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" <wireless@wispa.org>
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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