// reference · wlan requirements · cwna ch.10
WLAN Requirements and Design
Every WLAN design starts with a question: are you solving a coverage problem or a capacity problem? The answer dictates AP placement, channel plan, power settings, and minimum data rate configuration. Getting this wrong is the single most common source of enterprise Wi-Fi complaints. CWNA-109 Chapter 10.
// coverage vs capacity - two different design problems
Coverage Design
Maximize the physical area served by each AP. Use fewer, more powerful APs with high-gain antennas. Cell size is the primary design variable. Works well in warehouses, parking garages, campuses, large open areas with few concurrent users.
Approach
APs placed for maximum coverage area. Lower AP density.
AP TX power
High (20 dBm+ common)
Channel reuse
Less frequent - fewer APs, larger cells
Client density
Low - few users per AP acceptable
Min data rate
Lower acceptable - edge clients need lower rates
Risk
Capacity hit when more users arrive than planned
Capacity Design
Maximize concurrent users and throughput per area. More APs at lower power, smaller cells, more frequent channel reuse. Works in conference rooms, lecture halls, stadiums, trading floors, dense open offices with many concurrent devices.
Approach
APs placed for even client distribution. Higher AP density.
AP TX power
Lower (14-17 dBm typical) - limit cell size on purpose
Channel reuse
Frequent - same channel reused across nearby cells
Client density
High - 30-50+ clients per AP in dense deployments
Min data rate
Higher - disable low rates to prevent airtime hogging
Risk
CCI if channel planning is wrong; AP count cost
// channel planning - co-channel and adjacent channel interference
Co-Channel Interference (CCI)
Two APs on the same channel within range of each other. Both can hear each other's frames and must defer transmission. This increases contention but is NOT a collision - it's sharing. A 19 dB signal separation between APs on the same channel is the design target for acceptable co-channel performance.
2.4 GHz: only 3 non-overlapping channels (1, 6, 11). Any 4th AP in range creates CCI - unavoidable in dense 2.4 GHz. 5 GHz: 24+ non-overlapping channels available. 6 GHz: 59 channels at 20 MHz. CCI is solvable with proper channel planning.
Adjacent Channel Interference (ACI)
Two APs on overlapping channels (e.g. channels 1 and 3 in 2.4 GHz). Partially overlapping channels cause actual signal corruption - frames can be partially heard but not decoded. Worse than CCI. Never configure adjacent channels on nearby APs.
2.4 GHz rule: ONLY use channels 1, 6, 11. Never 2, 3, 4, 5, 7, 8, 9, or 10 - these overlap. 5 GHz: all 20 MHz channels are non-overlapping. 40/80/160 MHz bonding creates channel groups that need separation.
19 dB CCI Separation Rule
An 802.11 receiver can decode a frame if the desired signal is 19 dB above an interfering co-channel signal. This is the design target: APs on the same channel should be placed such that by the time you reach the second AP's coverage area, the first AP's signal has dropped 19+ dB.
Typical enterprise rule: same-channel APs separated by >60 feet in open office. More in dense multi-floor buildings. RRM (Radio Resource Management) on controllers handles this automatically via dynamic channel assignment.
// minimum data rates - why disabling low rates improves capacity
Every 802.11 management frame (beacons, probe responses) must be sent at the lowest enabled data rate. A client at -80 dBm operating at 1 Mbps occupies 54x more airtime than a client at 54 Mbps transmitting the same amount of data. One weak client can starve all other clients.
Recommendation: Disable rates below 12 Mbps for high-density deployments. Disable 1, 2, 5.5, 6 Mbps. Set 12 or 24 Mbps as minimum. This shrinks cell size (which is the point - forces clients to roam to a closer AP).
Airtime consumption by rate
1 Mbps (legacy) 54x more airtime than 54 Mbps
2 Mbps (legacy) 27x more airtime
5.5 Mbps (legacy) 10x more airtime
11 Mbps (legacy) 5x more airtime
12 Mbps 4.5x more airtime
24 Mbps 2.25x more airtime
54 Mbps (baseline) 1x airtime
// client density guidelines
| Environment | Max clients/AP | Concurrent active | Design notes |
|---|---|---|---|
| Standard office (data) | 30-40 | 10-15 | Cell overlap 15%. Most clients inactive at any moment. |
| High-density office (open plan) | 20-30 | 15-20 | Lower TX power. Disable low rates. 5 GHz preferred. |
| Conference room (shared) | 10-20 | 10-20 | High concurrent active ratio. Dedicated AP often needed. |
| Lecture hall / classroom | 20-30 | 20-30 | Nearly all active simultaneously. Very high density APs. |
| Stadium / event venue | 50-100+ | 50-100+ | 802.11ax BSS Coloring + OFDMA mandatory. Dense AP grid. |
| Warehouse / retail (IoT focus) | 30-50 | 5-15 | IoT devices mostly idle. Coverage primary. Lower data rate OK. |
| Healthcare (voice + data) | 15-25 | 10-15 | -67 dBm cell edge mandatory. Voice prioritization required. |
| Hospitality / hotel rooms | 5-10/room | 2-5/room | One AP per floor or per 2-4 rooms. Guest vs staff SSID. |
// cwna exam field notes
01 "Coverage design" = fewer APs, higher power, larger cells. "Capacity design" = more APs, lower power, smaller cells. Exam questions will present a scenario and ask which approach applies. Client density + application type determine the answer.
02 Disabling low data rates is a capacity improvement, not a coverage reduction (or rather, intentional coverage reduction to force clients to a closer AP). The exam often frames "disable low data rates" as a best practice for high-density environments.
03 Cell overlap percentages: 10-15% for data, 15-20% for voice. The overlap is needed so a roaming client can transition without losing coverage. Too much overlap = more CCI. Too little = dead zones between cells during roam.
04 2.4 GHz can only carry 3 non-overlapping cells (Ch 1, 6, 11). If your design needs more than 3 APs within range of each other, either use 5/6 GHz or accept CCI. This is the primary reason 5 and 6 GHz are preferred for high-density designs.
05 SSID proliferation: every additional SSID adds beacon and probe response overhead. 3-4 SSIDs per radio maximum is a widely cited rule. Each extra SSID reduces airtime available for data. "Virtual APs" (multiple SSIDs) have a real capacity cost.
See client density and retry patterns in your PCAP
WiFi Analyser surfaces client count per BSS, retry rates, and MCS distribution - the PCAP evidence that reveals coverage vs capacity design failures.