Antennas, Coverage, Roaming, Interference, and Site Surveys
Key Takeaways
- Omnidirectional antennas cover broadly; directional types (patch, Yagi, parabolic) focus energy and gain in one direction.
- SNR is a better quality indicator than RSSI alone, and high channel utilization signals airtime congestion.
- Roaming is client-driven, but AP placement, ~15-20% cell overlap, matched SSID/security, and power tuning enable seamless handoff.
- Interference can crush throughput even at full signal bars - from neighboring APs, Bluetooth, microwaves, or building materials.
- Survey types build on each other: predictive (modeling), passive (listen), active (test traffic), then post-deployment validation.
RF Design, Not Just "Turn On the AP"
Wireless implementation accounts for building materials, client density, antenna patterns, application needs, and interference. The exam expects you to reason about coverage versus capacity and to pick antennas and survey methods for a stated scenario.
Antenna Types
Antennas do not add power - gain simply focuses the same energy into a tighter shape, raising signal in one direction while shrinking it elsewhere. Gain is measured in dBi.
| Antenna | Pattern | Use case |
|---|---|---|
| Omnidirectional | Broad horizontal donut | Ceiling-mounted general office coverage |
| Patch / panel | Directional, flat | Wall-mounted coverage into one room/zone |
| Yagi | Focused directional | Medium point-to-point, targeted aisles |
| Parabolic / dish | Very narrow beam | Long-distance point-to-point links |
A long warehouse aisle wants a directional antenna firing down the aisle; an open office wants omnidirectional ceiling APs. A useful mental model: increasing gain on an omnidirectional antenna flattens the donut, extending horizontal reach but creating a dead zone directly below a ceiling mount, which matters in multi-story buildings. Directional antennas concentrate the same energy into a cone, which is why a Yagi or parabolic dish can bridge two buildings across a parking lot where an omnidirectional AP would never reach.
Always confirm both ends of a point-to-point link use aimed, line-of-sight directional antennas of compatible gain.
Coverage vs Capacity Metrics
| Metric | Meaning | Why it matters |
|---|---|---|
| RSSI | Received signal strength (negative dBm; -65 is better than -80) | How loud the AP is at the client |
| Noise floor | Background RF energy | Higher noise erodes usable quality |
| SNR | Signal-to-noise ratio (dB) | Better quality indicator than RSSI alone; aim for 20-25 dB+ for voice |
| Channel utilization | Percent of airtime in use | High values cause latency and retries |
| Retry rate | Frames resent | Flags interference, weak signal, or contention |
Strong signal everywhere can still fail if too many clients share one channel's airtime. Voice and video need low latency and jitter plus fast roaming, not just bars. RSSI is reported as a negative number in dBm, where closer to zero is stronger: -50 dBm is excellent, -70 dBm is the rough usable floor for data, and signals weaker than about -80 dBm drop. But two cells at the same RSSI can perform very differently if one sits in a noisier environment, which is exactly why SNR - the gap between signal and noise floor - is the better quality metric.
Voice deployments typically target 20-25 dB of SNR and under 30 percent channel utilization; exceed those and calls choppy even at full bars.
Roaming
Roaming is a client moving between APs. The client decides when to roam, but infrastructure shapes the decision. Common failures:
- AP power set too high, so a sticky client clings to a distant AP at poor SNR.
- Insufficient cell overlap (aim for roughly 15-20%) creates dead spots mid-roam.
- Slow re-authentication causes voice gaps during handoff.
- Mismatched SSID names or security settings block seamless movement.
Fast-roaming standards reduce re-auth time: 802.11r (fast BSS transition), 802.11k (neighbor reports), and 802.11v (BSS transition management). They require compatible clients. For most questions, focus on fundamentals: placement, power, overlap, and consistent SSID/security.
Interference Sources
| Source | Symptom | Response |
|---|---|---|
| Neighboring APs | High utilization / co-channel | Re-plan channels and power |
| Bluetooth | 2.4 GHz contention | Steer clients to 5/6 GHz |
| Microwave oven | Periodic 2.4 GHz dropouts | Relocate APs or change band |
| Metal racking / concrete | Dead zones, multipath | Move AP or change antenna |
| Excessive AP power | Sticky clients | Lower transmit power |
Do not equate interference with weak signal - it degrades throughput at full bars.
Site Surveys
| Survey type | Purpose |
|---|---|
| Predictive | Models coverage from floor plans before install |
| Passive | Listens to existing RF and beacons (no association) |
| Active | Associates and tests roaming, throughput, application behavior |
| Post-deployment validation | Confirms the live WLAN meets requirements |
Outputs include heat maps for signal, SNR, channel utilization, and coverage gaps, plus documented AP locations, mounting height, antenna orientation, and cable paths. The exam wants you to match the survey type to the project phase: a predictive survey is the only option before any hardware is racked, a passive survey shows what RF already exists (including rogue APs and neighbor interference) without joining the network, and an active survey is the only one that measures real throughput, retries, and roaming because the laptop actually associates and passes traffic.
Post-deployment validation closes the loop by proving the installed WLAN meets the documented requirement, not just the model.
PBQ-Style Survey Scenario
Facts: a hospital wing has new wireless medical carts; users report drops when moving between rooms; heat maps show strong 2.4 GHz but high channel utilization; some APs sit above metal equipment.
- Run an active survey using the actual carts.
- Review SNR, retries, utilization, and roaming logs.
- Reposition APs blocked by metal (multipath/dead zones).
- Tune transmit power to break sticky-client behavior.
- Prefer 5/6 GHz where clients and requirements allow.
The goal is not maximum signal everywhere - it is enough clean signal and capacity for the application, with predictable roaming.
A warehouse needs focused wireless coverage down a long aisle. Which antenna type is most appropriate?
Which findings can indicate Wi-Fi interference or airtime contention? Choose two.
Select all that apply
Voice-over-Wi-Fi calls cut out for a second whenever a user walks between two APs on the same controller. Which improvement most directly addresses slow handoffs?