WBCHSE HS Vocational Telecom cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In a series circuit current is common and voltages add; in a parallel circuit voltage is common and currents add.

Resistors in series add directly (R = R1 + R2); in parallel 1/R = 1/R1 + 1/R2, so two equal resistors in parallel give half the value.

A capacitor stores charge Q = C × V and blocks DC while passing AC; reactance Xc = 1/(2πfC) falls as frequency rises.

An inductor opposes change of current and passes DC while opposing AC; reactance Xl = 2πfL rises with frequency.

Impedance Z combines resistance and reactance: Z = √(R² + (Xl − Xc)²); at resonance Xl = Xc and Z is purely resistive.

Kirchhoff's laws: the sum of currents at a node is zero (KCL) and the sum of voltages around a closed loop is zero (KVL).

A diode conducts when forward biased (anode positive) above about 0.7 V for silicon and blocks reverse current; a Zener diode regulates by operating in reverse breakdown.

A transistor has three terminals (base, emitter, collector for BJT); a small base current controls a larger collector current, giving current gain hFE = Ic/Ib.

A transformer changes AC voltage by the turns ratio Vs/Vp = Ns/Np; power is approximately conserved, so a step-up in voltage steps down current.

A signal's bandwidth is the range of frequencies it occupies; channel capacity is bounded by the Shannon limit C = B log2(1 + S/N).

Decibel definitions: power ratio in dB = 10 log10(P2/P1); voltage or current ratio = 20 log10(V2/V1); +3 dB is about double power, −3 dB about half.

dBm is an absolute power level referred to 1 mW: P(dBm) = 10 log10(P/1 mW), so 0 dBm = 1 mW and 30 dBm = 1 W.

Sampling must satisfy the Nyquist criterion: sample at more than twice the highest signal frequency to avoid aliasing; the bit rate of PCM is samples/s × bits/sample.

Modulation types: AM varies amplitude, FM varies frequency, PM varies phase; digital schemes include ASK, FSK, PSK and QAM.

Multiplexing: FDM shares a medium by frequency slots, TDM by time slots, WDM by optical wavelengths, and CDMA by orthogonal codes.

Noise figure expresses degradation of signal-to-noise ratio through a stage; a lower noise figure in the first amplifier improves overall receiver sensitivity.

Analog signals vary continuously; digital signals take discrete levels, which allows regeneration without accumulating noise and enables error correction.

Twisted pair categories: Cat5e supports 100 MHz and 1 Gbps to 100 m; Cat6 supports 250 MHz and 10 Gbps to 55 m; Cat6A supports 500 MHz and 10 Gbps to 100 m.

UTP uses unshielded twisted pairs; STP/FTP adds a shield that must be grounded at one end to avoid ground loops.

Characteristic impedance is 100 Ω for Cat5e/6, 120 Ω for typical RS-485, and 50 Ω for common coaxial and RF systems; mismatches cause reflections.

Coaxial cable has a centre conductor, dielectric, braid shield and jacket; it carries higher bandwidth than twisted pair over short runs but is bulkier.

Single-mode fibre has a core of about 8–10 µm and carries long-haul traffic; multimode fibre has a core of 50 or 62.5 µm and suits short links.

Attenuation is measured in dB per km and rises with frequency for copper; fibre attenuation is lowest at 1310 nm and 1550 nm windows.

Crosstalk (NEXT/FEXT) is coupling between pairs; separation, twist rate and shielding reduce it, and it is tested during cable certification.

Cable bend radius, pulling tension and separation from power cables must be respected; kinked copper changes impedance and degrades performance.

The local loop connects the subscriber to the exchange and carries DC loop current (typically around 20–40 mA) plus speech and signalling.

Plain old telephone service uses −48 V DC nominal battery at the exchange; the ring signal is a high AC voltage (commonly around 75–90 V rms) at 20–25 Hz.

Dual-tone multifrequency (DTMF) dialling sends a pair of tones, one low-band and one high-band, for each key, giving 16 combinations.

Pulse or loop-disconnect dialling sends a train of breaks at about 10 pulses per second; DTMF is faster and is now standard.

The telephone handset contains a carbon or electret microphone and a receiver; a hybrid circuit converts two-wire local loop to four-wire transmission.

