RBSE Sec Voc Telecom cheat sheet

Ohm's law V = IR; power P = VI = I²R = V²/R. In series, R_total = R1 + R2 + … and current is common; in parallel, 1/R_total = 1/R1 + 1/R2 + … and voltage is common.

Kirchhoff's laws: KCL states the algebraic sum of currents at a node is zero; KVL states the algebraic sum of voltages around a closed loop is zero.

Diode: forward biased when anode is positive relative to cathode (barrier ≈ 0.7 V for Si, 0.3 V for Ge); reverse biased it blocks until breakdown. A Zener diode in reverse breakdown regulates voltage.

Rectifiers: half-wave gives ripple frequency f; full-wave and bridge give 2f. Capacitor filter ripple V_r ≈ I_load /(f_ripple × C).

Transistor as amplifier: emitter junction forward biased, collector junction reverse biased for active mode; β = I_C/I_B and I_E = I_B + I_C.

AC fundamentals: RMS = peak/√2, average of full sine = 0.637 × peak, form factor = 1.11, peak factor = 1.414; f = 1/T.

Impedance: X_L = 2πfL, X_C = 1/(2πfC), Z = √(R² + (X_L − X_C)²); resonance occurs when X_L = X_C, giving f₀ = 1/(2π√(LC)).

Electrical units: ampere (current), volt (potential difference), ohm (resistance), watt (power), farad (capacitance), henry (inductance); 1 kWh = 3.6 × 10⁶ J.

AM: carrier amplitude varies with modulating signal. Modulation index m = V_m/V_c; total power P_t = P_c(1 + m²/2); bandwidth = 2 × highest modulating frequency.

FM: carrier frequency varies; modulation index β = Δf/f_m; Carson's rule gives bandwidth ≈ 2(Δf + f_m). FM has better noise immunity than AM but wider bandwidth.

DSB-SC and SSB suppress the carrier; SSB also removes one sideband, halving bandwidth and suiting voice links. VSB keeps a partial sideband for TV video.

Noise: thermal noise power N = kTB; noise figure NF(dB) = 10 log₁₀(F); SNR in dB = 10 log₁₀(P_signal/P_noise).

Sampling theorem: sample at f_s ≥ 2f_max to avoid aliasing; PCM bit rate = f_s × n where n = bits per sample.

Quantization: SNR_q(dB) ≈ 6.02n + 1.76 for an n-bit quantizer with full-scale sine input; each added bit improves SNR by about 6 dB.

Multiplexing: FDM splits the spectrum into frequency slots; TDM assigns time slots; WDM is optical-domain FDM; CDMA separates users by orthogonal codes.

Superheterodyne receiver: RF amplifier → mixer with local oscillator → IF amplifier (455 kHz for AM, 10.7 MHz for FM) → detector → audio amplifier.

Switching hierarchy in Indian PSTN: local exchange (LE) → secondary switching area (SSA) → primary centre → tertiary centre (TAX) → main automatic exchange (gateway).

Space-division switching uses a crosspoint matrix (N×N needs N² crosspoints); time-division switching interchanges time slots via memory, so the SPC exchange handles both voice and control.

Erlang: traffic A = (calls per hour × mean holding time in hours); Erlang B gives blocking probability for a loss system, Erlang C for a delay (queuing) system.

PCM E1 frame: 32 time slots of 8 bits at 8000 frames/s = 2.048 Mbit/s; TS0 carries framing/alignment, TS16 carries signalling, TS1–TS15 and TS17–TS31 carry voice.

Signalling: CAS uses the traffic channel or TS16 bits; CCS (SS7) uses a separate signalling link with MTP, SCCP, ISUP and TCAP layers.

Numbering: Indian mobile numbers are 10 digits; country code +91; NLD/ILD prefixes route national and international calls; STD codes identify SSAs.

Traffic formulas: A = C × T / 3600 when C is calls per hour and T is holding time in seconds; grade of service is the fraction of calls blocked.

Nyquist bit rate for a noiseless channel: C = 2B log₂M bit/s, where B is bandwidth and M is signal levels; Shannon capacity C = B log₂(1 + S/N).

Line coding: NRZ has DC content; Manchester encodes each bit with a mid-bit transition for clock recovery; AMI and HDB3 limit DC wander on PCM lines.

