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Re: WiMax whats up?
From: David Farber <dave () farber net>
Date: Mon, 1 Sep 2008 15:08:45 -0400



Begin forwarded message:

From: Christian Huitema <huitema () windows microsoft com>
Date: September 1, 2008 12:18:35 PM EDT
To: "dave () farber net" <dave () farber net>
Subject: RE: [IP] Re:     WiMax whats up?

To feed the {I'd guess} several hundred WiMAX towers in say the DC/ Balt market, you'd need 100MB/s backhaul from each to the Big Kuhuna. Is that
going to come from the Bandwidth Fairy, nightly leaving a bucket of
packets under the site's pillow?

That may explain the interest in wireless backhaul, which I understand is what Clearwire uses (http://www.unstrung.com/document.asp?doc_id=154063&f_src=unstrung_FinancialContent ).

We heard a lot about wireless limitations, but we can also look at wireless successes. The progress of the Wi-Fi technology has been impressive. First, we saw the transition from direct sequence (802.11b) to OFDM (802.11a, 802.11g), getting about 5 times more bit per Hertz in the same condition. With 802.11n, we are getting MIMO, which uses multiple simultaneous radios and antennas in the same frequency to further increase the bandwidth. The interesting part about MIMO is that with multiple antennas we can obtain multiple independent channels between two points, and effectively bypass the single channel limit predicted by Shannon's theorem. The visible result is that, for short range networks, Wi-Fi bandwidth is likely to follow Moore's law.

The same technologies are being used in 3G, 4G and WiMax systems. Indeed, the competition with WiMax has significantly accelerated the technology adoption curve by 3G and 4G providers. We are not at Wi-Fi speed there, and probably never be, since long range transmission always will be more difficult than short range. Spectrum sharing is also an issue, since contention based access like Wi-Fi is highly sensitive to transmission latency. In practice, that means that cellular system need to operate on licensed spectrum, on relatively narrow channels. But Moore's law is at work there as well, and we can expect the cellular bandwidth to easily match copper based DSL, and eventually bypass it.

The good old point-to-point microwave transmissions are evolving as well, and can be used between tall buildings, or between towers. There are multiple systems available today, some operating on licensed frequencies and some operating on unlicensed spectrum, e.g. 60GHz. Again, these technologies improve with Moore's law. You can buy systems today that provide Gigabit transmission, and I expect to see more in the future. Cellular and WiMax are clearly creating a market for these products.

So, I can see that in quite a few places, people are going to drop the DSL and use wireless instead, just like they are dropping landlines for cell phones. We might get some competition after all.

-- Christian Huitema





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