oh oh. This one's gonna be fun. I'll warn ya now Tom, this is nothing
personal.....
Marlon
(509) 982-2181 Equipment sales
(408) 907-6910 (Vonage) Consulting services
42846865 (icq) And I run my own wisp!
[EMAIL PROTECTED]
www.odessaoffice.com/wireless
www.odessaoffice.com/marlon/cam
----- Original Message -----
From: "Tom DeReggi" <[EMAIL PROTECTED]>
To: "WISPA General List" <[email protected]>
Sent: Tuesday, December 26, 2006 12:53 PM
Subject: Re: [WISPA] Alvarion Comnet Radios have arrived
marlon,
I have to disagree, and state the opposite.
I've always been a fan of TDD, especially when combined with DSSS to be
able to survive the noise, with better SNRs.
OK, there's a problem here. Lets make sure we're talking the same acronyms
and such.
TDD = Time Division Duplex. In our case, this part really doesn't mean much
of anything.
DSSS = Direct Sequence Spread Spectrum,
SNR = Signal to Noise Ratio. This is the one that you fine tune on a CB
radio to get the his to go away.
For these and many more kindly take advantage of work I did years ago:
http://www.odessaoffice.com/wireless/definitions.htm
The problem occurs when DSSS is not enough to get above the noise.
This is a problem when using DSSS, FHSS, OFDM, FM or any other modulation
scheme we're using today.
When the noise is other OFDM
OFDM is NOT DSSS or FHSS. It's Orthogonal Frequency Division Multiplexing.
"I totally don't know what that is but I want it!" roflol
or Wifi contention gear,
WiFi is an interoperability standard based on IEEE standards. Today WiFi
can be either DSSS or OFDM, I'm not aware of any WiFi FHSS product. 802.11b
is DSS, 802.11a and g are OFDM.
possibly louder than your own signal, using CSMA/CA actually performs much
better in the severe interference environments.
Define better. No, I'm not trying to pull a Clinton here. If you want to
compare DSS to FHSS then, yes in certain types of noisy conditions, DSS can
overcome the noise by spreading it's data packets over a larger area. It's
able to rebuild damaged data packets or to just ignore some times of noise
that would cause an FHSS signal to back off and retransmit on a different
freqency, causing a rise in latency and a drop in speed.
A DSSS signal spreads the data over (in the WiFi example you site) 22 MHz of
spectrum. An FHSS signal spreads that same data over 1 MHz, but it hops
around interference.
I remember seeing a couple of graphs years ago. They showed an ever
increasing noise level and it's impact on DSSS and FHSS. The DSSS stayed at
or near full speed longer than the FHSS but once the noise got too high it
totally dropped off line.
The FHSS system, on the other hand, showed the noise as an overall slowdown
but kept on going long after that DSSS system rolled over and wet on it's
self. I'm hearing mixed results about OFDM. Some say it works better yet
in interference, some say it dies much sooner. I really don't know. It
would be nice to see someone run all three systems in a lab so we could see
the same tests. In fact it would be fun to see that same test with several
proprietary systems too. If only I had more time and money! That's exactly
the kind of tinkering that I live for!
The reason is TDD is guaranteed to transmit during the noisy period, some
percentage of time.
Nope. Not true at all. Been there, done that. I have more than one
T-shirt. It TOTALLY depends on the type of noise and it's levels in
relation to your carrier to interference ratios (also known as SNR).
If you have narrow band interference DSSS can (and OFDM should) work around
it. It'll be able to recreate the missing data bits and deliver a good data
packet. Or, if the noise is far enough off of the center frequency (the
middle part of the 22 MHz wide channel) it'll likely just completely ignore
the noise. Lets say, for example that you are running a WiFi based system
and your customers radio is hitting your AP in the B mode with a -65 signal.
WiFi radios need around a 15 dB c/i radio. So as long as your noise level
was below -80 this system should work pretty well. If the noise hit -75
though I'd expect to see some service degredation.
Canopy requires a roughly 3dB c/i ratio. It would still be working at a -69
dB noise floor. Hit -65 with the noise, and neither of them will work.
With CSMA/CA the radio waits for FREE time, or at minimum retransmits
until it gets FREE spectrum. This can increase latency significantly, but
it does reduce packet loss, which is more important.
Remember, CSMA/CA is WiFi!!!! That's the backoff mechanism that makes it so
easy to co-locate so many systems in a confined area like an office or
appartment complex.
The problem one runs into is that when there is a noise floor above your c/i
there is NEVER free air to transmit in.
TDD w/ ARQ,
Now we're talking apples and oranges. TDD is still Time Division Duplexing
(vs. an FDD Frequency Division Duplexing) mechanism. ARQ is an advanced
means of correcting errors that already took place during transmission. The
error could have been caused by any number of things including interference.