PCM chain: anti-alias filter → sampler → quantizer → encoder → line driver; at the receiver, regeneration, decoding and reconstruction filters follow.

Digital modulation: ASK varies amplitude, FSK varies frequency, PSK varies phase, QAM varies amplitude and phase (16-QAM = 4 bits/symbol, 64-QAM = 6 bits/symbol).

Bit error rate depends on E_b/N₀; coherent PSK outperforms non-coherent FSK by roughly 3 dB at the same BER.

Error control: parity detects single-bit errors; CRC detects bursts; Hamming code corrects single-bit errors with redundancy bits r where 2^r ≥ m + r + 1.

Scrambling and equalisation: scramblers avoid long runs of identical bits; equalisers compensate inter-symbol interference caused by channel dispersion.

OSI layers (1–7): physical, data link, network, transport, session, presentation, application. TCP/IP maps to link, internet, transport and application layers.

IPv4 header is 20 bytes minimum with a 4-bit version, 4-bit header length, 8-bit TTL, 8-bit protocol and 32-bit source/destination addresses; TTL prevents routing loops.

Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; loopback 127.0.0.0/8; APIPA 169.254.0.0/16. Subnet mask /24 gives 254 usable hosts.

TCP is connection-oriented with a three-way handshake (SYN, SYN-ACK, ACK) and sliding-window flow control; UDP is connectionless with 8-byte header and no retransmission.

Common ports: HTTP 80, HTTPS 443, FTP 20/21, SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67/68, POP3 110, IMAP 143.

Routing: static routes are manual; RIP uses hop count with a maximum of 15; OSPF is link-state with cost metric; BGP carries inter-domain routes between autonomous systems.

DHCP DORA sequence: Discover, Offer, Request, Acknowledge; DNS resolves names to addresses using A, AAAA, CNAME, MX and PTR records.

Total internal reflection occurs when light travels from a denser core (n₁) to a rarer cladding (n₂) and the incidence angle exceeds the critical angle θ_c = sin⁻¹(n₂/n₁).

Numerical aperture NA = √(n₁² − n₂²); acceptance angle θ_a = sin⁻¹(NA). Relative refractive index difference Δ = (n₁ − n₂)/n₁.

Fibre types: single-mode (core ≈ 8–10 µm, 1310/1550 nm, lowest dispersion), multimode step-index and graded-index (core 50/62.5 µm, 850/1300 nm).

Attenuation in dB = 10 log₁₀(P_in/P_out); typical windows are 850, 1300 and 1550 nm, with 1550 nm giving the lowest loss (about 0.2 dB/km).

Dispersion: modal dispersion dominates multimode fibre; chromatic dispersion (material plus waveguide) and polarisation mode dispersion limit single-mode links. Dispersion-shifted fibre moves zero dispersion to 1550 nm.

Link power budget: received power = transmitted power − (fibre loss × length) − connector losses − splice losses; the result must exceed receiver sensitivity by the margin.

OTDR measures loss and locates faults by backscatter; fusion splicing gives lower loss than mechanical splicing; WDM/DWDM increases capacity by using multiple wavelengths.

Cellular concept: frequency reuse with cluster size N = i² + ij + j²; reuse distance D = R√(3N); co-channel interference falls as D/R rises.

Handover transfers an active call between cells; roaming allows service on a visited network; location update keeps the HLR/VLR tracking area current.

GSM: 900/1800 MHz, 200 kHz carrier spacing, 8 time slots per TDMA frame of 4.615 ms, GMSK modulation, SIM-based authentication with A3/A5/A8 algorithms.

Multiple access: FDMA splits spectrum, TDMA splits time, CDMA separates by spreading codes (WCDMA uses 5 MHz carriers), OFDMA is used in 4G/5G downlink with SC-FDMA in LTE uplink.

LTE: all-IP, scalable bandwidth 1.4–20 MHz, MIMO, 20 MHz carrier gives up to 100 resource blocks; E-UTRAN comprises eNodeB and EPC (MME, S-GW, P-GW).

5G NR: sub-6 GHz (FR1) and mmWave (FR2, 24.25–52.6 GHz); flexible numerology with subcarrier spacing 15 × 2^µ kHz; peak rates of 20 Gbit/s downlink in ITU IMT-2020 targets.

