To be clear, Mikrotik us being used, and the 4 remote building are in wds
station mode and only configured to talk to the 1 central master WDS AP, the
four client WDS radios are not configured to talk to each other. So all the
CPE radios only have one hop to the APconnected to the Internet backhaul.
My theory for design was...
I had a 10 mbps backhaul. The WDS PtMP would have 16mbps (54 mbps
modulation), to help with waste from re-transmissions. All clients are
bandwidth managed (priority weighted method) centrally on other end of
backhaul, to also assist with fair transmission time. Also radios use
CDMA/CA, with the CA also assisting. The question is, is this enough to let
it work well with only four buildings.
I'm starting to think that it might not be. But the problem shouldn't be
that they hear each other. we want them to hear each other, so they don't
transmit at the same time. Thats what 802.11 needs. Hidden node happens
because CPEs don't hear each other, and don;t know someone else is
transmitting, from my understanding.
Part of my question is, Does WDS work differently when in Mikrotik Station
WDS mode than a normal WDS AP?
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
----- Original Message -----
From: "Anthony Will" <[EMAIL PROTECTED]>
To: "WISPA General List" <[email protected]>
Sent: Saturday, October 07, 2006 11:57 AM
Subject: Re: [WISPA] WDS PtMP
It would seem to me that as your load increased your WDS/APs are
transmitting over each other as clients are trying to transmit to the
central AP. client -->WDS/AP transmitting carrier beacons or other data to
client and passing onto to -->WDS/AP<--WDS/AP<--Client (transmitting to
local AP)
In this scenario you have the two clients talking and one AP all trying to
talk at the same time and thus raising your noise floor because they are
all on the same channel.
There is not a feature in standard WDS to coordinate who can talk and who
can not talk other then the standard CDMA layer of the 802.11 protocol.
This will create issues as the more load you have on this setup the more
self interference and retransmissions you will incur. The big thing the
mesh brings to the table is the ability to help coordinate all of this
traffic so that you can utilize the spectrum more efficiently. At least
that is my opinion as soon as someone actually does it. You likely are
going to have to switch to a station /AP solution for this setup because
everything is to close and can hear each other. This will destroy your
bridge setup unless you change to a propitiatory system such as Trango,
Canopy, etc. One other thing to note is that this is all half duplex so
you might have two many hops and thus running out of bandwidth.
Anthony Will
Broadband Corp.
Tom DeReggi wrote:
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.
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?
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). 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.
Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
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