Butch,
I do not disagree with your statements.
I'm jsut saying readers may misinterperete the post if my statements were
not added.
If the intend is to do FDX, Using OFDM to accomplish it is one easy way to
do it.
The question that I was challenging is in what cases FDX is appropriate.
It can be risky to rely on two channels in a noisy environment. What
happens if those channels develop interference? What channels would you move
to?
Anyone can get a good link on day one, but what is the plan for preventing
future disasters?
I'm starting to justify FDX now that 10Mhz channels are becoming available.
Two 10Mhz channels has less risk and more options than one 20 Mhz channel.
When we first got into this business, we really thought we were smart. We
were trying all kind of neat things. But at the end of the day, we learned
reasons why the rest of the world wasn't doing it already. Its humbling for
me to admit that publically, but I'm a smarter person for realizing it. We
were doing a lot with OFDM in the early years. The idea was to use two
10mbps Trangos on two freqs to reach 20 mbps. Sometimes Full Duplex, Some
times bonded Half Duplex. The end result is the noise floor got to high,
and it was to hard to move channels around, when needing to cater to what
channel was deployed adjacent, taking up two channels for a single link.
Selecting FDX might have meant not serving a particular direction. The
problem is that when a channel gets packet loss, OSPF doesn't know what to
do, when its up and when its down. We were setting up OSPF so that they
took different paths with full duplex immulated but if one of the links went
down, the second path (by OSPF) was the other channel converting it to half
duplex in the emergency state.
In general we deliver packet loss less links. But its not just a factor of
the technology to accomplish that in noisy areas. Its strategy of the WISP.
Using more spectrum for a task than one needs to, can be wasteful, and give
WISPs less options for selecting the channels that will allow them to use
radios that will prevent the packet loss.
Again, excellent arguements have been made on this list, specifically by
Matt Liotta and Lonnie, on how using 10Mhz channels or GPS syncing with Full
Duplex for a link can actually be more spectrum efficient than 20Mhz
channels in Half Duplex. In real world I have not seen that yet, but in
theory it all made sense. Its actually those debates that got me thinking
to start doing more PtP links on my network and attempting spectrum re-use,
that I am now more effectively accomplishing.
My general rule is.... Do you need 20 mbps for the link? Do you demand Full
Duplex? If the answer to both of these question is "yes", and it may be for
dominately VOIP applications, then it may very well be worth using OSPF for
FDX deployments. But there is risk in doing it. Because if I really only
need 10 mbps, or can survive with half duplex, I'd rather know that if any
of my links encounter interference, that it only takes down half the
customers, not all the customers, because I have customers spread out across
more radios. Having two radios operating independantly using half duplex,
allows redundancy on the fly, when needed.
My end of the day conclusion was, if in Rural or Licensed, go for it, but
otherwise I wouldn't do FDX unless doing it with 10Mhz channel size. The
exception to this is that when 5.4Ghz gear is legal, there are many more
available channels where it is less risky to take two channels for a link.
5.4Ghz will be the spectrum that revolutionizes FDX and PTP links in
Urban/Suburban America. 5.4Ghz is almost useless in PtMP on small sectors.
But it allows PTP links to go 7 miles with margin. Can you think of it now,
10Mhz channels on 100% clear fresh spectrum on day 1, thats 50 new channels,
including polarity. Or 25 new channels using smart Dual Pol NLOS antennas.
It will be the year of 2 ft dishes, to go the distance with low power
spectrum.
2007 will be the year of smart routing. Not MESH as the world typically
knows as MESH, but MESH as the definition, " a network with two paths or
more". PTP reduces latency over PTMP systems, allowing more hops to deliver
the same QOS. Networks will be designed to go to building to buidling more
often. WISPS will start to install two antennas as a requirement for every
new install. Networks will be run layers on top of each other in parallel,
so customers have fewest number of hops to the transit locations, but so
more buildings can be served. In other words it won't be one large mesh, it
will be many small mesh segments, with engineered PTP paths.
This is not a new idea, just equipment options and costs have changed to
allow it to become viable, and previous mass deployment and adoption,
demanding it from the providers. The secret to it all is smart routing. It
will come, or peoiple will leave routing and go back to layer2 bridging end
to end on networks. Its technologies like OSLR that are bringing hope back
to routing for wireless. OSPF is not adequate anymore on its own.
2007 will be the year for network redesign. It will be the year that Telcos
deploy Wi-Max on every cell tower to flood the air waves, buying into the
dream of Super Cell PtMP that will not work to the level that they thought
it would. It will be the science experiment that fails, and forces the
Telcos to spend all their money, and it will be the time that it sperates
the men from the boys in network design. The PTP Mesh, will be neededto
allow providers to quickly adapt, so they can fight the war of spectrum
claim, without the subscriber feeling the pain. The only way to fight mass
scale WiMax is to use PTP smarter, and have more spaces to broadcast from.
(Interference happens at Radio not in the air.), It will be the year that
challenges WISP contracts that exist. It will be the year to learn how loyal
property owners are to their contract with WISPs. It will be the year that
the WISP with the BEST and MOST agreements with MTU property owners at the
lowest cost, will have the upper hand to be able to afford to install the
many PTP links needed to build the MESH above the cities. Its the year Muni
will crash and Burn, when the world learns its not the street light poles
that are needed, but the roof in the sky. Its the year that property owners
will start to re-get inflated values of what their roof are worth, as more
and more providers fight for the colocation. The first in, will ahve the
upper hand, if they do their job right.
It will be the year that companies like Trango Broadband shine, and release
their new WiMax gear at industry low prices, and forcing other Wi-Max
vendors to drop their pricing to compete.
It will be the year that investment pays off, and value of assets get
proven. It will be the year of acquisition, because everyone will want the
first choice to buy the first in WISPs, as aquirers will learn the hard way
how important the first-in WISP's assets are needed for optimized success.
I tell you, its been a hard five years. And this next year is going to be
the hardest year of all, and not for the weak at heart. But those that make
it through it, will become the legends of the WISP movement, and either cash
out because they are worn out, or become part of a dominient entity that
rules the local wireless frontier.
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message -----
From: "Butch Evans" <[EMAIL PROTECTED]>
To: "WISPA General List" <wireless@wispa.org>
Sent: Thursday, September 21, 2006 10:42 PM
Subject: Re: [WISPA] PTP Link Recommendations
On Thu, 21 Sep 2006, Tom DeReggi wrote:
Just remember OSPF does not detect packet loss, and does not properly
switch to backup channels or switches to frequently between channels on
marginal links. So when you use two spectrum channels for 1 link, you
double your chance that the link will get interference and degration.
OSPF detecting packet loss? Switching channels? That's the job of the
radios. OSPF will simply use the links, regardless of the channel or even
type of radio. ANY FDX radio is going to be 2 radios (and therefore 2
channels).
--
Butch Evans
Network Engineering and Security Consulting
573-276-2879
http://www.butchevans.com/
Mikrotik Certified Consultant
(http://www.mikrotik.com/consultants.html)
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