WBCHSE HS Vocational Telecom common mistakes
WBCHSE HS Vocational Telecom · Understand the traps and how to avoid them.
When asked to compute power, resistance, or current in a series/parallel telecom circuit, the candidate plugs numbers into a memorised formula without first writing the circuit's equivalent resistance, so series and parallel branches are mixed up and the final value is off by a factor of the branch count.
Basic Electricity is taught early and often revised only as isolated formulas (V=IR, P=VI, series add, parallel reciprocals). Candidates practise formula recall rather than redrawing the network, and under exam pressure they grab the first formula that mentions the given quantities.
What to do instead
Adopt a fixed four-step routine for every circuit item: redraw and label the network, reduce it to one equivalent resistance branch by branch, compute total current from the supply voltage, then work back to the asked branch quantity. Do ten mixed series-parallel drills under timing, and always state the equivalent resistance in the answer even when not asked.
The candidate describes a signal only by its frequency or only by its amplitude, and cannot state how bandwidth, bit rate, and channel capacity relate, so questions asking why a wider bandwidth carries more data or why a higher carrier frequency suits a given service get vague, circular answers.
Signals are learned as vocabulary (frequency, wavelength, amplitude, modulation) without the linking relationships, and the candidate has never converted between period and frequency or between bandwidth and symbol rate by hand.
What to do instead
Build a one-page relationship sheet: period = 1/frequency, bandwidth as the range between upper and lower frequencies, and Shannon/Hartley-style capacity statements in words. Practise converting kHz/MHz/GHz to Hz and microseconds to seconds until unit slips disappear, then answer three 'why does this service need this bandwidth' questions in full sentences.
In cabling questions the candidate recommends or connects the wrong medium for the environment, for example specifying unshielded twisted pair for a long noisy industrial run, or joining copper pairs without regard to gauge, or confusing the pinout of an RJ-45 straight-through with a crossover lead.
Media are memorised as a list (twisted pair, coaxial, fibre) with a single attribute each, so selection becomes name-matching rather than matching distance, bandwidth, interference, and cost to the requirement.
What to do instead
Make a comparison table with columns for maximum distance, bandwidth, immunity to interference, cost, and typical use, and fill it for twisted pair categories, coaxial, and multimode/single-mode fibre. Then practise five scenario items where you must justify the choice in one sentence citing distance and interference, and redraw T568A and T568B pinouts from memory.
The candidate cannot trace a call path through a telephone exchange, so questions on subscriber line to local exchange, trunk between exchanges, and the role of the switching stage are answered by describing a mobile call or an internet call instead of the fixed-line sequence.
Telephone systems and switching are studied as separate chapters, and the candidate has never drawn the end-to-end path, so the terms subscriber loop, trunk, and exchange stay as unconnected labels.
What to do instead
Draw the full path on one sheet: telephone instrument, subscriber loop, line circuit, switching network, trunk, distant exchange, called subscriber, and annotate what each stage does (signalling, switching, supervision, ringing). Then practise labelling the same diagram for a local call, a trunk call, and a call to a private branch exchange.
On switching questions the candidate confuses circuit switching with packet switching and asserts that packet switching guarantees a fixed path or that circuit switching needs no call setup, and cannot say what a stored-program control exchange or a time-division switch actually does.
The two switching philosophies are memorised as two definitions with no operational picture, so under exam wording that asks for a comparison the candidate swaps their properties.
What to do instead
Write a side-by-side comparison covering path reservation, setup delay, suitability for voice versus bursty data, and efficiency, and give one real example of each. For exchange technology, explain in your own words how stored-program control separates control from switching and how a time-slot interchange moves a sample from one slot to another.
The candidate answers cellular questions by treating every cell as an independent system, missing frequency reuse, handover, and the role of the base station controller, and gives handover as 'the phone changes tower' without saying what triggers it or what happens to the call.
Mobile communication is learned as a list of generations (2G, 3G, 4G, 5G) and acronyms, without the cellular architecture diagram, so reuse distance, co-channel interference, and handover have no place to attach.
What to do instead
Draw a cluster of cells with a reuse pattern, mark co-channel cells, and explain why reuse multiplies capacity while creating interference. Then write the handover sequence in order: measurement report, decision by the network, resource allocation in the target cell, and release of the old link, and name the network element that makes the decision.
When asked about addressing or the path of data, the candidate mixes up the OSI layer at which a device operates, saying a switch routes between networks or a router forwards by MAC address, and cannot say which address type is used at which layer.
The layered model and the device list are memorised separately, so the mapping between layer, protocol data unit, address, and device is never practised as a single link.
What to do instead
Build one grid with rows for each layer and columns for PDU name, addressing used, and example devices and protocols. Fill it from memory weekly. Then answer 'which device and which address' for five scenarios: same-subnet delivery, inter-network delivery, broadcast domain, collision domain, and port-to-port forwarding.
During practical or viva work the candidate reports a measurement without stating the instrument range, the expected value, or the safety step taken, and skips continuity and insulation checks before energising a circuit, so the recorded result cannot be defended.
Practical sessions are often watched rather than rehearsed aloud, so the candidate performs steps silently and has never had to narrate the procedure, and safety checks are treated as formalities rather than part of the measurement record.
What to do instead
Rehearse a spoken procedure for each core task: state the instrument and range, the expected reading, the safety check performed, the reading obtained, and the conclusion. Keep a lab notebook with a column for expected versus measured values and a signed safety-check line, and practise the routine until it is automatic.
On optical fibre and broadband items the candidate confuses attenuation with dispersion, quotes a single fibre type for all distances, and cannot explain what a splitter, optical network unit, or digital subscriber line access multiplexer does in the access network.
Fibre is taught with heavy emphasis on drawing the structure of the cable, leaving loss budget, dispersion, and access-network architecture as names without function, and the candidate has never computed a simple power budget.
What to do instead
Compute three link budgets by hand: transmitter power minus connector and splice losses minus fibre attenuation times length, compared with receiver sensitivity. Then draw a passive optical network from exchange to subscriber, labelling optical line terminal, splitter, and optical network unit, and state one sentence on the function of each.
In occupational safety, entrepreneurship, and customer-service questions the candidate gives generic moral statements instead of the specific control, document, or communication step, for example answering an electrical hazard question with 'be careful' rather than isolation, locking, testing, and permit procedures, or answering a customer complaint with 'be polite' rather than an acknowledgement, fact-finding, and resolution sequence.
These domains are assumed to be common sense and are revised last, so answers are improvised from general attitude rather than from the hazard-control hierarchy, statutory safety provisions, or a service-recovery procedure.
What to do instead
Prepare short structured templates: for hazards, eliminate-substitute-engineer-administer-protect with a telecom example at each level plus isolation and lock-out steps; for enterprise, a cost, market, and finance-requirement outline for a small telecom service; for service, greet-acknowledge-clarify-resolve-confirm. Answer past items using the template headings so the response names concrete actions and documents.