Marlon,
For clarification....
1) Yes 5.250-5.350 is for outdoor, but I temporarilly put my radio to a
channel under 5.25 which is in the 5.1 band for indoor only use, for the
temporary testing.
2) My primary goal in the original post was to learn the difference between
Wifi Station/client and Wifi WDS at the protocol level on how the protocol
makes communications. For example, can they both do CTS/RTS? Unless the WDS
protocol is fully understood, its not possible to design networks optimally
using WDS.
3) Mikrotik actually has several WDS modes. They may not all necesarilly
operate the same at the protocol level.
4) Also, the reason the network was done this way was that only one of the
five buildings had LOS to our network. All clients within the building are
done with wires. Normally we would have done this site with Trango PtMP, but
when it was installed (1.5years ago), Trango had a short range packet loss
problem and no Omni AP option. Cosmetic requirements from Property owner
for the main site, would not allow Sector AP antennas for each remote
buildings, so Omni was required. WDS was required as Standard Wifi was not
true bridging. This was actually an excellent case study site for Mikrotik
acting as both the radio and VLAN switch w/9 ethernet ports on CPEs.
5) There are many ways to improve the network, the problem, is I'm looking
to be as least disruptive as possible, and don;t want to use the customer
base as guinee pigs, so looking to better understand WDS at the protocol
level. One of our consideration, is that we may leave the Mikrotiks as the
Building routers, and repalce the outdoor stuff with Trango, not that it has
good short range gear. But there is no reason to do that unless WDS is truly
the cause. We have not proven that for certain yet. We can also solve it,
by adding a second WDS Master AP, and then we'd split the load and have
redundancy.
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: Friday, October 06, 2006 1:27 PM
Subject: Re: [WISPA] WDS PtMP
----- Original Message -----
From: Tom DeReggi
To: WISPA General List
Sent: Friday, October 06, 2006 9:09 AM
Subject: [WISPA] WDS PtMP
Background....
In standard WIFI, a principle exists called hidden note, where two CPEs
transmit at the same time and colide because they do not hear each other.
There are three ways to get around that, using WIFI between Client and AP.
1) Polling (Karlnet, Nstream, Proprietary), 2) Use Omnis, so radios can
hear each other if in close proximity, 3) RTS/CTS which effectively solves
the problem at a significant performance degregation. A well know problem
with well known solutions.
mks: Close. It's when two CPE talk at the same time and the AP can't
hear one of them because the other one is louder. This is part of why you
should never build a network using the same size antennas everywhere. And
why more power isn't always better. I try to keep all of my cpe within
about 10 dB of each other.
mks: It can ALSO be where two cpe talk at the same time because they
don't know each other exists. This causes a collision at the ap (it can't
understand either one of them) and after a random backoff time they'll
each try again.
mks: The easy fix to that problem is usually to just add another ap as
you've filled up the one you already have :-).
Issue.....
How does this play our with WDS? AP to AP communication. Sure in PtP its a
non-issue, because there are only two radios involved to complete the
link. But WDS allows PtMP operation.
How does WDS commuication work? Does the Hidden Node problem exist with
PtMP WDS? And if so, is there a way to address it? If so, will it help to
make the CPE's Omnis, so they hear each other?
mks: As I understand it, wds is simply a way for a cpe unit to ALSO act
as an ap. Much like AdHoc mode. Except this time you can put in WDS
units only where needed so that you can go around a corner or two. With
AdHoc the whole network would have to be that way.
My confusion is how WDS/WDS works compared to Station/AP modes.
Example application:
Using 802.11a gear.
5 seperate MTU buildings, spread out within 300 yards of each other.
1 is a Master AP Site, with an Omni, and a second backhaul radio to the
Internet.
4 of the 5 have a direction CPE style antenna pointing to the Master
Antenna.
WDS is used to allow the radios to operate as true transparent bridges,
and to pass per client (5-10 clients per MTU) large packet VLAN traffic.
(Note: There is a reason we did not select Nstreme w/ Polling. It may have
been an incompatibilty with WDS or inabilty to do transparent bridging
with large packets, which standard 802.11 station mode does not support
under protocol. May have been early version of Firmware, not sure if still
an issue)
Why I thought it might be an issue:
Surveys show low noise. However, as more clients have been taken on (2
mbps average sustained throughput all combined), the Link quality started
to degregate as if the noise floor was rising.
As a tempoirary measure, we switched to 5.2Ghz (indoor only FREQ, which
appeared not to have any detectable noise in standard 802.11 based survey
tools, and was chosen because non-detectable carrier grade gear would not
use those channels). Its hard to believe that the noise floor would be
that high using that freq. So I'm wondering if the noise that I'm hearing
is actually my own CPEs within this project?
The symptom was sparatic higher latency, what typically would happen if
802.11a had frequent retransmissions (native prorocol ARQ).
mks: 5.2 gig is also usable outdoors. I use 5.2 and 5.3 anywhere I can!
Because most others don't :-). But the smart ones do. It's the 5.1 ghz
band that's indoor only.
mks: I think what you are probably seeing is indeed the effects of all
mesh networks that use single radio systems. They all use the same
channel and try to at the same time. That's why I've never liked standard
mesh systems. I don't think they (and feedback such as yours seems to
uphold this) will ever scale to any real use. Sure, put it in an office
and feel free to do email and an occasional print job, but don't do much
more than that with mesh.
I can look at stats to see if there are re-transmissions, but that data is
pointless, as what I want to know is, is the retransmisison because my own
noise or someone elses. Its hard to tell with WiFi, as WiFi doesn't
transmit when its not in use. So testing in the middle of the night, when
clients and users in town are off, may not be meaningful. Its also
possible, that I just have a failing radio card or two, and a totally
different cause.
mks: Well, first, try changing channels around and see if it has any
measureable effect. Next, get ahold of a spectrum analyzer (Bob M. isn't
that far from you, or I can ship mine out to you).
mks: Next, build a proper network! grin. Put in a 5 gig ptmp and/or ptp
system to link up all of the buildings back to the internet. Then use
your 2.4 for the inbuilding work. Better yet, use wires to get to the
customers and just build each of them that wants a wlan his own wlan.
You'll have a MUCH happier customer base!
laters,
marlon
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
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