CWEng flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Free space path loss formula

FSPL in dB equals 32.44 plus 20 log10 of distance in kilometers plus 20 log10 of frequency in MHz; it models unobstructed loss only.

Three dB rule

Adding 3 dB doubles the power in milliwatts and removing 3 dB halves it; a 10 dB change is a tenfold change in power.

EIRP calculation

Effective isotropic radiated power equals radio transmit power in dBm plus antenna gain in dBi minus cable, connector, and lightning arrestor losses in dB.

Non-overlapping 2.4 GHz channels

Channels 1, 6, and 11 are the only three that do not overlap in 2.4 GHz, because each roughly 22 MHz channel spans about five 5 MHz channel numbers.

First Fresnel zone radius

Radius in meters equals 17.32 times the square root of distance in km divided by four times frequency in GHz; keeping 60 percent clear avoids diffraction loss.

Six dB rule

Doubling the distance between radios in free space adds approximately 6 dB of path loss; indoor environments with higher path-loss exponents add more.

Multipath

Reflected signal copies arrive at slightly different times, causing intersymbol interference and deep fades; OFDM and MIMO mitigate this using multiple subcarriers and antennas.

Voice grade RSSI target

Design to at least -67 dBm with 25 dB SNR at the cell edge for voice, while data-only designs typically accept -72 dBm or better.

Cell overlap for roaming

Adjacent access point coverage should overlap about 15-20 percent so a client can complete a roam before its signal falls below the usable threshold.

Predictive site survey

A simulation using floor plans and wall attenuation values to estimate access point placement and power; fast and inexpensive but requires on-site validation.

Passive site survey

The survey tool listens only, recording beacon RSSI and SNR from access points already on air to produce a coverage and interference map.

Active site survey

The survey adapter associates to each access point and measures throughput, packet loss, and retries at each location, reflecting real client behavior.

AP on a stick survey

An access point is temporarily mounted at planned locations and the area is walked to validate a predictive design and refine power and channel plans.

Wall attenuation values

Drywall attenuates about 3-4 dB, brick near 10 dB, concrete 12-15 dB, metal 20 dB or more, and tinted or low-E glass 20-35 dB.

Four-way handshake

In RSN the client and access point derive the pairwise transient key from the PMK, exchange nonces and a MIC, and receive the group key encrypted.

CCMP encryption

AES counter mode with CBC-MAC supplies confidentiality and integrity for WPA2; it is mandatory in RSN and replaced the deprecated TKIP cipher.

WPA2-PSK key derivation

A passphrase of 8 to 63 characters is stretched with PBKDF2-SHA1 using the SSID as salt and 4096 iterations to produce a 256-bit PMK.

WPA3 SAE

Simultaneous Authentication of Equals uses the Dragonfly exchange to provide forward secrecy and resistance to offline dictionary attacks on captured handshakes.

EAP-TLS

A certificate-based 802.1X method requiring certificates on both client and server; considered the strongest EAP type and immune to password theft.

Management frame protection

802.11w signs deauthentication, disassociation, and action frames with a message integrity check, blocking spoofed deauth denial-of-service attacks against clients.

SNR calculation

Signal-to-noise ratio equals received signal strength in dBm minus the noise floor in dBm; about 25 dB supports 64-QAM, and under 10 dB is unreliable.

Adjacent channel interference

Partially overlapping channels such as 1 and 2 share spectrum and interfere even though they are separate channel numbers; it can be worse than co-channel interference.

Sticky client

A device that stays on a distant access point at low data rates instead of roaming; remedies include minimum RSSI, band steering, and 802.11k neighbor reports.

Hidden node problem

Two clients that cannot hear each other but both reach the access point transmit at once and collide; RTS/CTS or smaller cells mitigate the effect.

High retry rate

When retransmissions exceed roughly 10-15 percent of frames, suspect low SNR, co-channel interference, excessive client load, or non-Wi-Fi noise.

Non-Wi-Fi interference sources

Microwave ovens, 2.4 GHz cordless phones, Bluetooth, Zigbee, and wireless cameras raise the noise floor in the ISM band and reduce SNR.

Beacon interval

Defaults to 102.4 ms; beacons advertise the SSID, supported rates, and security settings, and clients use them for discovery and roaming decisions.

WMM access categories

802.11e EDCA defines voice, video, best effort, and background queues with different AIFS, contention windows, and transmit opportunity limits on the same channel.

802.11r fast transition

Fast BSS Transition pre-establishes keys with target access points, shrinking the roam exchange to a few frames and preserving latency-sensitive traffic like voice.

802.11ax OFDMA

Orthogonal frequency-division multiple access splits a channel into resource units so several clients transmit within the same time slot instead of contending for the whole channel.

CAPWAP ports

Controller-to-access-point control traffic uses UDP 5246 and encapsulated data uses UDP 5247; both must be allowed through any intervening firewall.

6 GHz band capacity

The 5925-7125 MHz band adds 59 twenty-MHz channels in the US, requires WPA3 and protected management frames, and excludes legacy devices from the band.