channel interferance

[mark78]

How much will performance suffer if say .05% of the frequency is being shared? ie. 2450 - 2455 are being used by 2 separate AP's/channels?

I have to give a test group a guaranteed channel. If I split my building in to 4 channels at 1, 4, 7, and 11; then 1, 4, and 7 each have a small overlap, but it leaves 11 isolated for testing.

We have a ton of SSID's, a few I can control with power reduction though.

If this doesn't work out, and we were to build a small isolated wifi test lab, what material could the walls be lined with to absorb but not reflect the signals? I was thinking that special wallboard used for home theaters...I would think sheet metal under the drywall may reflect the signals?

[M/Q]

I would not recommend that approach as there is more overlap than you think. That specification is only used to show the frequency range that is at a certain attenuation level. Which means that a device can be using a larger frequency range if its output power level is set higher.

My thoughts would be to use 1, 6 and 11. To help isolate the test network only use non interfering channels adjacent to its coverage area.

As for isolating a room for RF, check out Faraday rooms.

[WiFi Guru]

Do NOT use any channels other than 1, 6, and 11. Arrange them so that you get little to no overlap between the matching signals.

[M/Q]

That advise is not exactly true IMO. You need to be more specific or people will get confused. First, for example if you have only two devices there is no reason why you could not use channels 1 and 11 or any two that are not overlapping.

Second two devices that are on two different and non interfering channels can coexist right next to each other quite happily. That is the whole point of using different channels.

[cwilli]

I think it depends on the situation. You may be ok with a slight overlap, and thus slight interference, if you are not ok with co-channel interference. Especially if the co-channel interference is with an AP that you dont control. If it is on the same channel as you, you will share the channel and thus when that ap is talking, your ap will wait for the channel to be clear.

On the other hand, if you slightly overlap with say channels 1 and 4, then channel 4 will interfere with channel 1 only when the edge of the band is used, which is not all the time. When transmitting, the entire RF range of that channel is not used, so there is only a percentage of time that the edge of that channel (the overlapping edge) will be used. If it is being used when channel 1 is talking, then you get interference and thus retransmits on both channels.

So the question becomes, could you accept sharing the channel with another AP over accepting some level of retransmits. Its a design decision, but you do have a choice.

Here is a good whitepaper on this subject;
http://www.imconf.net/imc-2005/papers/imc05efiles/mishra/mishra.pdf

[M/Q]

Cwilli, I suggest that you are inaccurate in some instances with your conclusions. First, only APs of the same make and model may be aware of other APs in the same channel, if and only if they have enough intelligence. I can almost guarantee that consumer grade products will not be aware and I have come to that conclusion through some rather intensive testing of my own.

Second, I have read the paper you referred to a few times now and contest their findings to a certain extent. They did not specify which devices or firmware they were using. I would venture a guess that they were indeed using devices that had the intelligence to be aware of other APs in their immediate vicinity and that they were on interfering or identical channels. That is not always applicable to real life and I would never recommend to anyone to using channels that interfere.

Also your mention of the fact that even though the channels overlap there is a strong chance that they will not be at the same frequency at the same time. That is not accurate either when all circumstances are considered. The Spread Spectrum technology envelopes the entire channel frequency range in a matter of microseconds and the probability of interference is much more prevalent than you suggest.

[marlehole]

Hello M/Q,

Just a question I meant to ask you for sometime; but the opportunity never presented itself.

Have you ever attended or studied at YDI (Young Design Institute) now Terabeam?

Thanks!

[M/Q]

Hello Don,

Terabeam is a great company if memory serves me. They own Proxim and Ricochet, if I remember correctly.

I wish I had the money to attend classes and seminars, I pretty much am self taught. All of the certs and such have come that way. I was fortunate to attend university which endowed me with the thirst for knowledge as well as proper study and research habits. My employer Orange Business Services has a great training program for us field engineers but that is mainly in the WAN/Networking area. Recently though I have been doing quite a bit of wireless for MS campuses so I am fortunate in that regard.

[golfnut]

M/Q,

Need to get you out here to the Redmond campus sometime :-)

Greg

[M/Q]

Hey Greg,

Man would that be a dream come true. I can only imagine. The field offices are nothing to complain about, but the main campus must be very cool.

How is the migration going? I can only imagine what kind of device quantities you are looking at. I am a huge fan of Aruba now. Cisco had better watch out in the area of managed wireless networks.

[marlehole]

Hello M/Q,

Thanks for the response, and congratulations on your achivements. Terabeam Inc is a great company! When I wanted to transition into the Wireless Industry after being caught in the downsize market trend, and between jobs some four years ago; I approached a company on the East Coast to do an Internship. They offered to teach me, but I would have had to pay them $20 thousand (yes 20G's); I said, there must be another way.

After doing the kind of research you talked about, I found a then "small" company YDI Inc. (Mr. Young a visionary) in the state of Virginia holding seminars and willing to teach for a fraction of the cost if one was prepared to fly/drive there. Yes, now www.proxim.com.

The reason I asked: It is not the first time I find you display the philosophy and teachings of that institution. In fact that's what drew me to this forum, along with Greg (Golfnut, I am a golfer as well), and Bobby777, with whom I had the same interest at the time (D-Link authentication portal); who by the way I haven't seem posted in a while. (Bobby where are you?)

This thread just reminded me of those days. Thanks again, for the trip back.

[M/Q]

Hey Don, Thanks.. I certainly do not mind being remotely associated with stand up organizations like that.

I realize we hijacked this thread and I apologize to the OP, maybe we should start a new one on this subject. Greg has an interesting project right now that I was involved with locally as well. I am sure it would be interesting to others and it points to his expertise to be sure.

I think Bobby is busy with his new WISP venture and that might be the reason he is not posting.