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« Why WiMAX? | Main | Planning for Failure »

WiMAX vs. WiFi

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.

Why WiMAX? is about the advantages of that technology.

Internet 2.0 is Open Spectrum is an argument against licensed spectrum.

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Comments

yousif aboutalib ahmed

please inform me of leatest information of wirless technology ...like wise wifi and wimax...etc thanx for help

samwyse

I just want to point out that I see no reason why WiFi couldn't scale indefinitely, just like cell phones. Start out with big cells and as they experience congestion, you build more, smaller, cells. The only time you'll have problems is if someone doesn't reduce their power to the minimum needed to talk to their access point. (This is why we're discouraged from using cell phones on airplanes. Too many cells see the phone, all at the same power level. The solution in that case is to extend the cell network into the 3rd dimension by putting a cell on each aircraft.)

Craig Plunkett

I love ya Tom, but have to disagree with you on the latter half of your penultimate paragraph. History repeats itself all the time, socially and technologically. Those of us old enough to remember Thick and ThinWire Ethernet recall that if you wanted to squeeze more performance out of your network, you needed to go to an FDDI or SONET ring as your core, which are deterministic protocols, ( like WiMAX )as opposed to the non-deterministic nature of Ethernet. As each of your Ethernet LANs grew, they had do be chopped up into smaller "collision domains" and connected to the rest of the world by protocol gateways that would go from Ethernet to your core protocol and back. Until 10BaseT Ethernet and multi-port ethernet bridges ( now called switches ) came along, courtesy of Moore's law and companies like Kalpana, 3COM, and Artel, you needed to use this kind of multi-protocol scheme to make your net work as it grew. If you didn't, you would suffer from congestive collapse. The Ethernet layer 2 protocol itself still doesn't scale, but the faster and faster clock speeds, combined with the change over to the more suitable physical layer of fiber and twisted-pair ( 2-node ) LANS has masked this intrinsic scalability problem. It has allowed us to get link speeds of 10 Gigabits.

A wi-fi lan is the same thing as a piece of old ThickWire, just without the wire. When we take away the wire, we turn back the clock, protocol-wise and have to repeat history. The wi-fi LAN can only grow to to a certain size before it has to get broken up into smaller cells, just like grandpa's LANs used to have to be segmented with bridges and then switches. Wi-Fi at this level doesn't scale. You need to have lots of little pools of wi-fi connected by backhaul for it to scale, and that's why muni wi-fi is a dead man walking. The limits of the technology blow the business case out of the water. We have to think about using different Layer 1 and 2 technologies when we bring the edge of the internet out of the telco wire centers and cableco head-ends. Licensed 700 MHz (layer 1) solves the range and customer premise equipment problems, but the scalability problems of 802.11 protocols still remain if you stick with Ethernet, and they become exacerbated as your LANs get larger. This combo might give a provider a chance to get a start and grab some customers with a relatively small deployment, but they need to plan for segmenting the network as soon as they start to grow, which is a good problem to have.

Richard Bennett

This is a fair enough summary, but it's a hard subject to cover in a few words. The difference between WiFi and WiMax with respect to transmits from the station side needs some clarification, for example.

In WiFi, stations can initiate a transmit in two (actually three) different ways:

1. Send an RTS packet to reserve a slice of bandwidth, and wait for a CTS packet from the destination device (typically the Access Point)/

2. Send a CTS packet to reserve a slice, and don't wait for a response.

3. Wait a random period of time, from 1-32 slot times, and then send the data packet.

1 and 2 are optional, and mutually exclusive.

In WiMax, the procedure is very much like it is in DOCSIS, the cable modem protocol:

1. The station makes a request for bandwidth, which can either ask for one or more packet times. This request competes with other requests and can die due to collision.

2. When the request is granted, the station is given a precise time in which it can access the air without collisions. For periodic requests, such as phone calls, this can cover the duration of the call and is therefore highly efficient.

Now to complicate things, in 802.11e we have mechanisms whereby stations can ask for and get recurring reservations that function the same way that WiMax reservations do, and we have a priority system that gives Voice priority over BitTorrent, for example.

So the point is that the protocols aren't really all that different from a "command and control" perspective. The differences that do exist are in respect of different methods of seeking the same goal, namely a high ratio of productive transmit attempts to collisions.

The best methods of doing this depend on the propagation delay from user to access point, the number of users per access point, and the traffic mix.

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