Antennas, Coverage, Roaming, Interference, and Site Surveys
Key Takeaways
- Antenna type and placement shape the RF cell and affect coverage, throughput, and roaming.
- Omnidirectional antennas radiate broadly; directional antennas focus coverage in a specific direction.
- Roaming depends on client behavior, AP placement, power levels, authentication speed, and controller features.
- Interference can come from other WLANs, non-Wi-Fi devices, building materials, and poor channel planning.
- A wireless site survey validates coverage, noise, signal-to-noise ratio, channel use, and capacity assumptions.
Wireless implementation is not just turning on an access point. RF design must account for building materials, client density, antenna patterns, application needs, and interference.
Antenna Types
| Antenna | Pattern | Use case |
|---|---|---|
| Omnidirectional | Radiates in many horizontal directions | General office coverage from ceiling APs |
| Directional | Focuses energy in a direction | Hallways, warehouses, point-to-point links |
| Patch or panel | Directional flat antenna | Wall-mounted coverage into a room or zone |
| Yagi | More focused directional pattern | Longer point-to-point or targeted coverage |
| Parabolic | Highly focused | Long-distance point-to-point links |
Antenna gain does not create unlimited power. It focuses energy into a pattern. A higher-gain directional antenna may improve a link in one direction while reducing coverage elsewhere.
Coverage and Capacity
Signal strength matters, but capacity matters too. A design can show strong signal everywhere and still fail if too many clients share the same airtime.
| Metric | Meaning | Why it matters |
|---|---|---|
| RSSI | Received signal strength | Indicates how strong the AP signal is at the client |
| Noise floor | Background RF energy | Higher noise reduces usable signal quality |
| SNR | Signal-to-noise ratio | Better indicator of usable quality than signal alone |
| Channel utilization | Airtime already in use | High utilization causes latency and retries |
| Retry rate | Frames that must be resent | Indicates interference, weak signal, or contention |
Voice and video require more than basic coverage. They need low latency, low jitter, and reliable roaming.
Roaming
Roaming occurs when a client moves from one AP to another. The client usually decides when to roam, but infrastructure design influences the decision.
Common roaming problems:
- AP transmit power is too high, so clients stay connected to a distant AP.
- AP cells do not overlap enough, causing dead spots.
- Authentication takes too long during roaming.
- Different SSIDs or security settings prevent seamless movement.
- Sticky clients remain associated to a weak signal.
Fast roaming features can help, but they require compatible clients and correct configuration. The exam often focuses on fundamentals: placement, power, overlap, and consistent SSID/security settings.
Interference Sources
| Source | Symptom | Response |
|---|---|---|
| Neighboring APs | High channel utilization or co-channel interference | Adjust channel plan and power |
| Bluetooth devices | 2.4 GHz contention | Prefer 5 GHz or 6 GHz for capable clients |
| Microwave ovens | Periodic 2.4 GHz disruption | Move APs or shift clients to other bands |
| Metal shelving or concrete | Dead zones or multipath | Change AP placement or antenna type |
| Excessive AP power | Sticky clients and roaming problems | Reduce transmit power and balance cells |
Do not assume interference only means weak signal. Interference can cause poor throughput even when the client shows full bars.
Site Surveys
| Survey type | Purpose |
|---|---|
| Predictive survey | Uses floor plans and modeling before installation |
| Passive survey | Measures existing RF energy and AP beacons |
| Active survey | Tests association, roaming, throughput, and application behavior |
| Post-deployment validation | Confirms the installed WLAN meets requirements |
Survey outputs often include heat maps for signal strength, SNR, channel utilization, and coverage gaps. A proper survey also documents AP locations, mounting height, antenna orientation, cable paths, and environmental constraints.
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 signal but high channel utilization.
- Some APs are mounted above metal equipment.
Recommended actions:
- Perform an active survey using representative medical carts.
- Review SNR, retries, channel utilization, and roaming logs.
- Move or reorient APs affected by metal obstructions.
- Tune transmit power to avoid sticky clients.
- Prefer 5 GHz or 6 GHz where client support and requirements allow.
The practical goal is not maximum signal everywhere. The goal is enough clean signal and capacity for the application while supporting 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 contention? Choose two.
Select all that apply
Order the steps for validating wireless coverage after AP installation.
Arrange the items in the correct order