CWT cheat sheet

Wavelength: λ (m) = 300 / f (MHz). 2.4 GHz ≈ 12.5 cm, 5 GHz ≈ 6 cm, 6 GHz ≈ 5 cm; shorter wavelength means smaller Fresnel zone and more attenuation through obstacles.

Free-space path loss (dB) = 32.44 + 20log10(f in MHz) + 20log10(d in km); doubling distance adds about 6 dB of loss, and doubling frequency also adds about 6 dB.

Fresnel zone radius at midpoint r (m) ≈ 17.32 × √(d1·d2 / (f·D)) with distances in km and f in GHz; keep at least 60% of the first zone clear of obstructions.

Link budget: received power (dBm) = transmit power (dBm) + transmit antenna gain − cable/connector loss + receive antenna gain − path loss. EIRP = TX power + antenna gain − cable loss.

dB rules of thumb: +3 dB = 2× power, +10 dB = 10× power, +6 dB = 2× distance in free space; +3 dBi doubles antenna gain relative to isotropic.

Attenuation by material at 2.4/5 GHz: drywall ~3 dB, wood door ~3 dB, brick wall ~6–10 dB, concrete ~12–15 dB, metal mesh or foil-backed insulation blocks nearly all signal.

Multipath causes fading, nulls, and delay spread; OFDM (used by 802.11a/g/n/ac/ax) tolerates delay spread via cyclic prefix and subcarriers, while DSSS spreads energy across a wide band.

Noise floor rises with channel width: 20 MHz ≈ −95 dBm, 40 MHz ≈ −92 dBm, 80 MHz ≈ −89 dBm (typical 5 GHz reference); SNR (dB) = signal (dBm) − noise floor (dBm).

2.4 GHz has only three non-overlapping 20 MHz channels (1, 6, 11 in North America); 5 GHz offers many 20 MHz channels, and 6 GHz adds up to 59 × 20 MHz channels with no legacy overlap.

802.3af PoE supplies up to 15.4 W at the PSE (12.95 W at the PD); 802.3at (PoE+) up to 30 W (25.5 W at PD); 802.3bt Type 3 up to 60 W and Type 4 up to 90 W at the PSE.

PoE voltage is nominally 44–57 V DC; check the AP's rated draw against the switch port budget, and remember 2-pair vs 4-pair operation affects available wattage on bt.

Cabling: Cat5e supports 1 GbE to 100 m; Cat6/6A support 10GBASE-T to 55 m/100 m. Solid-core cable for permanent runs, stranded for patch cords; T568A or T568B consistently on both ends.

Antenna types: omnidirectional (dipole) radiates in a donut pattern with ~2–5 dBi; patch/panel is directional with ~6–14 dBi; Yagi and parabolic grid give high gain for long point-to-point links.

Antenna gain trades coverage area for distance: higher dBi narrows the beamwidth. EIRP = radio power (dBm) + antenna gain (dBi) − cable loss (dB); regulatory EIRP limits vary by band and country.

Mount APs at planned locations, below ceiling obstacles, away from metal ductwork, elevator shafts, and fluorescent/LED drivers; keep antennas vertical and consistent for predictable coverage.

Survey kit: site survey software, calibrated adapter, spectrum analyzer, floor plans, laser distance meter, and a PoE injector for testing AP placement before permanent mounting.

Cable test after installation: length, wire map, insertion loss, and near-end crosstalk; a PoE tester confirms the PD receives correct class and voltage before the AP is mounted.

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

Beacon interval default 100 TU (102.4 ms); DTIM period controls how often buffered multicast/broadcast is announced - DTIM 1 wakes clients every beacon, saving less battery but improving latency.

CSMA/CA with DCF: station senses the medium, waits DIFS, backs off a random slot count, transmits, and waits for ACK; RTS/CTS mitigates the hidden-node problem.

802.11n/ac/ax features: MCS index sets modulation and coding, channel width (20/40/80/160 MHz), spatial streams (MIMO), and guard interval (400/800 ns). Higher MCS needs better SNR.

Roaming: clients decide when to roam; 802.11k provides neighbor reports, 802.11v sends BSS transition management frames, and 802.11r enables fast BSS transition (FT) for quicker reauthentication.