A private branch exchange (PBX) switches calls inside an organisation and connects them to the public network via trunks.

Signalling is either in-band (within the speech channel) or common-channel (separate path); SS7 is a common-channel signalling system.

Caller identification, call waiting and call forwarding are supplementary services implemented by the exchange, not by the handset itself.

A switch builds a temporary path between an inlet and an outlet; blocking occurs when no path is free, and a non-blocking switch always finds one.

Space-division switching uses physical crosspoints; time-division switching interchanges time slots in a memory, so many calls share one path.

A Strowger step-by-step exchange uses electromechanical selectors; crossbar and stored-program digital exchanges replaced it.

In a digital exchange the codec converts analog speech to 64 kbps PCM (8000 samples/s × 8 bits), the standard G.711 rate.

Traffic intensity is measured in erlangs: A = (number of calls × mean holding time)/observation period; one erlang is one circuit busy continuously.

The Erlang B formula gives blocking probability for a loss system with no queuing; Erlang C applies where calls wait in a queue.

Grading and trunking principles: a graded multiple gives high-usage subscribers more outlets, reducing cost while keeping blocking acceptable.

A PBX or EPABX performs internal switching, least-cost routing and trunk management; the main distribution frame terminates external and internal cabling.

Cellular reuse divides an area into cells; a cluster of N cells uses all channels, and the reuse distance grows with √N so co-channel interference falls.

Handover transfers a call between cells or sectors; roaming lets a subscriber use a visited network through agreements and a home location register.

GSM uses 200 kHz carriers with TDMA (8 time slots) and a SIM for subscriber identity; the HLR and VLR store subscription and location data.

A base transceiver station serves one cell; the base station controller manages several BTSs and handles radio resource and handover decisions.

Path loss grows with distance and frequency; indoor penetration is worse at higher bands, which is why low bands give better rural coverage.

Multiple access: FDMA gives each user a frequency, TDMA a time slot, CDMA a code, and OFDMA allocates subcarriers, as in 4G LTE.

Antenna gain is directional: a higher-gain antenna concentrates energy in a narrower beam, trading coverage angle for range.

Wi-Fi (IEEE 802.11) uses the 2.4 GHz and 5 GHz bands; channels 1, 6 and 11 are the non-overlapping set in the 2.4 GHz band.

The OSI model has seven layers: physical, data link, network, transport, session, presentation, application; TCP/IP collapses these into four.

IPv4 addresses are 32 bits written in dotted decimal; private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 and are not routed on the internet.

A subnet mask separates network and host bits; /24 means 24 network bits and 254 usable hosts after removing network and broadcast addresses.

TCP is connection-oriented and reliable with acknowledgements and retransmission; UDP is connectionless with lower overhead, used for voice and video.

Ports identify services: 20/21 FTP, 22 SSH, 23 Telnet, 25 SMTP, 53 DNS, 80 HTTP, 110 POP3, 143 IMAP, 443 HTTPS.

A switch forwards frames by MAC address at layer 2; a router forwards packets by IP address at layer 3 and separates broadcast domains.

DNS resolves names to IP addresses; DHCP assigns addresses automatically using a four-step exchange (discover, offer, request, acknowledge).

Error detection: parity catches single-bit errors, checksum is weak, and CRC is the standard for frames because it detects burst errors well.

Before installation, verify site power (voltage, earthing resistance, UPS backup) and confirm the equipment's rated input and heat dissipation.

Earthing resistance for telecom installations is typically kept below about 1–5 Ω depending on the standard; a poor earth causes noise and shock risk.

Use a multimeter for voltage, continuity and resistance; use an earth tester for electrode resistance and a clamp meter for loop current.

A time-domain reflectometer locates faults in copper or fibre by sending a pulse and timing the reflection; distance = velocity × time / 2.

Optical power is measured in dBm with a power meter; a light source and power meter pair measures link loss against the loss budget.

Preventive maintenance includes cleaning connectors, checking battery specific gravity or voltage, tightening terminations and reviewing alarm logs.

Fault isolation follows a layered order: check power, then physical connection, then configuration, then the far end, before replacing hardware.

