We have a few 450 AP's with 30-40 subscribers and have been getting several slow speed complaints lately. I just chaulked it up to issues with the SM since the AP rarely got over 20mb/s downlink. We upgraded to 13.4 recently so we could watch our frame utilization. We started graphing it over night and as you can see, we are hitting 100% for sustained periods of time. During that time the AP is only doing approx. 23mb/s. This particular AP has 42 registered SM and the majority show 6x and 4x with 4 or 5 SM's at 2x. When we put these AP's in last year, we really expected them to top out at around 80mb/s not 20mb/s. This is a major disappointment. Anyone else have similar experiences?
AP configuration: 20mhz channels, 2.5ms frame, 10 miles, 75% downlink, 3 contention slots.
I would encourage you to read a couple other threads on this topic that may be pertinent...
First, a quick explanation of the "shared media" aspect of the system. This knowledge base post summarizes the explanation that the SMs that are far away (or poorer signal levels) may be using more of the frame to complete their data transmissions, thereby degrading the overall sector capacity.
Second, there are some other customer experiences that you can read about here. These guys discuss some of the same points you've made here.
Providing the frame utilization statistic is not changing the performance of the sector in any way, but allowing you to see when you might need additional capacity or make changes to add capacity to a given sector.
It's practically impossible to get the full 125mbps throughput in a real world environment. It would require every single client to have stable 8x modulation on the downlink and uplink. Most real world AP's have a mixture of clients at different modulation levels, furthermore, these modulation rates may change due to various enviornmental factors like interference. Also take into account that while you might be seeing some clients at 6x, they might only be achieving 6x only part of the time, they'll send a small number of fragments at 6x, while the rest of them are being sent at 4x and below.
Let's say we average out all the clients connected and pretend that they're all at a strong 4x modulation. If we do a little math, we see that @ 4x modulation, the maximum aggragate throughput available at the AP would be around 62mbps... but we have a fixed downlink/uplink ratio... in your case 75% downlink... that gives us a maximum available throughput in the downlink of 46.5mbps. So, again, assuming everyone is not experiencing ARQ's, or dealing with interference and having to retransmit at lower modulation... we have 46mbps to play with. Now if we add a handful of 2x or 1x clients, and they also happen to be very active clients... that will further drag down that aggragate... and you'll see your frame utilization skyrocket.
So what to do? You'll either need to split up the AP, or increase the channel width (which you can't do because you're already using 20MHz), or increase the modulation rate to those clients that have poor signal or that have poor signal and are using the most bandwidth.
We experienced the same thing and I can tell you a few tips we went by
1) See who you're heaviest hitters (who uses most bandwidth) as they effect total throughput. The more bandwidth they use the more they drag down the overall modulation of the tower since it's not just how many x each person is in but how much airtime each is using in what modulation.
2) If you're using stinger style antennas stop. Reflectors or domes may be ugly but use them for the best modulation.
3) Consider 6 sector deployments if needed (if using 4 sector) or using 4 if using omni. It costs more but the frequency re-use has been quite good for us and we've been able to use 40 mhz total spectrum (2 20 mhz channels) for both 4 90' sectors and 6 60' sectors.