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Recent Entries

5 GHz or Bust
Quiet Addition of 255 MHz in 5 GHz for 802.11a, Backhaul
Apple Adds Atheros
Belkin Uses Atheros Tech for A+G
NewLogic Avoids Radar with 802.11h
Intel's Tri-Mode Chip May Arrive This Week

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Recently in 802.11a Category

January 24, 2007

5 GHz or Bust

By Glenn Fleishman

George Ou has some interesting points about 5 GHz, but there's more to the story: Ou hates the 2.4 GHz band with something greater than a passion--it's crowded, there aren't enough non-overlapping channels, and it's just so out of fashion. He's right about all that. The 5 GHz band has lots of possibilities, including 23 channels open for use. Before we get too excited, though, let me point out a few things that Ou didn't cover--primarily, the signal power output restriction for 5 GHz. 5 GHz 802.11 standards can't send signals as far as 2.4 GHz for a good hunk of the band. (23 channels are available for 802.11 specs; there are technically 24 possible in a different configuration.)

One of N's possible advantages of double-wide channels--instead of 22 MHz, they can use 40 MHz channels, which effectively doubles throughput. When you combine a newly efficient design for encoding, two or more radios, and double-wide channels, that's when you get the high symbol rate of 300 Mbps, with effective throughputs that could go well over 100 Mbps. The 100 Mbps throughput factors in--as I understand it--the expectation that N devices will have brief periods in which they can bond two channels.

Here's my executive summary so you don't have to read my entire analysis:

While 5 GHz is uncrowded and has more clear, non-overlapping channels that can be combined for the highest speeds in 802.11n, the rules governing 5 GHz for indoor use with omnidirectional antennas mean that only two double-wide channels in 5 GHz are available at anywhere near the maximum power (and thus potential range) of interior Wi-Fi. A number of 5 GHz channels offer power levels that are comparable to 2.4 GHz--and that assumes manufacturers will allow their 5 GHz radios to output enough juice to produce ranges similar to 2.4 GHz. But the lack of competing networks in 5 GHz could mean that 5 GHz N networks will almost certainly work much better in crowded RF environments--apartment buildings, for instance--than 2.4 GHz N networks which have very little chance to use double-wide channels. (Proviso: Some countries don't allow double-wide channels in 5 GHz.)

Ou says that "802.11b was supposed to have given way to its sibling standard 802.11a which operated in the 5 GHz range." That's not my recollection back at the time this was happening. 802.11b and a were seen as having somewhat different purposes: 802.11a had the potential for high-speed, short-range indoor connections, and long-range point-to-multipoint outdoor hookups. This was largely due to the higher cost of A, as Ou points out, but also due to the shorter range possible in indoor applications due to lower signal strength allowed. (Also, there were doubts about A's ability to be produced using CMOS chip processes, which Atheros put to rest, even as it moved into G for competitive reasons.)

He notes that while 802.11g boosted speed, "the problem was that in order to maintain backward compatibility, 802.11g also had to operate in the limited 2.4 GHz space and worse it had to switch to 802.11b if even one legacy 802.11b device joins the party." This isn't strictly correct as described, although it's widely stated this way. An 802.11b device forces a G network to drop down to B speeds only when the B device is transmitting or receiving data. There's a bit of extra network overhead, as well, if you choose for your G device to support B that does produce a single digit reduction in throughput even on G devices. The same will be true with N: older devices will occupy disproportionate amounts of time while transmitting or receiving, but if they aren't heavy network users, the N network should still work well.

I would argue that dual-band gateways didn't succeed in the consumer marketplace because of an absence of a compelling reason to use them. Throughput wasn't a big motivator. There were initially few adapters, none affordable, for 802.11a when 802.11g hit its stride. Remember Steve Jobs telling us 802.11a was dead? (Hey, their new device supports A, so I guess it got better.)

If you had mostly 802.11b/g devices and wanted to use A, you'd have to switch all your adapters over because no consumer devices supported simultaneous dual-band operation in 2.4 GHz and 5 GHz. Some enterprise hardware did (and still does) allow both bands either through two baseband chips or through two separate radios.

Ou complains that MIMO's leap into the market caused the lack of 5 GHz expansion. "Since it already required multiple radios for single band operation, adding an additional set of radio on the access point would have increased the already-high prices even higher." But there are few except very high end devices that would have duplicated radios; typically, as I just noted, it's one radio with two frequency ranges supported for either 2.4 GHz or 5 GHz operation.

Now here we get to the crux: Ou writes, "802.11n in my opinion should have NEVER permitted 2.4 GHz operation in the first place and should have only used the 5 GHz band." However, that idea has two flaws, one of which Ou admits and addresses. First, it breaks compatibility, which means that it's not a solution for people with B and G that want to gradually move to N. Ou suggests that a cheap single-radio could have been inserted into access points to handle B/G clients and the N system could have worked just in 5 GHz. Let's leave out all the radio engineering issues involved in that--like antenna coordination, cost of manufacture, firmware support, and so forth. The second flaw is more critical: 5 GHz has too many limitations in range.

