On thing I forgot to mention is that every single packet transmitted is
going to be retransmitted on all the WDS/AP connected together on the
wireless side. With sustained traffic that would mean that all of them
are transmitting and receiving the 2 megs mentioned. And we can assume
that these units are not exactly all the same distance or under the same
exact load so there will be very tiny differences when each unit will be
retransmitting that 2 meg of traffic.
I am not real happy with the way I explained this let me know if it
makes any sense :)
Anthony Will
Broadband Corp.
Anthony Will wrote:
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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