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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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