Matt Liotta wrote:
Jack Unger wrote:
1. The attenuation between 2.4 GHz nodes is not enough to prevent each
node from hearing multiple other nodes as noise (thus more packet
retransmissions and more reduced throughtput). This requires
understanding link budgets, signal-to-noise ratios, and receiver
threshold specifications.
Luckily for us we happen to be a WISP that understands these issues. We
have deployed several Tropos-based networks with sufficient attenuation
between nodes.
I'm glad you understand RF and that you have deployed several Tropos
networks. I'm VERY interested in hearing about any big-city,
"wireless-for-all" (shared residential-business-muni-public safety)
voice-video-date deployments that do now (or that do in the future)
successfully meet the original city-government and city-residents
expectations.
Please share with us the following information about your most
successful Tropos deployment:
1. The end-user throughput expectations
2. The end-user application expectations
3. The number of nodes
4. The backhaul architecture (point-to-multipoint or meshed)
5. The number of end-users
6. The geographical coverage
7. The obstructions in the environment
8. The interference environment
9. The design and installation costs for hardware and labor
10. The throughput-delivery performance over time
11. The support costs
12. Tips and suggestions for others who would like to deploy mesh networks.
Sharing your real-world data here will be of immense value to all WISPs.
2. Metricom is not a good comparison because:
a. They were frequency hoppers on 900 MHz.
Physics applies on all spectrum.
This statement glosses over the issue of different propagation
characteristics at different frequencies and the issue of different
modulation robustnesses. The same propagation characteristics don't
apply to all spectrum nor does interference immunity apply the same to
frequency hopping vs. direct-sequence spread spectrum. The narrower the
channel (and FHSS uses narrow channels) the easier it is for a receiver
to recover a signal in the face of interference. Comparing the narrow
Metricom FHSS channels robustness to the current-day wideband DSSS
channels is comparing apples to oranges even if both systems were
operating in the same frequency band. Further, comparing the propagation
characteristics of 900 MHz to 2.4 GHz is (again) like comparing apples
to oranges. The longer wavelength of the 900 MHz signal undergoes less
attenuation from obstructions when compared to the attenuation that a
2.4 GHz signal experiences from those same obstructions.
b. They promised low (128kbps and then 256kbps, if memory serves)
throughput. This doesn't compare to today's expected throughput levels.
It was stated that the problems occurred for hams at 1200 baud.
1200 baud, 128 kbps, or 11 Mbps - when same-frequency packet collisions
occur, throughput is reduced however, the higher the data rate (speed),
the more complex the modulation mode and the more easily the packet
payload can be mangled by interference.
c. They eventually went to a two-band node that backhauled on 2.4 GHz.
so they could increase throughput.
Only in select areas; the vast majority of the network was single band.
Right, and therefore their network was severely throughput limited over
the vast majority of the network.
d. Metricom then went out of business.
The network did work and it was profitable in a number of cities. The
fact that there was a market bust or that company built more cities than
they had cash flow to support isn't a technical concern.
Their network was always slow, perhaps 128 kbps tops when only a single
user was active. The network never served many customers and was
therefore never heavily loaded. Metricom never had enough customers to
become profitable. Being "ahead of their time" and building out in too
many unprofitable cities were the "non-technical" reasons that they
failed. Combining these reasons with the technical fact of the low
network throughput capabilitiy limited the number of end users that they
could serve thereby denying them the chance to be profitable. Their
investors finally stopped giving them money and they had to close their
doors.
Physics is still physics and companies need to but don't yet
understand wireless physics. They need this understanding before
bidding on muni projects and before they make these high-expectation,
wireless-for-all, triple-play (voice, video, data) promises to public
officials. Once a muni network is engineered incorrectly and deployed
incorrectly, it may well take as much additional money to fix it (if
it even can be fixed) as it took to deploy it in the first place.
Math is still math and companies need to but don't yet understand
advanced mathematics. This generalization is just as accurate as your
statement, but hopefully seems more absurd. Some companies understand
wireless physics. Some of these same companies even deploy wireless
networks that work. Some markets meet the correct criteria to have a
muni Wi-Fi network that can be successful; some even exist today. How do
any of these statements specify the success of muni Wi-Fi in general?
With all due respect Matt, I don't understand your question. Could you
restate it with less over-generalization and with more precision please?
Thanks,
jack
-Matt
--
Jack Unger ([EMAIL PROTECTED]) - President, Ask-Wi.Com, Inc.
Serving the License-Free Wireless Industry Since 1993
Author of the WISP Handbook - "Deploying License-Free Wireless WANs"
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Our next WISP Workshops are April 12-13 and April 26-27
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