Troubleshooting order: verify physical link and PoE, confirm channel and power, check for co-channel and adjacent-channel interference, then inspect client association, DHCP, and authentication logs.

Common causes of low throughput: retries from interference, low SNR, channel width mismatch, legacy 802.11b clients at 1–11 Mbps, excessive clients per AP, and upstream wired bottlenecks.

WLC/AP modes: local (centralized) for normal service, monitor for IDS/scanning, sniffer for packet capture, Rogue Detector, and FlexConnect for local switching at branch sites.

WEP uses RC4 with 24-bit IVs and is broken; WPA used TKIP with per-packet key mixing; WPA2 requires CCMP/AES; WPA3 uses SAE (Simultaneous Authentication of Equals) to resist offline dictionary attacks.

802.1X/EAP: supplicant, authenticator (AP/WLC), and authentication server (RADIUS). EAP-TLS uses certificates on both sides; PEAP and EAP-TTLS use a server certificate plus inner MS-CHAPv2 credentials.

WPA2/WPA3-Enterprise requires a RADIUS server; WPA2/WPA3-Personal uses a pre-shared key. WPA3-Personal replaces the PSK handshake with SAE and adds forward secrecy.

Rogue AP detection: authorized APs are known by BSSID; a rogue is an unauthorized AP on the wired network, while a neighbor AP is simply nearby. Use WIPS, monitor mode, and switch port security.

Site survey types: predictive (software modeling on floor plans), passive (walk-around listening to existing APs with a survey tool), and active (associating and measuring real throughput/coverage).

Survey metrics: target RSSI around −67 dBm for voice/video, about −72 dBm for data, SNR of 25 dB or better, and overlapping cells of 15–20% for seamless roaming.

Channel planning: reuse 1/6/11 in 2.4 GHz, prefer 5 GHz for capacity, set 20 MHz in dense areas, avoid DFS channels where radar is present, and lower transmit power to shrink cells.

Validate after deployment: post-installation survey, verify coverage in all areas, confirm roaming boundaries, check for co-channel interference, and document AP locations, channels, and power settings.

Wavelength: λ (m) = 300 / f (MHz). 2.4 GHz ≈ 12.5 cm, 5 GHz ≈ 6 cm, 6 GHz ≈ 5 cm; shorter wavelength means smaller Fresnel zone and more attenuation through obstacles.

Free-space path loss (dB) = 32.44 + 20log10(f in MHz) + 20log10(d in km); doubling distance adds about 6 dB of loss, and doubling frequency also adds about 6 dB.

Fresnel zone radius at midpoint r (m) ≈ 17.32 × √(d1·d2 / (f·D)) with distances in km and f in GHz; keep at least 60% of the first zone clear of obstructions.

Link budget: received power (dBm) = transmit power (dBm) + transmit antenna gain − cable/connector loss + receive antenna gain − path loss. EIRP = TX power + antenna gain − cable loss.

dB rules of thumb: +3 dB = 2× power, +10 dB = 10× power, +6 dB = 2× distance in free space; +3 dBi doubles antenna gain relative to isotropic.

Attenuation by material at 2.4/5 GHz: drywall ~3 dB, wood door ~3 dB, brick wall ~6–10 dB, concrete ~12–15 dB, metal mesh or foil-backed insulation blocks nearly all signal.

Multipath causes fading, nulls, and delay spread; OFDM (used by 802.11a/g/n/ac/ax) tolerates delay spread via cyclic prefix and subcarriers, while DSSS spreads energy across a wide band.

Noise floor rises with channel width: 20 MHz ≈ −95 dBm, 40 MHz ≈ −92 dBm, 80 MHz ≈ −89 dBm (typical 5 GHz reference); SNR (dB) = signal (dBm) − noise floor (dBm).

2.4 GHz has only three non-overlapping 20 MHz channels (1, 6, 11 in North America); 5 GHz offers many 20 MHz channels, and 6 GHz adds up to 59 × 20 MHz channels with no legacy overlap.

802.3af PoE supplies up to 15.4 W at the PSE (12.95 W at the PD); 802.3at (PoE+) up to 30 W (25.5 W at PD); 802.3bt Type 3 up to 60 W and Type 4 up to 90 W at the PSE.