5G service classes: eMBB (enhanced mobile broadband), URLLC (ultra-reliable low-latency, 1 ms), and mMTC (massive machine type, 10⁶ devices/km²); network slicing and beamforming are key enablers.

Isotropic radiator is the reference; antenna gain G(dBi) = 10 log₁₀(G); effective isotropic radiated power EIRP = P_t(dBm) + G(dBi) − cable loss(dB).

Half-wave dipole length ≈ 0.5λ and has 2.15 dBi gain; a quarter-wave monopole over a ground plane is ≈ λ/4 with about 5.15 dBi gain.

λ (m) = 300 / f (MHz). Antenna aperture A_e = Gλ²/(4π); free-space path loss FSPL(dB) = 32.44 + 20 log₁₀(d_km) + 20 log₁₀(f_MHz).

Radiation resistance and VSWR: VSWR = (1 + |Γ|)/(1 − |Γ|); a VSWR of 1.5 corresponds to about 4% reflected power; return loss RL(dB) = −20 log₁₀|Γ|.

Propagation modes: ground (surface) wave follows the earth at LF/MF; sky wave refracts off the ionosphere at HF; space wave (line of sight) serves VHF/UHF and microwave; ducting and tropospheric scatter extend range.

Microwave components: klystron and magnetron oscillators, reflex klystron, TWT amplifier, Gunn diode, circulator (routes power by port), isolator, magic tee, directional coupler, cavity resonator.

Waveguide: cutoff frequency f_c = c/(2a) for the TE₁₀ mode of a rectangular guide of broad dimension a; below cutoff the wave is evanescent. Microwaves use S-band 2–4 GHz, C 4–8, X 8–12, Ku 12–18, Ka 26–40 GHz.

Telecom Regulatory Authority of India (TRAI) regulates tariffs and quality of service; the Department of Telecommunications (DoT) licenses operators and manages spectrum; WPC issues spectrum and equipment type approvals.

Telecom licences in India: Unified Licence (UL) and Unified Licence (VNO) cover access, national long distance, international long distance and ISP services; licence fees and spectrum charges are separate.

DoT's TEC (Telecommunication Engineering Centre) issues interface and testing requirements; equipment must meet mandatory testing and certification before sale or connection.

Safety: before working on a line or equipment, isolate and lock out, verify dead with a tester, discharge capacitors, and use insulated tools; treat every conductor as live until proven dead.

Electrical safety basics: earth resistance should be low (commonly ≤ 5 Ω for telecom earth pits); use ELCB/RCCB protection; never work alone on live high-voltage plant; use PPE such as helmets, gloves and safety belts.

Optical fibre safety: never look into a fibre end or connector without verifying no source; laser light at 1310/1550 nm is invisible and can damage the retina. Dispose of bare fibre offcuts in a sharps container.

Tower and RF safety: RF exposure limits follow DoT norms (ICNIRP-based); observe exclusion zones near antenna main beams, and use fall arrest while climbing. First aid for electric shock: switch off supply, then CPR if breathing has stopped.

Competency-based curriculum: write learning outcomes as observable actions (install, test, troubleshoot) tied to National Skills Qualification Framework (NSQF) levels and industry job roles.

Instructional sequence for a practical session: demonstration → guided practice → independent practice → assessment; state safety rules before tool distribution.

Assessment tools: checklist for procedure, rating scale for skill, rubric for fault-finding logic, and viva for underpinning knowledge; record evidence in a portfolio.

Laboratory management: maintain an inventory and calibration register, log equipment issue, keep a stock of consumables, and display emergency and first-aid contacts.

Lesson planning: state objectives, prerequisites, teaching aids, time allocation, demonstration steps, student activity and evaluation in a written lesson plan.

Fault-based teaching: introduce a deliberate fault (open wire, wrong polarity, faulty connector) and require the learner to apply a systematic test sequence rather than guess.

Feedback and remediation: use short quizzes and practical observation to identify gaps; reteach the sub-skill, then reassess; keep records for progressive assessment.

Rajasthan: capital Jaipur; state formed on 30 March 1949; largest state by area; districts include Jodhpur, Udaipur, Kota, Bikaner, Ajmer and Alwar; the Thar Desert lies in the west.

