be acceptable for a homeowner contemplating installing one or two access points and a few network client adapters might not be tolerable for a large enterprise.
Beyond that, enterprise WLANs are primarily used for transferring data and Web traffic, not HD video streams. Dulaney says that existing .11a/g infrastructure isnt being overtaxed. It used to be that enterprise networking gear was big, brawny, industrial-strength stuff, while home networking products were toys, by comparison. Now, in some respects, its the developing residential media networks that are pushing hardest on the performance envelopes for networking throughput and quality of service.

It would be prudent to lay out the risks and potential rewards for your client, when helping them make a decision about Draft-.11n gear. A client who opts to purchase such equipment knowing theres a possibility that he or she may need to replace it when the final products come out will be gratified if they later learn that only software upgrades are needed. But someone who hasnt been apprised of the risks (no matter how small) wont be happy if they find that their Draft-.11n products arent fully compliant with the final .11n gear, and cant be software-upgraded.

IEEE 802.11n has been an audacious technical undertaking for its developers (802.11 Task Group n), and its been a long time coming. Although distributed-video applications wrapped around wireless applications are still in their infancy, its highly likely that video-oriented application developers are watching these developments with great anticipationand so are the telcos.

Alan R. Frank ([email protected]) is a networking consultant and freelance writer covering the fields of communications, networking, and digital entertainment.