The issue: CCI (Co-channel Interference) on 2.4GHz.
The Environment: 40 seats per classroom. Higher-ed, so extremely mixed client base. AP-on-a-stick set to 24Mb lowest Mandatory data rate & lowest Tx power setting (2 dBm)
The Problem: Needing to disable 2.4GHz radio's to achieve the required density does not seem optimal. Am I going about this wrong?
I've used the following formula for AP density and number of radios:
- TTc = Target TCP/UDP Throughput, per client
- TCc = Throughput Capabilities of client
- CUm = Maximum realistic Channel Utilization
c = Number of clients
TTc / TCc / CUm * c = # of Radios
At 40 seats per classroom, assuming each student has a laptop and a smartphone or tablet, with a target throughput of 3Mbps for laptops & 1Mbps for smartphone/tablets:
- Laptops: 3Mbps / 150 / .8 * 40 = 1
- Smartphone/Tablet: 1Mbps / 30 / .8 * 40 = 1.6
Total radios needed per classroom = 2.6. Each AP having 2 radios, this brings me to 1 AP per classroom; if rounding down. All classrooms are in use, and fully populated every day. Current client base shows about 30% 5GHz capable, campus-wide. I expect this number to increase rapidly as most clients are becoming 5GHz capable.
Antenna in use: AIR-ANT2566P4W-R (Dual-band patch)
PDF picture is result of placing the AP/Antenna at the front of classroom (facing East, or away from the hallway). There is very little attenuation between the rooms (sheetrock walls). This is a 3-story building, with same layout on each floor.
One AP per classroom, with patch antenna mounted facing east, for classrooms which have an exterior wall (see diagram), except using Mechanical down-tilt on first floor to reduce the signal in the rooms 'behind' the Anetanna. 2nd & 3rd floors have antenna mounted as drawn, w/ no downtilt. Using 20MHz 5GHz channels, and disabling some of the 2.4GHz radio's, to reduce CCI.
Interior classrooms will utilize same patch antenna, again using down-tilt on first floor and not on upper floors. Disabling some of the 2.4GHz radio's. If down-tilt is utilized on 2nd/3rd floors, the signal penetrates the floors too much.
Options I've considered:
- Purposefully creating attenuation 'behind' the antenna (mesh on wall, RF blocking paint etc.) to reduce the signal 'behind').
- Using non-Cisco antenna, with a better front-to-back ratio, which would reduce CCI as a result of the radiation of signal 'behind'. Not sure if an equivelant one exists. Essentially, I'd benefit from a similiar radiation patter, minus the back-lobes.
- Using internal antenna, and just disabling more 2.4GHz radio's (ea. antenna is ~500$)
The picture represents the results of having an antenna mounted in room 222, pointing out/east/towards the exterior wall; and client RSSI readings from various locations.