RBSE SS Voc Telecom cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. Units: volt, ampere, ohm, watt.

Series: R_total = R1 + R2 + ...; current is the same everywhere. Parallel: 1/R_total = 1/R1 + 1/R2 + ...; voltage is the same across branches.

DC vs AC: DC has constant polarity (batteries, -48 V exchange plant); AC reverses periodically, described by frequency (Hz), peak and RMS values. For a sine wave, V_rms = V_peak / √2 ≈ 0.707 × V_peak.

Telecom DC plant: exchange battery nominal -48 V DC (float ~53.5 V, end-of-discharge ~43.5 V); positive terminal is earthed so the -48 V rail is the 'live' feed.

Capacitor impedance X_C = 1/(2πfC) - falls as frequency rises, so capacitors block DC and pass AC (coupling, decoupling, filter).

Inductor impedance X_L = 2πfL - rises with frequency, so inductors pass DC and oppose AC (chokes in power feeds).

Semiconductors: diode conducts one way (rectification, protection); transistor as switch/amplifier (BJT: base current controls collector current; FET: gate voltage controls channel).

Signal basics: decibel dB = 10 log10(P1/P2) for power, 20 log10(V1/V2) for voltage; +3 dB ≈ double power, +10 dB = ten times power, -3 dB is half power.

Voice band: 300–3400 Hz, band-limited to 4 kHz before sampling. Nyquist: sampling rate ≥ 2 × highest frequency, so PCM uses 8000 samples/s.

PCM: 8-bit companded (A-law/µ-law) samples at 8000/s = 64 kbit/s per voice channel (DS0). 32 channels × 64 kbit/s = 2.048 Mbit/s E1 (30 voice + 2 signalling/framing).

Transmission media: twisted pair (balanced, twist cancels crosstalk; categories limit bandwidth), coaxial (shielded, wideband), optical fiber (immune to EMI, huge bandwidth), radio (line-of-sight/antenna).

Attenuation is loss over distance (dB/km); distortion changes shape; noise (thermal, crosstalk, impulse) sets the error floor. Repeaters/regenerators restore level or bits.

Two-wire vs four-wire: subscriber loop is 2-wire; trunk networks are 4-wire (separate transmit/receive), so hybrids convert and can cause echo.

Signalling: off-hook/loop current, dial tone, DTMF (dual-tone multi-frequency, two tones per digit from low/high groups), ringing ~25 Hz/75 V, busy tone.

Crosstalk: near-end (NEXT) measured at the transmitting end, far-end (FEXT) at the receiving end; twisting and shielding reduce it.

PSTN hierarchy: subscriber → local exchange (LE) → tandem/trunk exchange → toll/transit; international gateway for cross-border calls.

Switching functions: concentration, switching (space or time division), and expansion; stored-program-controlled (SPC) digital exchanges use time-slot interchange (TSI).

Numbering plan (India example): STD code + subscriber number; total significant digits fixed per area; mobile numbers are 10 digits. Numbering must be unambiguous and routable.

Routing: a call is routed on the destination number's prefix; routing tables map prefixes to trunks. Alternate routing provides a second path when the primary trunk group is busy.

Call setup phases: seizure → addressing (digits) → routing → alerting (ringing) → answer → conversation → release (clear-forward/clear-back).

Grade of Service (GoS): probability that a call is blocked at busy hour; Erlang B formula relates offered traffic (in erlangs) and number of trunks to blocking.

Signalling systems: in-channel (CAS, e.g. R2) vs common-channel (CCS7/SS7) with separate signalling links; SS7 enables intelligent network services and fast setup.

OSI seven layers: physical, data link, network, transport, session, presentation, application. TCP/IP model: network access, internet, transport, application.

IP addressing: IPv4 is 32-bit dotted decimal; IPv6 is 128-bit hexadecimal. Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Subnet mask separates network and host bits.

TCP is connection-oriented and reliable (sequencing, ACK, retransmission, flow control); UDP is connectionless with no delivery guarantee (used for voice/video, DNS).

Broadband access: ADSL/VDSL over copper (frequency-split with voice), cable (DOCSIS), FTTH (GPON: 2.488 Gbit/s down / 1.244 Gbit/s up), fixed wireless, mobile data.

Key metrics: bandwidth (capacity), throughput (achieved rate), latency (delay, RTT), jitter (variation), packet loss. Broadband commonly defined as always-on, higher-than-dial-up speed.

DNS resolves names to IP addresses; DHCP assigns IP configuration automatically; NAT translates private addresses to a public address.

Ethernet (IEEE 802.3) uses MAC addresses and CSMA/CD historically; modern switched full-duplex Ethernet has no collisions. VLANs segment a LAN logically.

Cellular concept: geographic area split into cells with frequency reuse; a cluster of N cells uses distinct frequency sets, and reuse distance depends on N (hexagonal geometry).

Handover transfers a call between cells; roaming lets a subscriber use another operator's network; the HLR/VLR store subscription and location data.

Generations: 2G GSM (TDMA, 900/1800 MHz), 3G UMTS (WCDMA), 4G LTE (OFDMA, all-IP), 5G NR (sub-6 GHz and mmWave, higher bandwidth, lower latency).

Antenna basics: gain (dBi/dBd, dBd = dBi − 2.15), beamwidth, polarization, VSWR (good if < 1.5:1). Directional antennas focus energy; omni antennas radiate 360°.

RF propagation: free-space path loss increases with frequency and distance; losses from buildings, terrain and rain (especially above ~10 GHz). Fresnel zone clearance matters for line-of-sight links.

Frequency bands and duplexing: FDD uses separate uplink/downlink frequencies; TDD shares one band with time slots. Guard bands prevent adjacent-channel interference.