The four spectrum hunks of legal 5 GHz channels each carry restrictions that don't dog 2.4 GHz, and that's why 5 GHz hasn't caught on. Where 802.11b/g/n devices can transmit as much as 1 watt of power at the antenna and use any channel indoors or outdoors, rules for 5 GHz prescribe in the U.S. and some other markets which channels may be used indoors only, and has much lower power levels for omnidirectional indoor use than 2.4 GHz allows.

There are now four bands in 5 GHz channelized for 802.11 in the US, although they're numbered somewhat strangely. In brief, there is total of 555 MHz across 23 channels in 802.11a/n. The lower four are indoor only; the higher 19 are indoor/outdoor. The lowest four (5.15 to 5.25 GHz) can have 50 mW of output power, the next four (5.25 GHz to 5.35 GHz), 250 mW; the next 11 (5.47 to 5.725 GHz), 250 mW; and the top four (5.725 to 5.825 GHz) up to 1 W. (There are further restrictions on 5.25 GHz to 5.725 GHz in terms of detecting and avoiding stepping on military radar transmissions, which share those bands. And the 802.11a spec specifies 40 mW/200 mW/800 mW instead of 50, 250, and 1,000, just to make it even more complicated.)

There are an enormous number of details about effective output, antenna gain, and so forth, but most of that affects the use of directional antennas and point-to-multipoint outdoor connections, not the use of interior omnidirectional antennas.

Because 5 GHz signals have shorter wavelengths than 2.4 GHz signals, at the same amount of power, they propagate shorter distances. They're also worse at penetrating solid objects. This means that even if you use the top four channels for 802.11a or single-wide channels for 802.11n in 5 GHz, you will only be able to send data less than half as far if that. There are only two double-wide channels possible in that top band.

In the 250 mW restricted range of 5 GHz, you could achieve the same range by using higher power in 5 GHz than in 2.4 GHz. But many of the devices that offer 2.4 GHz and 5 GHz radios don't compensate in 5 GHz by having higher-powered signal output. Thus a device that gives you 100 interior feet in any direction in 2.4 GHz could span less than 50 feet for this reason in 5 GHz. The lack of interference from competing networks could compensate for the shorter distance, however.

(Another issue: Some 802.11n device makers may not let you use double-wide channels in 2.4 GHz. Apple's new AirPort Extreme with 802.11n says in its advanced configuration manual--online already long before the product ships--that what it dubs the Use Wide Channels options is only available in 5 GHz. Conversely, Apple is promoting its AirPort Extreme with N in some European as only offering 20 MHz channels in 5 GHz because of regulatory limits. Thanks to Iljitsch van Beijnum for pointing to the manual and the European issue.)

Still, 5 GHz does offer some hope, and while Ou thinks the boat was missed, I see Apple's support in clients and adapters for 5 GHz in N, and Intel's support in its Centrino client for 5 GHz as a sign that that band will pick up steam. Note that Intel is certifying access points and routers with a Connect with Centrino label--and those devices will likely have to support 5 GHz, like this Intel-co-branded Buffalo router.

Let me reiterate one point here at the end: Manufacturers often limit their devices to 100 mW or even much less of output power for 2.4 GHz for reasons of cost. The problem in using 5 GHz will come entirely from whether those manufacturers decide to use the same power output limits for 2.4 GHz and 5 GHz even though they don't have to, or whether they'll actually take advantage of 5 GHz by boosting its power to put its range into parity.

Posted by Glenn Fleishman at 2:30 PM | Permanent Link | Categories: 802.11a, 802.11n, Future, Spectrum | 4 Comments | No TrackBacks

February 17, 2006

Quiet Addition of 255 MHz in 5 GHz for 802.11a, Backhaul

By Glenn Fleishman

The FCC seems to have quietly approved the rules necessary to allow 11 more 802.11a channels to the 5 gigahertz (GHz) band: Joanie Wexler of Network World reports the 255 megahertz (MHz) chunk added to unlicensed 5 GHz use comes as a result of compromise between the Department of Defense and the "industry," which wasn't well defined. Public process doesn't seem to have been broadly involved here, but it may just be less transparent to someone not trained in the intricacies of FCC interaction with other parts of government.

The new rules went into effect Jan. 20, and allow the use of 5.47 to 5.725 GHz--11 channels in the 802.11a version of Wi-Fi--with a couple of key signal usage modifiers. The rule also changes the requirements for the next-down stretch of spectrum, 5.25 to 5.35 GHz to conform to those modifiers.

(Update: It turns out this story is incorrect: the 5.47 to 5.2725 GHz band was available about a year ago. This new order adds the signal modifiers below to the 5.25 to 5.35 GHz band. There was a lag of some kind between these new channels being available for legal use and chips and equipment supporting them, but no regulation gap.)

Both bands must now use dynamic frequency selection (DFS) to avoid in-use spectrum, and transmit power control (TPC), which throttles power to the minimum necessary for given communication. Older equipment using 5.25 to 5.35 GHz is exempt, although one might expect manufacturers to push out firmware upgrades. These requirements are part of 802.11h, which extended 802.11a for legal European operation.

