You are right to be concerned about interference between AP's because of the effect it can have on throughput. Two AP's that interfere with each other will have the roughly the same total throughput capacity as a single AP, and who wants one for the price of two. You are also partially correct that the extent of the interference is related to the distance between AP's, but not for the reason most people think. In an AP only environment (let's forget clients for a minute), two AP's on the same channel will either interfere or not based on the receiver sensitivity and the received signal strength. Once two AP's are close enough to interfere, the interference will not be made worse by moving them closer together. For example, lets assume we have two identical AP's on the same channel (AP-1 & AP-2) with a received signal strength (RSSI) from AP-2 at AP-1 of -80dB. If the receiver sensitivity of AP-1 is less than -80dB, say -90dB, AP-2 will interfere with AP-1. If the receiver sensitivity is greater than -80dB, say -70dB, AP-2 will not interfere with AP-1. If the RSSI is greater than the receiver sensitivity, it doesn't really matter by how much. Be aware that this is a simplified account of what actually happens. Most late-model enterprise type radios employ some manner of DSP to eliminate some of this interference, but that doesn't change the yes/no nature of interference, only the signal level at which interference is a factor.
Unfortunately, our networks don't consist solely of AP's, and network clients complicate the issue tremendously. For a given radio, the factors that will determine the achievable throughput are the transmission rate and the size of the collision domain. Unlike the wired networks of old, where the collision domain was the same for every node on the network segment, WLAN collision domains are determined by RF signal propagation and are different for each radio. Consider again our AP's from above. If the AP's are far enough apart so that the coverage areas (region within which a client can associate with the AP) don't overlap, AP-1's collision domain is not affected by the presence of AP-2 or it's clients. If however the coverage areas do overlap, the clients in the overlapping region are in the collision domain for both AP's. So, if there are 30 clients associated to each AP and coverage areas don't overlap, the collision domain for each AP is 31 nodes. If, with the same number of clients, the coverage areas overlap by 1/3, each collision domain is 41 nodes. In the latter case, network performance will be worse than the number of users associated to each AP would suggest. This is why moving AP's closer together can negatively affect throughput. It's not so much the magnitude of the interfering signal, but the size of the collision domain. Finally, you must also consider the size of each client's collision domain. This is especially true if high-gain directional antennas are employed. These narrow beam antennas are capable of producing non-overlapping cells with large amounts of area in close proximity to each other. Picture the coverage area of our two AP's as adjacent oversized lanes on a bowling alley. It's quite possible to have 15 clients (30mW radios with omni antennas) in each lane that can only see one AP, but can see every other client. In that case the collision domain for each AP is 16 nodes, but for each client it's 31. Again, performance will be worse than the number of users associated to each AP would suggest. The good news is that for low client densities using applications with modest bandwidth & QOS requirements, throughput may not be as important as everyone makes it out to be. However, may need to consider all of the factors in a high client density environment, especially if 802.11 phones or other streaming apps are being used. My advice is to keep your AP's from interfering with each other, since that's easily controllable, and the extent to which you worry about clients is up to you. I know I didn't exactly answer your question, but I strongly believe that only you can. Every design in the WLAN business is a one-off, where performance, expense, ease of use, ease of administration, etc. are traded off against one another. You should be suspect of anyone who says they have "THE" answer. Chuck Enfield Sr. Communications Engineer PSU, Information Technology Services 110 USB 2 UP, PA 16802 Ph. 814.863-8715 Fx. 814.863-9851 -----Original Message----- From: James Savage [mailto:[EMAIL PROTECTED] Sent: Thursday, November 13, 2003 4:02 PM To: [EMAIL PROTECTED] Subject: Re: [WIRELESS-LAN] AP radio interference I'd love to use 5g radios but that's not going to happen. I've considered preaching the 11a gospel to the other institution on the chance they might convert ;+) ...thanks for the response........J On Thu, 13 Nov 2003, John J. Brassil wrote: > It works OK - our engineering school recently completed a new wing > that has a large central atrium similar to the one you describe. This > is an extremely dense environment for RF - 84 APs for this single > wing! They wanted as much throughput as possible, so our contractor > designed a picocell network with as many as 3 APs in a single room, > one on 1, one on 6 and one on 11, dialed to 1mW and in some cases > futher attenuated with inline 10 and 25 dB resistors (I know resistors > aren't measured in dB but that's the net effect of the attenuation.) > > Unfortuanely, depspite their best efforts, there is still a lot of > leakage outside the intended coverage areas - not a lot of signal > strength, but > single- and low double-digit signal strength from non-primary APs in lots > of areas, the worst of which is the atrium which has at least 5 devices > visible to Netstumbler or AirMagnet in most places on all three channels! > > We haven't benchmarked the throughput rates but I would imagine they > are not full rate (we fix our APs at 11mbps) but my laptop at least > has been able to hold a connection in there every time I have tried. > Others' MMV. > > The good news is that it all still works. For the situation you are > describing, I wouldn't screw around with 4 channels, that's just a > hack since the frequencies are what they are and playing games with 1 > 5 9 & 11 or some other such silliness doesn't change that. Keep your > same channel APs as far apart as possible and do the best you can. > > Or buy some 5MHz radios and do it properly. :) > > John > > John J. Brassil | Network Engineer, Vanderbilt Data/Video Engineering > voice 615.322.2496 | ICQ 9660375 > > --On Thursday, November 13, 2003 3:33 PM -0500 James Savage > <[EMAIL PROTECTED]> wrote: > > > Hi, > > We're sharing a building with another institution and must also > > share the airspace. We've agreed to go to the four non-overlapping > > channel model instead of three. Each institution will use two > > channels. This is my first shot at full building coverage and > > foresee channel interference issues as I'm restricted to two instead > > of three channels. Presumably, this can be addressed with > > directional antennas and adjustment of signal strength but I thought > > I might ask more experienced folks who might have already > > experimented with channel interference. If two APs operating on the > > same channel can 'hear' each other, is it simply a throughput hit or > > do things just not work at all? Presumably, there's a relationship > > to how close the APs are to each other (signal > > strength-wise)...'closer' means more interference? Also, the amount > > of traffic is a factor as well? This particular building has a > > central open area surrounded by offices/classrooms with glass > > windows overlooking the open space. The signal seems to easily > > penetrate the glass and cover the open space as well.....ie....I > > have multiple APs operating on the same channel bleeding into the > > open space. Is this a show-stopper for the open space or is it > > possibly a slow but liveable scenario? > > > > ....advice or comments are greatly appreciated. > > > > ......thanks in advance.........Jamie > > > > James Savage York University > > Senior Com. Tech. 108 Steacie Bldg. > > [EMAIL PROTECTED] 4700 Keele Street > > phone: 416-736-2100 ext.22605 Toronto, Ontario > > fax: 416-736-5701 M3J 1P3, CANADA > > /\ /\ /\ /\ > > / \ / \ / \ / \ > > \ / \ / \ / > > \/ \/ \/ > > > > ********** > > Participation and subscription information for this EDUCAUSE > > Constituent Group discussion list can be found at > > http://www.educause.edu/cg/. > > > > > > ********** > Participation and subscription information for this EDUCAUSE > Constituent Group discussion list can be found at > http://www.educause.edu/cg/. > James Savage York University Senior Com. Tech. 108 Steacie Bldg. [EMAIL PROTECTED] 4700 Keele Street phone: 416-736-2100 ext.22605 Toronto, Ontario fax: 416-736-5701 M3J 1P3, CANADA /\ /\ /\ /\ / \ / \ / \ / \ \ / \ / \ / \/ \/ \/ ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/cg/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/cg/.