Cell site elements: BTS/eNodeB/gNodeB, antenna feeder (coax or fiber-to-antenna), tower/mast, power and backhaul. Sectorization (3×120°) increases capacity.

Fiber structure: core (glass, carries light), cladding (lower refractive index, total internal reflection), coating/primary buffer, strength members, outer sheath.

Single-mode fiber: ~9 µm core, 1310/1550 nm windows, long haul, low loss (~0.2 dB/km at 1550 nm). Multimode: 50 or 62.5 µm core, 850/1300 nm, short reach, modal dispersion limits distance.

Splicing: fusion splicing (arc welds fibers, lowest loss) vs mechanical splice. Cleave angle and cleanliness determine splice quality; typical fusion splice loss < 0.1 dB.

Connectors: SC, LC, ST, FC; APC (angled, 8°) has lower back-reflection than UPC/PC. Always clean before mating; use one-time wipes.

Testing: OTDR measures loss vs distance and locates faults/splices (launch and receive dead zones); power meter + light source measures end-to-end loss; VFL finds breaks.

Copper cabling: UTP categories (Cat5e 100 MHz, Cat6 250 MHz, Cat6A 500 MHz), 100-ohm impedance; punch-down on IDC blocks; test for continuity, length, wire map, NEXT.

Bend radius and handling: never exceed manufacturer's minimum bend radius; protect fiber from tight bends (macrobending loss) and from tensile stress; keep dust caps on unused ports.

Exchange/rack practice: 19-inch or 21-inch racks, ESD wrist strap when handling boards, cable routing separated (power vs signal), labelled cables at both ends.

Power plant: rectifiers convert AC to -48 V DC, battery bank floats on the bus, load distribution via MCBs/fuses; verify polarity before connecting equipment.

Earthing: separate protective earth and telecom earth bars; low earth resistance (commonly ≤ 5 ohm for telecom earth) protects people and equipment.

Installation sequence: site survey → cable/rack plan → mechanical fixing → cabling and termination → power-up and commissioning tests → as-built documentation.

Preventive maintenance: check battery specific gravity/voltage, tighten terminals, clean filters, inspect antenna VSWR and feeder, verify alarm operation, log readings.

Fault alarms: fuse/MCB trip, rectifier fail, battery low voltage, high temperature, door-open, mains fail; each maps to a documented response.

Test equipment: multimeter, earth tester, insulation resistance tester (megger), OTDR, optical power meter, cable analyzer, RF power meter, spectrum analyzer.

CPE examples: telephone instrument, ONT/ONU, ADSL router, set-top box, IP phone, Wi-Fi access point. CPE is the operator's demarcation boundary; beyond it is customer wiring.

Telephone instrument checks: handset cord, dial tone, DTMF keys, ringer, transmitter/receiver - swap handset or instrument to isolate.

ADSL/VDSL faults: check line sync LED, SNR margin and attenuation on the modem page; low SNR/high attenuation indicates loop problems; filter/splitter must be fitted on every extension.

FTTH CPE: ONT optical power must be within spec (e.g. -8 to -27 dBm receive); LOS LED red = no light → check patch cord, connector, splitter port, fiber break.

Wi-Fi issues: channel congestion, weak signal, wrong password, DHCP failure; verify with a client device, change channel, check router WAN status.

Structured diagnosis: verify power → check physical connections → read status LEDs/logs → test with known-good spare → escalate with measured data.

No dial tone flow: test at NTP/demarcation; if dial tone exists at demarcation, fault is in customer wiring/CPE; if not, fault is in the network.

Electrical safety: treat circuits as live until proved dead; use lock-out/tag-out, insulated tools, rubber gloves; -48 V DC can still cause burns and arcing.

Work at height: use full-body harness with double lanyard, inspect ladders/scaffolds, never work alone on a tower, maintain 3-point contact, check weather and lightning.

RF hazard: do not stand in front of an active antenna; respect exclusion zones; power down transmitters before working on antennas; RF exposure limits vary with frequency.

Fiber safety: never look into a fiber or connector end - laser light is invisible and can damage the eye; use an OTDR/VFL only as instructed; dispose of fiber scraps (splinters) in a sharps container.

Chemical and battery safety: lead-acid batteries release hydrogen (no flames/sparks), electrolyte causes burns (eye wash, acid-resistant PPE); handle solvent/MSDS per label.

Environmental practice: e-waste (boards, batteries, cables) goes to authorized recyclers; do not burn cable or dump fiber scrap; spill kits for electrolyte and fuel.

Emergency response: know first aid/CPR, emergency numbers, and the site's emergency plan; report every incident and near-miss.

Telecom sector segments: network operators (mobile/fixed), infrastructure/tower companies, ISPs, equipment vendors, cable/contracting firms, OTT and system integrators.

Regulation (India): DoT (policy/licensing), TRAI (tariffs and interconnection), TEC (equipment certification); ISP/licence categories define what a business may offer.

Business basics: fixed cost vs variable cost, break-even point = fixed cost / (price − variable cost per unit), cash flow, margin, GST invoicing.

Small telecom business ideas: cabling contractor, FTTH last-mile partner, CPE sales/service, Wi-Fi hotspot provider, telecom test-equipment rental, AMC services.

Employability skills: technical (installation, testing, fault finding), customer handling, documentation, teamwork, safety compliance, basic English and local-language communication.

Career paths: technician → team lead/supervisor → project/field manager; or NOC/transmission/optical specialist; certifications (fiber splicing, tower climbing, CCNA) improve prospects.

Emerging trends: 5G/FWA, FTTH expansion, IoT/M2M, cloud and data centers, cybersecurity, e-waste compliance - each creates service and employment demand.