Cross Channel Interference
Last Post: March 23, 2006:
I'm currently working towards my CWNA, but I have been assigned a task outside my experience and I was hoping to get some input.
My task is to connect a fifth building to an existing wireless network which is using Cisco Aironet 340 Bridges. I've been told that there is one main bridge and that each of the other three buildings have been configured to use separate channels (assuming 1,6,11).
My question deals with CCI. Can the new building be configured on one of the existing channels without too much fear of interference? What questions should I be asking? And while we're on the subject of CCI, if 1,6, and 11 are the most commonly used frequencies what about Joe Blow's access point down the street, won't that interfere with the city's network or vice versa depending on the power of the city's signal? Most of the CCI problems I've read assume that the network is in a bubble. With the popularity of wireless networks how do you account for the masses who set up wireless and keep it from interfering?
I know there will need to be clarification, but I'm hoping for some general guidelines first. Thanks.
I need some clarification on your situation. You say that you have one main bridge, but the other three buildings are using 1,6,11. Are you in a point to multipoint bridge or are there three separate bridges currently on your main building?
Here are some tips. If you already have three bridges on one building using channels 1,6,11 (correctly) then you have two choices. One is use an 11a set of bridges for your next connection or, change the polarity of your next 2.4Ghz set of bridges. Even with the polarity change you will want to maintain at least 5+ feet between co-located antennas, the more the better.
As far as your interference questions, one of the best solutions is go to 11a because it is used less in most areas. If that cannot be accomplished, then you will make sure that you are using highly directional antennas (parabolic dish/grid) for your point to point. There are many times you will see yagi antennas used for point to points, but I do not agree with this thinking. Yagi antennas have a wider beam width which can be beneficial, but in a point to point will cause more interference with others and intercept more interference from surrounding devices in the frequency range.
If you have a point to multipoint then that is a different issue. If that is your situation I will address that. Please post back with as much detail as possible and we'll be happy to help.
There is only one bridge on the main building servicing the other three. So I believe it is point to multipoint. I will try to get more details. Unfortunately, the engineer who originally set this up is no longer available and his documentation leaves much to be desired.
I cannot even confirm at this point that it is set up using the three channels. This information based on what a colleague told me who was around when the network was installed. Customer is looking for a quote to add the additional building and I'm being asked if its possible and how to go about it. So any help you guys can give would be greatly appreciated. Even if its just which questions I should be asking.
Ok. So that means that you don't have a channel problem because every bridge will have to be on the same channel. But, you could have a throughput problem. I take it that you are happy with the current performance. If you add another bridge of course you will be adding to the current load. Depending on the budget and how critical throughput is to the users, you may just want to add your fourth bridge in point to multipoint and see what happens. If the load is too great, then you can just go point to point with the bridge with the highest load. Hopefully all this makes sense.
That's the part that wasn't making sense to me then. Guess that's what I get for listening to third hand information. So if it's a single radio that's on the main building (which it is). It can only listen on one channel, correct?
But my sidebar question still stands... With the prevalance of wireless these days and channels 1,6, and 11 being the most commonly used, how can I avoid CCI from Joe Blow down the street with his AP set to the same channel as mine? (Assumes many Joe Blows with AP's set to 1, 6 and 11).
If we are still talking about your outdoor bridging application, the only wireless signals you really have to worry about are those with external antennas. For example, if you local area has a WISP, then you may have a problem. If you are just worried about some guy that has a SOHO AP set up 1/4 mile away, they won't have the damaging affect that you may think.
Although simple and not completely accurate, get on top of the building where your bridges are and see if you can "see" any other wireless networks in the area. Its not spectrum analysis, but it could shed some light.
How to avoid it if it is there? Again, using 11a is one way. Another way is to change from point to multipoint (PtMP) mode to 3 or 4 point to point (PtP) bridges. The main problem with the interference that you will have is with your radio at the main building because it is probably an omni, at best a sector antenna. It will be receiving the most interference from surrounding unfriendly AP's. With PtP using highly directional antennas, you are limiting the scope of what the radio will receive.