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WiMAX vs. WiFi

Tom Evslin

In fact WiFi (technically standard 802.11) and WiMAX (802.16) don't compete for broadband users or applications today. That's partly because WiFi is widely deployed and WiMAX is still largely an unfulfilled promise and partly because the two protocols were designed for very different situations. However, if WiMAX is eventually widely deployed, there will be competition between them as last mile technologies.

Some people describe the difference between WiFi and WiMAX as analogous to the difference between a cordless phone and a mobile phone. Wifi, like a cordless phone, is primarily used to provide a connection within a limited area like a home or an office. WiMAX is used (or planned to be used) to provide broadband connectivity from some central location to most locations inside or outside within its service radius as well as to people passing through in cars. Just like mobile phone service, there are likely to be WiMAX dead spots within buildings.

From a techie POV, the analogy is apt at another level: WiFi, like cordless phones, operates in unlicensed spectrum (in fact cordless phones and WiFi can interfere with each other in the pitiful swatch of spectrum that's been allocated to them). There are some implementations of WiMAX for unlicensed spectrum but most WiMAX development has been done on radios which operate on frequencies whose use requires a license.

Some more subversive types (they're subversive so I can't link to them) say that WiMAX is what you get when bellheads (not a nice term) try to reinvent WiFi the way they'd like it to be. It's true that WiMAX is much more a command and control protocol than WiFi. Oversimplified, in a WiFi environment every device within reach of an access point shouts for attention whenever it's got something to transmit. In that chaos, some signals tromp on other signals; the more powerful devices and those closer to the access point tend to get more than their share of airtime like the obnoxious kid who always has his hand up in the front of the class. In WiMAX devices contend for initial attention but then are assigned times when they may ask to speak. The protocol allows the operator more control over the quality of service provided — bellheads like control.

But it's not clear that more control means better service than contentious chaos (I'm talking about technology but the same may apply to economies or bodies politic). The Internet and its routing algorithms are chaotic; the routers just throw away packets if they get to busy to handle them. Bellheads (and even smart people like Bob Metcalfe) were sure that design or lack thereof wouldn't scale. They were wrong.

Same people said that voice would never work over the Internet — there's no guarantee of quality, you see. They were wrong although it's taken awhile to prove it. Now HD voice is available on the Internet but NOT on the traditional phone network (although it could be).

Lovers of an orderly environment and those who like to keep order were absolutely sure that WiFi couldn't work once it became popular. Not only is it chaotic; it also operates in the uncontrolled environment of unlicensed frequencies along with cordless phones, bluetooth headsets, walkie-talkies and the occasional leaky microwave oven. But somehow it's become near indispensable even in places where a city block full of access points contend for the scarce frequencies.

Net: I'm not convinced that WiMAX won't suffer from its own orderliness. Did you ever fume leaving an event when an amateur cop (or a professional one) managed traffic into an endless snarl? Fact is cars at low speed usually merge better without help than otherwise. Turns out that control comes at the expense of wasted capacity. The reason that the Internet or WiFi radios can work is that the computing power necessary to deal with chaos from the edge of the network is far cheaper and less subject to disruption or misallocation than the computing power (and communication) for central command and control.

WiMAX may be too well-controlled for its own good. Moreover, if it is used only in regulated spectrum where most frequencies are idle most of the time AND licenses for the frequencies have to be purchased, it will be even less efficient than if it could contend for unlicensed spectrum.

By the way, WiFi CAN operate at distances as great as WiMAX but there are two reasons why it doesn't. One reason is that radios operating in the unlicensed frequencies are not allowed to be as powerful as those operated with licenses; less power means less distance. These regulations are based on the dated assumption that devices can't regulate themselves — but the assumption MAY be correct over great enough distances. The second reason why WiFi access points don't serve as wide an area as WiMAX access points are planned to do is the engineering belief that the problem of everybody shouting at once, even if it's surmountable in a classroom, would be catastrophic in a larger arena. Maybe.

New licensed spectrum is being made available for WiMAX and other technologies NOT including WiFi — for example, the valuable 700MHz frequencies currently used by analog over the air TV. WiMAX could have a good run because it is allowed to operate in that efficient spectrum while WiFi will eventually run out of the pitifully little spectrum that's been allocated to it. That's policy and politics and not engineering but could still be a reason for WiMAX success.

By Tom Evslin. More blog posts from Tom Evslin can also be read here.

Related topics: Access Providers, Broadband, Mobile, Policy & Regulation, Telecom, White Space, Wireless

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Comments

Unmanaged wireless don't scale George Ou  –  Jan 04, 2009 3:03 PM PDT

Unmanaged wireless don't scale.  Simply look at what happens when we compare a unmanaged/unscheduled Wi-Fi network to a managed LTE network.

