CWT flashcards

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

Wavelength formula

Wavelength in meters equals 300 divided by frequency in MHz. At 2.4 GHz the wavelength is about 12.5 cm; at 5 GHz about 6 cm.

Free-space path loss

FSPL in dB equals 32.44 plus 20 log of frequency in MHz plus 20 log of distance in km. Each doubling of distance adds roughly 6 dB.

EIRP calculation

Effective isotropic radiated power equals transmitter power in dBm plus antenna gain in dBi minus cable and connector loss in dB. Regulations cap EIRP per band.

Fresnel zone clearance

The elliptical volume around a line-of-sight link where signal travels. At least 60 percent of the first zone should be unobstructed for reliable point-to-point links.

Decibel rules of thumb

Plus 3 dB doubles power, plus 10 dB multiplies power by ten, and plus 6 dB doubles free-space range. Minus 3 dB halves power.

Multipath and delay spread

Signals reflecting off surfaces arrive at different times, causing fading and intersymbol interference. OFDM counters this with a cyclic prefix and many subcarriers.

Signal-to-noise ratio

SNR in dB equals received signal in dBm minus the noise floor in dBm. Higher SNR supports higher MCS rates; about 25 dB is a common good target.

2.4 GHz channel reuse

Only three 20 MHz channels do not overlap in North America: 1, 6, and 11. Adjacent-channel overlap causes interference even when channels differ.

802.3af power budget

The original PoE standard delivers up to 15.4 W at the power sourcing equipment and guarantees 12.95 W at the powered device after cable loss.

802.3bt PoE

Type 3 delivers up to 60 W and Type 4 up to 90 W at the source using all four pairs. It powers access points, cameras, and high-draw devices.

Omnidirectional antenna pattern

A dipole radiates in a torus or donut shape, strongest perpendicular to the antenna axis. Gain is typically 2 to 5 dBi with wide horizontal coverage.

Directional antenna tradeoff

Higher gain concentrates energy into a narrower beam, extending range in one direction. Patch, Yagi, and grid antennas serve bridges and long corridors.

Cat6A 10GBASE-T distance

Cat6 supports 10 GbE to about 55 meters, while Cat6A reaches the full 100 meters. Both handle 1 GbE to 100 meters for access point uplinks.

T568A versus T568B

Two wiring pinouts for RJ45 jacks and plugs. Either is valid if used consistently on both ends of a cable; mixing them creates a crossover.

AP mounting best practice

Mount access points below ceilings and away from metal ducts, elevator shafts, and electrical drivers. Keep antennas vertical and consistent for predictable coverage.

PoE injector use

A midspan device adds power to an Ethernet run when the switch lacks PoE. Useful for testing AP placement before permanent cabling is installed.

Beacon interval

Management frames sent by the AP, default 100 time units or 102.4 ms, advertising the SSID, supported rates, and capabilities so clients can discover the network.

DTIM period

Indicates how many beacons pass before buffered multicast and broadcast traffic is announced. Lower values reduce latency; higher values save client battery.

CSMA/CA operation

Clients sense the medium, wait a DIFS interval, back off a random number of slots, then transmit and await an acknowledgment. Collisions are avoided, not detected.

RTS/CTS handshake

A request-to-send and clear-to-send exchange reserves the medium and mitigates the hidden-node problem, at the cost of extra overhead per frame.

MCS index

Defines modulation, coding rate, and number of spatial streams for 802.11n/ac/ax. Higher indexes need better SNR and deliver greater throughput.

802.11k neighbor reports

The AP supplies clients with a list of nearby access points and channels, helping them scan less and choose roam targets faster.

802.11r fast transition

Fast BSS transition pre-authenticates the client with the target AP so rekeying happens quickly. Reduces roaming delay for voice and video.

Hidden node problem

Two clients can each hear the AP but not each other, so they transmit simultaneously and collide. RTS/CTS or better AP placement resolves it.

WPA3 SAE

Simultaneous Authentication of Equals replaces the WPA2 four-way PSK handshake with a password-authenticated key exchange, resisting offline dictionary attacks and adding forward secrecy.

WPA2 CCMP encryption

WPA2 mandates AES with CCMP for confidentiality and integrity. It replaced the weaker TKIP used by WPA and the broken RC4-based WEP.

802.1X three roles

The supplicant is the client, the authenticator is the AP or controller, and the authentication server is RADIUS. EAP carries credentials between them.

EAP-TLS

A mutual certificate-based EAP method requiring certificates on both client and server. Strongest common enterprise method but requires a PKI.

PEAP inner authentication

PEAP creates a TLS tunnel using a server certificate, then runs an inner method such as MS-CHAPv2 for user credentials. Clients must validate the server certificate.

Rogue access point

An unauthorized AP connected to the corporate wired network. Distinct from a neighbor AP, which is merely nearby and not on the internal LAN.

Predictive site survey

Software models coverage on floor plans using wall attenuation values and AP specifications. Fast and useful before installation, but must be validated by an on-site survey.

Survey signal targets

Aim for about minus 67 dBm RSSI for voice and video, minus 72 dBm for data, SNR of 25 dB or better, and 15 to 20 percent cell overlap.