Interest in 5 GHz has increased in the last year or so with the recognition that having no 802.11b devices to support in that band makes voice over IP over WLAN (VoWLAN) easier to operate in corporations. The band is also being used for backhaul on mesh networks and will be one of the early profiles for WiMax.

Even though 5 GHz has restrictions on omnidirectional signal strength that makes it propagate less far than the highest-powered 2.4 GHz omni-driven gateways, the rules for directional antennas for the top four channels (highest frequencies) in the band give it parity. The lack of competing signals and now a total of 23 channels (some with limitations on outdoor vs. indoor use) make 5 GHz very appealing for these specific applications.

Posted by Glenn Fleishman at 1:08 PM | Permanent Link | Categories: 802.11a, Regulation | 1 Comment | No TrackBacks

January 14, 2006

Apple Adds Atheros

By Glenn Fleishman

It's not as big a move as IBM/Freescale to Intel, but it's a shift, nonetheless: Broadcom scored an early trifecta with 802.11g back in late 2002 and early 2003 by signing Apple, Belkin, and Linksys for a round of 802.11g-based products. They also swept in Buffalo and several other firms (notably missing D-Link and NetGear) in that heady run-up to 802.11g ratification.

In the latest Apple products, the first to be based on Intel processors using the Core Duo chips, sources outside of Apple told me that Atheros chips have been incorporated: it's true, but Broadcom hasn't been abandoned. Both Atheros and Broadcom chips are specified in Apple documents and are shown in FCC filings.

It's not odd that with a new system architecture Apple would have reviewed chip suppliers, and they may have chosen to work with both Broadcom and Atheros to have competition for their business. There's a limited number of PCI Express-based Wi-Fi chips, which is what the internal, included AirPort Express hardware uses.

The MacBook Pro (the PowerBook replacement) and the Intel-based iMac support 802.11a for the first time, as well. Apple isn't emphasizing the 802.11a inclusion, and the technical specifications only say "802.11g standard."

Although Steve Jobs declared 802.11a "dead" back in Jan. 2003, it was clear he thought it was a non-starter in the consumer market, and the enterprise was far from a win. In Jan. 2006, 802.11a's place as a larger spectrum swath without legacy slower equipment as a way to run more dense, faster enterprise networks and handle campus-wide VoIP is pretty clear. Apple adding 802.11a lets them sell more easily into enterprises and academia that are adopting 802.11a.

One rumor cited by AppleInsider is that the demonstration of the MacBook Pro's built-in iSight video camera was carried over 802.11a to avoid conflicting with the many ad hoc 802.11b networks running at the keynote venue.

Posted by Glenn Fleishman at 10:10 AM | Permanent Link | Categories: 802.11a, Chips | 2 Comments

January 5, 2005

Belkin Uses Atheros Tech for A+G

By Glenn Fleishman

Belkin has many dance partners these days: Tom's Networking reports that Belkin has chosen Atheros for its A+G products, which may be heating up in the marketplace these days. Belkin uses Broadcom for 802.11g and Airgo for its "pre-N" MIMO devices.

Posted by Glenn Fleishman at 4:10 PM | Permanent Link | Categories: 802.11a, Hardware | No Comments | No TrackBacks

December 1, 2004

NewLogic Avoids Radar with 802.11h

By Glenn Fleishman

Remember 802.11h? It's an extension to 802.11a that helps avoid trampling radar signals, among other issues: For 802.11a to work in Europe, the IEEE 802.11h task group had to be formed and ratified a standard that used two techniques to meet the continental guidelines: TPC (transmission power control) and DFS (dynamic frequency selection). TPC keeps signal strength efficient, using only enough power to reach active users rather than using a uniform power output. DFS ensures a reduction in interference with other systems.

NewLogic is emphasizing that it's DFS component will perform much more intelligent checks against radar signals, a point of contention in the U.S. in the 5 GHz range. A compromise between the U.S. military and industry--a compromise that was a little behind doors--resulted in more spectrum allotted to unlicensed use but with the caveat that radar could be operating in that spectrum and must not be trod upon. By reducing false positives for radar, NewLogic will offer better network performance and robustness.

Posted by Glenn Fleishman at 2:40 PM | Permanent Link | Categories: 802.11a | No Comments | No TrackBacks

August 23, 2004

Intel's Tri-Mode Chip May Arrive This Week

By Nancy Gohring

Analysts think that Intel plans to introduce its tri-mode chip on Thursday: The chip will support 802.11b, 802.11g, and 802.11a and will be Intel's first foray into 802.11a territory. The chips will be built into notebooks computers.

As usual, Intel isn't the market leader but once the company steps into the market with a product, the uptake surges. Perhaps home and enterprise networkers will opt for 802.11a networks now that access will be available from the most popular notebook chipmaker.

Posted by Nancy Gohring at 1:14 PM | Permanent Link | Categories: 802.11a | No Comments | No TrackBacks

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