Documentation for handover should include as-built drawings, cable schedules, IP plans, test results and equipment serial numbers.

Total internal reflection occurs when light travels from a denser core to a less dense cladding at an angle greater than the critical angle.

Numerical aperture NA = √(n1² − n2²) and, for multimode step-index fibre, determines the acceptance cone of light.

Single-mode fibre (ITU-T G.652) has a core around 9 µm and low dispersion at 1310 nm; dispersion-shifted fibre moves the zero-dispersion point to 1550 nm.

Optical windows: 850 nm for short multimode links, 1310 nm for zero dispersion, 1550 nm for lowest attenuation (about 0.2 dB/km).

Splicing by fusion gives lower loss (often below 0.1 dB) than mechanical splices; connectors typically add 0.3–0.5 dB per mated pair.

A link loss budget sums fibre attenuation, splice loss, connector loss and margin; the result must be less than the transmitter-to-receiver margin.

FTTH architectures include point-to-point active Ethernet and PON, where a passive splitter (commonly 1:32 or 1:64) shares one feeder fibre.

GPON uses 2.5 Gbps downstream and 1.25 Gbps upstream with 1490 nm downstream and 1310 nm upstream wavelengths; video overlay uses 1550 nm.

Broadband access technologies include ADSL (over copper, asymmetric), VDSL2, cable DOCSIS, and fibre PON; xDSL speed falls as loop length rises.

Electric shock severity depends on current path and magnitude: about 1 mA is perceptible, 10–30 mA causes muscle contraction, and above 50 mA can cause ventricular fibrillation.

A residual current device trips at a small earth-leakage current (commonly 30 mA for personnel protection) and is the key protection against electric shock.

First aid for electric shock: isolate the supply before touching the casualty, then check breathing and circulation and call emergency services.

Never work alone on live circuits; use lockout/tagout, prove dead with a tester, and discharge capacitors before touching equipment.

Laser safety: never look into a fibre end or connector; use an optical power meter rather than the eye, and keep caps on unused ports.

Work at height requires fall protection, a trained person and inspection of ladders and harnesses before use.

Battery rooms need ventilation because lead-acid charging releases hydrogen; avoid sparks and use eye protection when handling electrolyte.

E-waste such as batteries, PCBs and cables must go to authorised recyclers; burning cable insulation releases toxic fumes.

Ergonomics and noise control: keep noise below about 85 dB(A) for an eight-hour shift and use lifting aids for heavy equipment.

A business plan covers the product or service, market and competitors, operations, team, financial projections and funding requirement.

Break-even quantity = fixed costs / (selling price − variable cost per unit); above that volume the business makes a profit.

Working capital is current assets minus current liabilities; a telecom contractor needs it to cover wages and materials before payment.

GST registration thresholds and invoicing rules depend on turnover and state; a tax invoice must show GSTIN, HSN/SAC code and tax breakup.

For a small enterprise, common funding sources include own savings, bank term loans, Mudra-type schemes and supplier credit.

Customer service basics: greet, listen without interrupting, confirm the problem, give a time commitment, and follow up until closure.

A service-level agreement defines response and restoration times; exceeding them should trigger escalation and a documented explanation.

Employability skills for telecom work include reading a wiring diagram, using test instruments, documenting work, teamwork and basic numeracy.

Upselling succeeds when the customer's actual need is met; a complaint resolved well often creates more loyalty than a trouble-free sale.

Before powering a circuit, inspect for shorts with a continuity test and verify polarity; never assume a supply is off.

Crimp RJ-45 plugs to T568B order: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown; keep the same order both ends for a straight cable.

Verify a LAN cable with a cable tester; a crossed-pair fault shows as mismatched LED sequence at the far end.

Solder joints should be shiny and concave; a dull, ball-shaped joint indicates cold soldering and should be reheated.

When measuring current, connect the meter in series; when measuring voltage, connect it in parallel, and select a range above the expected value.

For a project report, record aim, apparatus, circuit or block diagram, procedure, observations table, calculations, result and conclusion.

Repeat measurements and take the mean to reduce random error; note the instrument's least count as the limit of resolution.

Fibre handling: clean and inspect connectors before mating, respect minimum bend radius, and never exceed the specified pulling tension.