Rajasthan geography: Aravalli range divides the state; Chambal, Luni, Banas and Mahi are main rivers; Sambhar Lake is a salt lake; Mount Abu is the highest peak area (Guru Shikhar).

Indian education policy: National Education Policy 2020 emphasises vocational education from school level, multidisciplinary learning, and integration of skill training with general education.

Rajasthan skill ecosystem: Rajasthan Skill and Livelihoods Development Corporation (RSLDC) and Rajasthan ILD Skills University (RISU) coordinate skill training; ITIs and polytechnics deliver trade courses.

Reasoning basics: number series (difference, ratio, square/cube patterns), coding-decoding, blood relations, direction sense, syllogism, and seating arrangement; always verify the rule with the given options.

Percentage and average shortcuts: percentage change = (new − old)/old × 100; average = sum/count; successive percentage changes of a% and b% give a + b + ab/100.

Indian polity: fundamental rights in Part III, fundamental duties in Part IVA (Article 51A), Right to Education under Article 21A; Parliament comprises President, Rajya Sabha and Lok Sabha.

TRAI regulates tariffs and quality of service; DoT licenses operators and manages spectrum; WPC issues spectrum and equipment type approvals.

Unified Licence (UL) and Unified Licence (VNO) cover access, NLD, ILD and ISP services; licence fees and spectrum charges are separate.

TEC under DoT issues interface requirements and mandatory testing/certification for telecom equipment before sale or connection.

Isolate and lock out before working on a line; verify dead with a tester; discharge capacitors; use insulated tools; treat conductors as live until proven dead.

Earth resistance for telecom earth pits is commonly kept ≤ 5 Ω; use ELCB/RCCB protection and PPE such as helmets, gloves and safety belts.

Never look into a fibre end without verifying no source; 1310/1550 nm laser light is invisible and can damage the retina; dispose of fibre offcuts in a sharps container.

RF exposure limits follow ICNIRP-based DoT norms; observe exclusion zones near antenna main beams and use fall arrest while climbing. For electric shock, switch off supply first, then CPR if breathing has stopped.

Competency-based curriculum: write learning outcomes as observable actions (install, test, troubleshoot) mapped to NSQF levels and industry job roles.

Practical session sequence: demonstration → guided practice → independent practice → assessment; state safety rules before tool distribution.

Assessment tools: checklist for procedure, rating scale for skill, rubric for fault-finding logic, and viva for underpinning knowledge; record evidence in a portfolio.

Laboratory management: maintain inventory and calibration registers, log equipment issue, stock consumables, and display emergency and first-aid contacts.

Lesson planning: state objectives, prerequisites, teaching aids, time allocation, demonstration steps, student activity and evaluation in a written plan.

Fault-based teaching: introduce a deliberate fault (open wire, wrong polarity, faulty connector) and require a systematic test sequence rather than guessing.

Feedback and remediation: use short quizzes and practical observation to find gaps; reteach the sub-skill, then reassess; keep records for progressive assessment.

Rajasthan: capital Jaipur; formed 30 March 1949; largest state by area; districts include Jodhpur, Udaipur, Kota, Bikaner, Ajmer and Alwar; the Thar Desert lies in the west.

Rajasthan geography: Aravalli range divides the state; Chambal, Luni, Banas and Mahi are main rivers; Sambhar Lake is a salt lake; Guru Shikhar near Mount Abu is the highest peak.

NEP 2020 emphasises vocational education from school level, multidisciplinary learning, and integration of skill training with general education.

Rajasthan skill ecosystem: RSLDC and Rajasthan ILD Skills University (RISU) coordinate skill training; ITIs and polytechnics deliver trade courses.

Reasoning basics: number series (difference, ratio, square/cube patterns), coding-decoding, blood relations, direction sense, syllogism and seating arrangement; verify the rule against the options.

Percentage and average shortcuts: percentage change = (new − old)/old × 100; average = sum/count; successive changes of a% and b% give a + b + ab/100.

Indian polity: fundamental rights in Part III, fundamental duties in Part IVA (Article 51A), Right to Education under Article 21A; Parliament comprises President, Rajya Sabha and Lok Sabha.