WBCHSE HS Vocational Telecom flashcards

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

Ohm's law

The current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, expressed as V = I × R where V is volts, I amperes and R ohms.

Capacitive reactance

Opposition a capacitor offers to alternating current, given by Xc = 1/(2πfC). It decreases as frequency or capacitance increases, so a capacitor blocks DC and passes high frequencies.

Decibel

A logarithmic ratio used to express gain or loss. For power, dB = 10 log10(P2/P1); for voltage, dB = 20 log10(V2/V1). A 3 dB change is roughly double or half.

Nyquist sampling criterion

To reconstruct a signal without aliasing, it must be sampled at more than twice its highest frequency component. Voice limited to 4 kHz is therefore sampled at 8000 samples per second.

Multiplexing

Sharing one transmission medium among several signals. Frequency division assigns separate bands, time division assigns recurring slots, and wavelength division assigns separate optical wavelengths.

Cat6 cable limit

Category 6 twisted pair has a 250 MHz bandwidth rating and supports 10 Gbps Ethernet up to 55 metres, or 1 Gbps up to the full 100 metre channel length.

Single-mode fibre

Optical fibre with a core of about 8 to 10 micrometres that carries only one propagation mode, giving very low attenuation and dispersion for long-distance and high-capacity links.

Numerical aperture

A measure of the light-gathering cone of an optical fibre, calculated as the square root of n1 squared minus n2 squared, where n1 is core and n2 cladding index.

Local loop

The pair of copper wires connecting a subscriber's telephone to the local exchange. It carries DC loop current for the set, the speech signal, and dialling and ringing signals.

DTMF signalling

Dual-tone multifrequency dialling sends two simultaneous tones, one from a low band and one from a high band, for each key, giving sixteen distinct key combinations.

Erlang

A unit of telephone traffic equal to one circuit occupied continuously for one hour. Traffic intensity equals the number of calls multiplied by mean holding time, divided by the observation period.

Blocking probability

The chance that a call attempt finds no free path and is rejected. Erlang B estimates it for loss systems where calls are not queued and simply fail.

PCM voice rate

Standard G.711 digital speech uses 8000 samples per second with 8 bits per sample, producing a 64 kbps channel, the basic building block of digital exchange switching.

Frequency reuse

The cellular principle of assigning the same radio channels to cells far enough apart that interference stays acceptable. Reuse distance increases with the square root of cluster size.

Handover

The process of transferring an active mobile call from one cell or base station to another as the subscriber moves, without dropping the connection or interrupting speech noticeably.

OFDMA

Orthogonal frequency division multiple access divides a wide channel into many narrow subcarriers and allocates groups of them to different users, the basis of 4G LTE downlink.

OSI model

A seven-layer reference model: physical, data link, network, transport, session, presentation and application. Each layer provides defined services to the layer above it.

Private IP ranges

Address blocks reserved for internal use and not routed on the internet: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They require network address translation to reach the internet.

TCP versus UDP

TCP is connection-oriented, acknowledges segments and retransmits lost data, suiting file transfer. UDP is connectionless with less overhead, suiting voice, video and DNS queries.

Subnet mask

A 32-bit value that marks which bits of an IPv4 address identify the network and which identify the host. A /24 mask leaves 254 usable host addresses.

Time-domain reflectometer

A test instrument that sends a pulse along a cable and times the reflection. Fault distance equals propagation velocity times round-trip time divided by two.

Earthing resistance

The resistance between an earth electrode and the general mass of earth, measured with an earth tester. Low values are needed so fault current trips protection and noise stays low.

Fusion splicing

Joining two optical fibres by melting their ends with an electric arc. It produces a permanent joint with very low loss, typically below 0.1 dB, and high mechanical strength.

Link loss budget

The sum of fibre attenuation, splice loss, connector loss and safety margin for a link. It must not exceed the difference between transmitter output and receiver sensitivity.

GPON

A passive optical network standard delivering 2.5 Gbps downstream at 1490 nm and 1.25 Gbps upstream at 1310 nm, sharing one feeder fibre through passive splitters.

Residual current device

A protective device that monitors the difference between live and neutral current and trips when earth leakage exceeds its rating, commonly 30 mA for personal protection.

Lockout and tagout

A safety procedure in which the supply is isolated, locked and tagged so it cannot be re-energised, then proved dead before work begins on the equipment.

Break-even quantity

The sales volume at which total revenue equals total cost, calculated as fixed costs divided by the difference between selling price and variable cost per unit.

Service-level agreement

A written contract between provider and customer specifying measurable service targets such as response time, restoration time and availability, plus remedies when targets are missed.

T568B wiring order

Pin order for RJ-45: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown. Using the same order at both ends makes a straight-through cable.

Meter connection rules

A voltmeter connects in parallel with the component and a high range is selected first. An ammeter connects in series and must never be placed directly across a supply.

Project report structure

A practical project record contains aim, apparatus, circuit or block diagram, procedure, observation table, calculations, result, conclusion and any precautions or sources of error.