PoE voltage is nominally 44–57 V DC; check the AP's rated draw against the switch port budget, and remember 2-pair vs 4-pair operation affects available wattage on bt.

Cabling: Cat5e supports 1 GbE to 100 m; Cat6/6A support 10GBASE-T to 55 m/100 m. Solid-core cable for permanent runs, stranded for patch cords; T568A or T568B consistently on both ends.

Antenna types: omnidirectional (dipole) radiates in a donut pattern with ~2–5 dBi; patch/panel is directional with ~6–14 dBi; Yagi and parabolic grid give high gain for long point-to-point links.

Antenna gain trades coverage area for distance: higher dBi narrows the beamwidth. EIRP = radio power (dBm) + antenna gain (dBi) − cable loss (dB); regulatory EIRP limits vary by band and country.

Mount APs at planned locations, below ceiling obstacles, away from metal ductwork, elevator shafts, and fluorescent/LED drivers; keep antennas vertical and consistent for predictable coverage.

Survey kit: site survey software, calibrated adapter, spectrum analyzer, floor plans, laser distance meter, and a PoE injector for testing AP placement before permanent mounting.

Cable test after installation: length, wire map, insertion loss, and near-end crosstalk; a PoE tester confirms the PD receives correct class and voltage before the AP is mounted.

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

Beacon interval default 100 TU (102.4 ms); DTIM period controls how often buffered multicast/broadcast is announced - DTIM 1 wakes clients every beacon, saving less battery but improving latency.

CSMA/CA with DCF: station senses the medium, waits DIFS, backs off a random slot count, transmits, and waits for ACK; RTS/CTS mitigates the hidden-node problem.

802.11n/ac/ax features: MCS index sets modulation and coding, channel width (20/40/80/160 MHz), spatial streams (MIMO), and guard interval (400/800 ns). Higher MCS needs better SNR.

Roaming: clients decide when to roam; 802.11k provides neighbor reports, 802.11v sends BSS transition management frames, and 802.11r enables fast BSS transition (FT) for quicker reauthentication.

Troubleshooting order: verify physical link and PoE, confirm channel and power, check for co-channel and adjacent-channel interference, then inspect client association, DHCP, and authentication logs.

Common causes of low throughput: retries from interference, low SNR, channel width mismatch, legacy 802.11b clients at 1–11 Mbps, excessive clients per AP, and upstream wired bottlenecks.

WLC/AP modes: local (centralized) for normal service, monitor for IDS/scanning, sniffer for packet capture, Rogue Detector, and FlexConnect for local switching at branch sites.

WEP uses RC4 with 24-bit IVs and is broken; WPA used TKIP with per-packet key mixing; WPA2 requires CCMP/AES; WPA3 uses SAE (Simultaneous Authentication of Equals) to resist offline dictionary attacks.

802.1X/EAP: supplicant, authenticator (AP/WLC), and authentication server (RADIUS). EAP-TLS uses certificates on both sides; PEAP and EAP-TTLS use a server certificate plus inner MS-CHAPv2 credentials.

WPA2/WPA3-Enterprise requires a RADIUS server; WPA2/WPA3-Personal uses a pre-shared key. WPA3-Personal replaces the PSK handshake with SAE and adds forward secrecy.

Rogue AP detection: authorized APs are known by BSSID; a rogue is an unauthorized AP on the wired network, while a neighbor AP is simply nearby. Use WIPS, monitor mode, and switch port security.

Site survey types: predictive (software modeling on floor plans), passive (walk-around listening to existing APs with a survey tool), and active (associating and measuring real throughput/coverage).

Survey metrics: target RSSI around −67 dBm for voice/video, about −72 dBm for data, SNR of 25 dB or better, and overlapping cells of 15–20% for seamless roaming.

Channel planning: reuse 1/6/11 in 2.4 GHz, prefer 5 GHz for capacity, set 20 MHz in dense areas, avoid DFS channels where radar is present, and lower transmit power to shrink cells.

Validate after deployment: post-installation survey, verify coverage in all areas, confirm roaming boundaries, check for co-channel interference, and document AP locations, channels, and power settings.