1,200 Seats, 2,400 Devices, and a 4-AP Pilot
A 1,200-seat auditorium hosts a conference where each attendee carries a laptop and a phone. The organiser has contracted for 1 Mbps of usable throughput per device. The room is 40 m × 30 m with a 9 m ceiling. There is no usable 2.4 GHz spectrum left: a neighbouring hotel's APs are visible at -60 dBm on channels 1, 6 and 11. The room is currently served by four ceiling-mounted APs running 80 MHz channels on 5 GHz at full power. Pilot feedback: connections succeed everywhere, but video stalls and file sync times out. The vendor's remedy is to add a second AP onto each of the four 80 MHz channels.
1. Do the capacity arithmetic: what aggregate throughput is required and roughly how many APs does that imply?
Read the worked answer
Devices: 1,200 attendees × 2 devices = 2,400 clients Aggregate demand: 2,400 × 1 Mbps = 2.4 Gbps of sustained usable throughput
Two independent constraints:
| Constraint | Working | APs needed |
|---|---|---|
| Throughput | 2.4 Gbps ÷ ~80 Mbps usable per 20 MHz 2SS AP | ~30 |
| Client count | 2,400 ÷ ~40 clients per AP for sustained 1 Mbps each | ~60 |
The client count is the binding constraint, not raw throughput. Throughput per AP is irrelevant if 300 handsets are contending for airtime; with CSMA/CA contention and per-client management overhead, efficiency per client falls faster than the channel capacity rises.
Conclusion: plan for roughly 60 APs at 20 MHz, mounted for physical cell isolation, not 4–8 APs at 80 MHz.
2. The vendor wants 8 APs at 80 MHz. Why does that fail on both capacity and channel reuse?
Read the worked answer
Capacity: 2,400 ÷ 8 = 300 clients per AP, each promised 1 Mbps. That is 300 Mbps of demand against a realistic 80 MHz 2SS usable ceiling of ~350–400 Mbps - nominally feasible, practically not. Three hundred clients contending under CSMA/CA each get a vanishing slice of airtime; the queueing delay alone breaks video, before any interference is counted.
Channel reuse: 5 GHz offers roughly six non-overlapping 80 MHz channels. Eight APs therefore means at least two pairs sharing a channel inside the same room, with no physical separation to isolate them - the definition of co-channel interference. An 80 MHz channel also aggregates the noise, interferers and any radar event across four 20 MHz sub-channels, so it is only as clean as its worst quarter.
Adding APs to a channel plan that is already over-wide makes the interference worse, not better. The fix is narrower channels, more APs, and lower power.
3. Draft the channel and radio plan you would actually deploy, and justify each choice.
Read the worked answer
- 5 GHz only for attendees. 2.4 GHz is unusable here (-60 dBm from the hotel on 1/6/11) and would only tempt clients into a doomed band. Disable 2.4 GHz radios for the attendee SSID, or at minimum band-steer aggressively.
- 20 MHz channels throughout. With ~60 APs, 80 MHz gives only six channels - far too few for reuse in a single room. 20 MHz gives ~25 channels, so a reuse pattern is even possible.
- Low power and downtilt directional antennas. Aim for cells that cover a seating block and die before they reach the next block; high power creates overlapping cells and sticky clients, not coverage.
- Disable low data rates (1/2/5.5/11 and the lowest OFDM rates). Legacy rates consume disproportionate airtime per frame and enlarge the effective cell.
- Enable 802.11k/v so clients are steered and can make informed roam decisions in a dense environment; 802.11r only if the client fleet supports it.
- Per-client limits and WMM to keep one device from monopolising a channel and to prioritise voice/video.
- Design for 50% airtime utilisation at peak, not 100% - a dense venue needs headroom for retries and bursty traffic.