So you walk into a large public venue (LPV), Stadium, or Lecture Hall and pull up your favorite mobile WiFi scanner app and there it is; CCI out the yang. Way more Co-Channel Interference/Contention (CCI, or CCC depending on which semantics camp your in) than you'd ever design your WLAN for.
We all know adding AP's which results in increased Co-Channel absolutely reduces capacity. The over-head introduced comes at no gain, and increases channel utilization. It's a shared medium, listen-before-talk, CSMA/CA etc. If you haven't done so yet, prove it to yourself. Two same-channel AP's in the same space, a couple clients and iperf. You will see an overall capacity reduction with the addition of the second AP.
So how much time during your WLAN design do you spend figuring out where to place an AP, without creating CCC? It depends, but for me, seems like >80% sometimes. Designing for Voice, Location, Capacity while keeping neighboring same-channel AP's at > -85 away in 2.4GHz is...hard!
So then, there's only two possibilities in our scenario:
- Someone was being too lazy to plan for proper spectral re-use, threw up a bunch of AP's, and just designed it wrong.
- Someone spent a lot of time, considered the use-case and RF environment, and actually designed it just right.
An example of number 2 is what I'll try to detail here, but first some background.
I'll keep this brief, and not detail CSMA/CA, why Co-channel is bad, or all the standard 'HD Tweaks'. See bottom of post for recommended considerations in an HD environment.
- The quality of an STA's connection, be it Client or AP, can not be determined by RSSI.
- You can not determine a clients achievable PHY data-rate, by knowing it's RSSI.
What does this mean in our HD scenario, with seemingly too much Co-Channel? Consider the following example:
Important distinction: no matter where I placed a patch antenna, or what amount of downtilt (including straight at the ground) there is no possibility of getting more than 3 2.4GHz radios in these rooms, without introducing excessive Co-channel contention.
Considering the target application, and use-case of these halls:
- 4 to 5 AP’s were required in the larger rooms
- Two or more AP’s having 2.4GHz radios disabled
- Directional Antennae on almost every AP in the building
Circles represent the ‘intended’ coverage areas. For drawing clarity, I’ve not included all the AP’s deployed in this building. These are four lecture halls, used simultaneously. Approximately 800 seats in total. In many cases, seats does equal associated and 'using' the wifi. In this case, it can. Each student has at least a smartphone, possibly a tablet, and most likely a laptop. In some cases laptops are a requirement, and usage of them during class is expected.
Important consideration is all four rooms are within RF range of each other. Just how strong you hear AP's in neighboring rooms depends on a couple factors, like location, client-type, etc. That's a LOT of 802.11 & non-802.11 (Bluetooth) in a relatively small space contributing to higher Noise Floor.
AP-1 and AP-4 with directional patch antennas at 90 degree downtilt are on the same channel (6). More specifically, even though a CMU wall exists between room 106 (top left) and room 112 (bottom left) a typical mobile client placed under AP-4 can hear AP-1 at or around -75. AP’s having higher Rx sensitivity, can hear each other at an even stronger signal. In many environments I would approach the design in such a way as to move AP-1 or AP-4 (or remove one of the AP’s) so that this CCI does not exist. As previously mentioned, -75 might yield an SNR of 20, in an office environment, but here it yields an SNR of at best 5. The Noise Floor in these halls can be in the -70's (or worse). Of course, it's not always that high, but low 80's is common, and what we are faced with is a situation where you either have CCI, or you have not enough SNR to achieve usable data-rates. Clients would end up with unstable connections at times where each hall is in session.
Here's the rub: CCI is taking place, not because I’m attempting to increase performance by simply ‘adding AP’s’, but because without the CCI introduced by AP-4, 2.4GHz clients in that area will be unable to maintain an acceptable connection. In short, in order for clients to obtain enough SNR in High-Density area’s CCI is sometimes unavoidable. It is a balancing act I often struggle with and is my favorite design challenge.
Recommended Considerations for HD environments:
- Raising 'Basic' & 'Supported' Data Rates
- Don't be scared of static power/channel assignments. Try seeing how "RRM would do it" and go from there (baseline & lock in)
- Optimized Roaming (if not possible, RSSI Low Check), ClientMatch or whatever your particular vendor does to help with situations like Sticky Clients and/or low-rate connections.
- Directional Antenna's. Use them! (AIR-ANT2566P4W-R= in use here)
- Watch your noise floor (remotely with "show ap auto-rf 802.11b <ap_name>") & locally (Metageek Chanalyzer Pro or similar)
- Typical SNR to MCS table: http://www.revolutionwifi.net/2014/09/wi-fi-snr-to-mcs-data-rate-mapping.html