This is in part a problem with the way 802.11 was designed.
It was not designed with having large numbers of simultaneous connections.
Example
You have an AP (5 GHz or 2.4 GHz, doesn’t matter) you test it with one client and get 12 Mbps
Conventional wisdom would tell you that you could have 12 x 1 Mbps connections and be fine, this is NOT true.
I have not really delved into the 802.11 design too much, but I theorize that the reaons for the drop in aggrogated speed has to do with:
CSMA - at both customer and AP, the more radios that are attempting to Tx the more likely you are to have backoff
Retransmits - one link on the entire AP can cause huge performance losses if it is a bad link, the more clients the greater the chance
HDX - most customer presence equipment is HDX (since AP can’t Tx and Rx simultaneously the more customers the more switching which takes time)
Design - staying connected to the AP required Tx from both AP and CPE, the more client the more ‘wasted’ transmissions
If I were to lay blame on any one thing it is the CSMA issue. I harp on this all the time, but I have tested extensively and found it to be very common. For anyone that doubts me run this simple test.
AP1 (2ghz-5mhz/2412, or 5ghz-5mhz/5800) → CPE1
AP2 (2ghz-5mhz/2432, or 5ghz-5mhz/5820) → CPE2
You can use 5 GHz or 2.4 GHz, all that really matter in the end is the strength of the signal at the opposite AP (if you use 5 MHz channel sizes the affect is even more pronounced), seperate the channels by a good 20 MHz. In theroy the links should be able to individually operate and max speed, but run a bandwidth-test Tx-only from AP to CPE. Test them individually (other link shut down), indivudally (other link operational but no data transmission), then simutanesouly.
Last time I did this you could acieve ~90% max bandwidth of each link when operating simutanesouly only when seperation was >30 MHz, any less and the percentages drops. Using alternate polarizations helps a bit, seperation helps, but any two tx antennas operating in ‘listening’ range will cause this.
I’ve completed this test in a lab enviroment and on 3 towers simultaneously to see how they were affecting eachother the results were not to my liking. I have not had the luxury of completing these tests with cavity filters to essestially negate the signals are much as possilbe before entering the radio, if anyone completes a test with those please post your results.
If you take a look at the protocols used in WiMax, in cell phone systems, and in the RouterOS competitions products, they attempt to account for these problems is various ways.
The problem is made worse do to the popularization of VoIP, online gaming, streaming video, etc. anything that cause as the previous poster mentioned continously small-packet transmission.
I would love to see a RouterOS expand Nstreme to allow two radio cards to function as one in the AP, ie. one card Rx only, the other Tx only BUT allow CPE’s to connect with only one card, this way the AP is FDX but clients are still HDX. Not sure is the current 802.11 radio technology could be expanded to do this or not, but it would be a big improvement in efficiency.
I would also like to see RouterOS be expanded to allow AP sync, this allows better utilization of the very limited spectrum most of us have to work with.
Cheers