20 MHz of unmanaged/unscheduled 802.11b Wi-Fi operating in an ideal environment with no external interference supports 4 simultaneous VoIP phone calls.  Any more than 4 and every call starts randomly colliding.

5 MHz of managed LTE supports 200 devices.  That's a 200-fold increase in spectral efficiency.  4-fold speed increase is due to the faster signaling rate and MIMO implementation, 50-fold speed increase is due to the scheduling algorithm.  So it's clear this "bellhead" command and control design is necessary if you want any kind of scalability for multiple users.  Unmanaged Wi-Fi might run well in a flat-out one-to-one scenario but it falls apart in a multiuser environment.

The Wi-Fi designers aren't stupid and they've put in advanced scheduled access mechanisms in to the 802.11e standard.  The reason it's not implemented in most Wi-Fi devices is because the benefits are relatively small in a 100 mW small-cell environment compared to the cost and complexity.  On a larger network or even in a small office network where voice capacity is critical, you better believe they're looking at intelligent managed network designs.

Lastly, the amount of spectrum assigned to Wi-Fi isn't pathetic at all.  We're talking about 80 MHz just in the 2.4 GHz range which is comparable to the holdings of the largest wireless cellular operators in America.  There's another 240 MHz (potentially up to 480 MHz) in the 5 GHz range available to Wi-Fi.  The problem for Wi-Fi isn't that it's being starved of spectrum; the problem is that it suffers from the tragedy of the commons.

Scaling unmanaged wireless Tom Evslin  –  Jan 06, 2009 3:51 PM PDT

George:

It's certainly possible that WiFi will evolve to have more control than it has today although I would guess that the control would remain very distributed and tolerant of contending systems with no control-communication.

I do stand by my statement that WiFi has been allocated miserable shards of spectrum both because of the other applications it has to share the spectrum with and - more importantly - because 80MHz at 2.4GHz isn't the same as 20MHz in 600-800 MHz for obvious engineering reasons. There's a reason why frequency allocation charts are drawn on a log scale. Similarly the 240MHz in the 5GHz range isn't much either.

Distributed "control" or participation is the less scalable option George Ou  –  Jan 06, 2009 4:17 PM PDT

"It's certainly possible that WiFi will evolve to have more control than it has today although I would guess that the control would remain very distributed and tolerant of contending systems with no control-communication."

Distributed "control" or participation is the less scalable option but it's easier to implement and that's all you need for small scale wireless networks.  This is the lighter form of ad hoc 802.11e where the clients participate and it allows an 802.11b network to scale from 4 calls per 20 MHz block to 12 calls.  If you want to entirely eliminate random collisions to scale up to 100+ calls or you want to cover larger cell sizes, the more advanced from of 802.11e requires a centralized scheduling mechanism which is similar to WiMAX and what you refer to as "bellhead" control.  Again, this is not a propagation or lack of spectrum issue, its a lack of network intelligence.

Your statement that Wi-Fi has miserable shards of spectrum is invalid because the propagation issues don't really come in to play because we don't want unlicensed bands to propagate that well anyways.  Imagine everyone within 2 city blocks of your house trying to use the same 80 MHz block of spectrum, you DON'T want that signal (which is interference to you) to propagate in to your home.  So 80 MHz of spectrum in this context is every bit as effective for carrying bits as 80 MHz of spectrum in the 700 MHz range.  Propagation is a good thing when you're talking about long-range licensed applications where you're trying to offer mobile coverage to a 7 mile radius.  Propagation becomes the enemy as noise when we're talking about unlicensed spectrum.

The biggest problem with Wi-Fi from a large-scale deployment standpoint is the fact that it's unlicensed "junk-band" that anyone can broadcast on.  But you're trying to envision an applications that unmanaged Wi-Fi was never designed for, and you fail to realize how important centralized network management is especially in the wireless space.

As for your claim (http://www.circleid.com/posts/20081126_fcc_white_space_regulations/) that 4 watt 700 MHz devices have an effective range of 30 miles, I'm afraid I'll have to burst that illusion because you're playing fast and loose with the engineering realities.  You'd be lucky to hit 30 miles with a clear line of site with well aimed large dish antennas on both ends.  You can forget about "WRANs" that make LTE and WiMAX obsolete.  Even a massive $650,000,000 cellular 3G tower operating with a lot more power within licensed bands have maybe 7 mile coverage radius if you want good enough signals for in-home and in-car coverage.

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