But ARQ (as I understand it) is NOT a way to prevent errors, rather it's a
way to recover from them, hopefully without the need for a retransmission.
can be even better, provided one has a high end radio, that can be
engineered for both ARQ and optimal link quality. But not all ARQ radio
can be optimized for best RSSI. I'd take 8 db of higher RSSI, than ARQ,
because their is no need for ARQ, if you are adequately above the noise.
Agreed.
Alvarion's strength is it empowers an operator to engineer a more durable
link, based on antenna quality and flexibility.
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message -----
From: "Marlon K. Schafer (509) 982-2181" <[EMAIL PROTECTED]>
To: "WISPA General List" <[email protected]>
Sent: Tuesday, December 26, 2006 12:46 PM
Subject: Re: [WISPA] Alvarion Comnet Radios have arrived
Got it. Thanks.
I guess my "beef" comes from being a wifi based wisp. I find it too
difficult to reject interference with a csma based product. Anything
with a "wait for clear air, then transmit" MAC is GREAT for collocation.
But sucks when there are products around that don't follow that
mechanism. That's (my personal belief) why Canopy went with it's GPS
sync. It doesn't care who's already out there, when it's time to
transmit it does. Trango does that to, just without sync'ing the AP's.
My REAL world experience so far is that csmak (or csma/ca, or whatever
collision avoidance scheme you want to use) is GREAT where there aren't
many other systems within ear shot of the radios. However, when there
are other devices in the area, especially those that don't have a
collision avoidance mechanism, the csma radio will pay a heavy price in
performance.
Having used both csma and polling products, I'm not putting in any wifi
type products at 5 gig. All of our next gen products will be polling as
long as we can keep things that way.
These days, I'm learning to sacrifice raw performance for reliability and
uptime. There's a balance, sure, but getting that last 10 to 20% out of
a product is less important to me than having a product that can survive
some of the games that my less scrupulous competitors play.
However, with EITHER technology choice, it's critical to design a network
that can, and does, physically (antenna choice and ap locations) isolates
your system as well as you possibly can. That seems to be the type of
trick that just can't be taught. Your network designer either gets it or
he doesn't. Heck, I've even done consulting gigs where I looked a guy
right in the eye and gave them several choices for site locations. Only
to have them pick something completely different, and sometimes
unworkable.
80 to 90% of people's problems with wireless are self inflicted. Either
outright or in a lack of forethought manner.
Here's an idea for you Patrick. Make this product work both ways. Give
it the option to be either csma or some fancy new version of token ring.
Then we could optimize performance for any environment that we find
ourselves in.
Oh yeah, I remember the big hubbub about GPS in the BreezeACCESS II line.
Why was it important for collocation then but not now?
Hope you guys all had a great Christmas!
Marlon
(509) 982-2181 Equipment sales
(408) 907-6910 (Vonage) Consulting services
42846865 (icq) And I run my own wisp!
[EMAIL PROTECTED]
www.odessaoffice.com/wireless
www.odessaoffice.com/marlon/cam
----- Original Message -----
From: "Patrick Leary" <[EMAIL PROTECTED]>
To: "WISPA General List" <[email protected]>
Sent: Tuesday, December 26, 2006 9:26 AM
Subject: RE: [WISPA] Alvarion Comnet Radios have arrived
I'd never call you a neophyte, Marlon. A jolly elf maybe, neophyte
never...
CSMA/CA. But the MAC has been substantially altered, especially with 4.0
and the WLP (wireless link prioritization) feature where all stations
can be made to wait while those stations with spooled up voice can
release their packets regardless of where they are in the cell. Also, in
VL an operator can adjust numerous values of the CSMA/CA, such as
contention window duration, contention levels, etc. It is more
sophisticated than your basic polling and more efficient.
Patrick Leary
AVP WISP Markets
Alvarion, Inc.
o: 650.314.2628
c: 760.580.0080
Vonage: 650.641.1243
[EMAIL PROTECTED]
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
Behalf Of Marlon K. Schafer (509) 982-2181
Sent: Tuesday, December 26, 2006 9:13 AM
To: WISPA General List
Subject: Re: [WISPA] Alvarion Comnet Radios have arrived
Got that part. I still didn't see in there anywhere, in plain English
that
a neophyte like me can understand, is this a polling or csmak product?
Marlon
(509) 982-2181 Equipment sales
(408) 907-6910 (Vonage) Consulting services
42846865 (icq) And I run my own wisp!
[EMAIL PROTECTED]
www.odessaoffice.com/wireless
www.odessaoffice.com/marlon/cam
----- Original Message -----
From: "Patrick Leary" <[EMAIL PROTECTED]>
To: "WISPA General List" <[email protected]>
Sent: Friday, December 22, 2006 1:54 PM
Subject: RE: [WISPA] Alvarion Comnet Radios have arrived
Marlon, I'll answer this with a re-post of a September post that
explains, in part, why VL is not just regular CSMA:
<<trim>>
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