CWSS cheat sheet

Wavelength in meters = 300 / frequency in MHz. A 2.4 GHz signal has a wavelength near 12.5 cm; a 5 GHz signal near 6 cm.

Free-space path loss (dB) = 32.44 + 20log10(f in MHz) + 20log10(d in km). Loss rises 6 dB per doubling of distance and 6 dB per doubling of frequency.

Fresnel zone radius at midpoint (m) = 17.32 x sqrt(d1 x d2 / (f x D)), with d1, d2, D in km and f in GHz. Keep at least 60% of the first zone clear.

Received power (dBm) = transmit power (dBm) + transmit gain (dBi) - cable/connector loss (dB) + receive gain (dBi) - path loss (dB).

dBm to mW: mW = 10^(dBm/10). 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W.

Every 3 dB change doubles or halves power; every 10 dB change multiplies or divides power by ten. A 6 dB gain quadruples power.

EIRP (dBm) = transmitter power (dBm) + antenna gain (dBi) - cable loss (dB). Regulatory limits apply to EIRP, not to radio output alone.

Higher frequency means shorter wavelength, smaller antennas, more path loss, and less penetration through walls and foliage at equal power.

Multipath causes fading, nulls, and intersymbol interference; diversity antennas, MIMO, and OFDM guard intervals mitigate it.

802.11 frame types: management (beacon, probe request/response, association, authentication, deauthentication), control (RTS, CTS, ACK, Block ACK), and data.

CSMA/CA with DIFS and random backoff precedes transmission; a positive ACK confirms delivery, and a missing ACK triggers retransmission.

RTS/CTS clears the channel around a hidden node; it adds overhead and is normally enabled only above a configurable frame-size threshold.

Roaming is client-driven: the station scans, chooses an AP by RSSI or 802.11k neighbor report, and re-associates; 802.11r speeds key exchange.

WPA2 uses CCMP/AES with a 4-way handshake; WPA3 adds SAE, forward secrecy, and protected management frames (802.11w).

802.11ax (Wi-Fi 6) adds OFDMA, uplink MU-MIMO, 1024-QAM, BSS coloring, and target wake time for dense and IoT environments.

Channel width trade-off: 20 MHz gives more non-overlapping channels and better range; 40/80/160 MHz raises peak throughput but consumes spectrum and raises noise floor.

In 2.4 GHz only channels 1, 6, and 11 are non-overlapping in North America; 5 GHz and 6 GHz offer far more clean 20 MHz channels.

A BSSID identifies one radio's basic service set; an SSID identifies the network and can map to multiple BSSIDs across APs.

Access point: bridges wireless clients to the wired LAN; modes include local, FlexConnect, monitor, sniffer, and mesh.

Antenna selection rule: omnidirectional for 360-degree coverage, directional (Yagi, panel, dish) for point-to-point links and long range.

Antenna gain is passive and reciprocal: it shapes coverage, adds no transmit power, and affects both transmit and receive performance.

PoE budget check: total draw of all powered devices must not exceed switch PoE budget; 802.3af gives about 15.4 W at the port, 802.3at about 30 W.

Controller-based vs. cloud-managed vs. autonomous: controllers centralize configuration, roaming, and RF management; autonomous APs are configured individually.

Site survey tools: passive surveys measure existing RF, active surveys test coverage and throughput, and predictive surveys model coverage from floor plans.

Spectrum analyzers show interference and non-Wi-Fi energy; protocol analyzers capture and decode 802.11 frames for troubleshooting.

A wireless LAN controller handles AP discovery via CAPWAP, image and config push, and client mobility; licensing limits joined AP counts.

Cabling and power: check cable loss at the operating frequency, connector types, and lightning arrestors on outdoor runs before mounting.

Requirement gathering order: define capacity (users and devices per area), coverage (target RSSI and SNR), applications, and security policy before selecting hardware.

Design targets commonly used: -65 to -67 dBm RSSI for voice, about -70 dBm for data, and SNR of 25 dB or better for high-density deployments.

Capacity planning formula: usable throughput per AP divided by per-client demand gives the client count each AP can support; high density needs more APs at lower power.

Business drivers to capture: uptime and SLA targets, guest access needs, BYOD and IoT growth, regulatory compliance, and budget/capital cycle.

Security policy decisions: WPA3-Enterprise with 802.1X/EAP for staff, isolated guest SSID with captive portal, and segmentation via VLANs and firewall rules.

Risk and compliance: PCI DSS restricts wireless in cardholder environments; HIPAA and similar rules require documented controls and monitoring.

Change management: stage firmware and configuration changes, keep a rollback plan, and schedule work in maintenance windows to protect availability.

Measure success with defined metrics: coverage heat maps, client counts, throughput, retry rates, roaming times, and help-desk ticket volume.

Total cost of ownership includes APs, controllers or subscriptions, licensing, cabling, installation, surveys, and ongoing support staff time.