WBBSE Madhyamik Telecom (Vocational) cheat sheet

A basic communication system has five blocks in order: information source → transmitter → channel → receiver → destination; noise enters mainly at the channel.

Simplex = one direction only (TV broadcast); half-duplex = both directions but not at once (walkie-talkie); full-duplex = both directions simultaneously (telephone).

Telephone voice band is 300–3400 Hz; sampling at 8 kHz satisfies Nyquist (fs ≥ 2 × 4000 Hz) and 8-bit PCM gives the 64 kbps DS-0 rate.

Modulation shifts a baseband signal onto a carrier so it can radiate from an antenna of practical size: a quarter-wave monopole is λ/4 = 75/f(MHz) metres long.

Power ratio in decibels: dB = 10 log10(P2/P1); for voltage or current dB = 20 log10(V2/V1). +3 dB ≈ double power, +10 dB = ten times, −3 dB = half.

Channel capacity (Shannon): C = B log2(1 + S/N) bps - bandwidth alone does not fix the data rate; the signal-to-noise ratio sets the limit.

Multiplexing lets many signals share one medium: FDM divides frequency, TDM divides time slots, WDM divides optical wavelengths.

Analogue signals vary continuously and accumulate noise; digital signals take discrete levels and can be regenerated cleanly at each repeater.

Standards and regulators: ITU-T (telecom standards), ITU-R (radio), IEEE (LAN); in India, DoT (policy and licensing), TRAI (tariff and QoS) and WPC (spectrum assignment).

Circuit switching reserves a dedicated end-to-end path (PSTN voice); message switching stores and forwards whole messages (telegraph); packet switching forwards independent labelled packets (internet), giving statistical sharing.

E1 = 32 time slots × 64 kbps = 2.048 Mbps (30 speech + 1 signalling + 1 frame alignment); T1 = 24 × 64 kbps + framing = 1.544 Mbps.

SDH hierarchy: STM-1 = 155.52 Mbps, STM-4 = 622.08 Mbps, STM-16 = 2.488 Gbps, STM-64 = 9.953 Gbps.

One Erlang = one circuit busy for one hour; traffic A (Erlang) = number of calls × mean holding time ÷ observation period. Grade of Service is the fraction of calls blocked at the busy hour.

Optical fibre structure: core (glass, refractive index n1), cladding (lower index n2), buffer coating, strength members, outer jacket; light is guided by total internal reflection when n1 > n2 and the incidence angle exceeds the critical angle.

Numerical aperture NA = √(n1² − n2²); a larger NA accepts a wider cone of light but increases modal dispersion in multimode fibre.

Single-mode fibre: core about 8–10 µm, 1310 nm and 1550 nm windows, used for long haul. Multimode: 50 or 62.5 µm core, 850/1300 nm, used for short premises runs.

Typical loss budget: 1310 nm ≈ 0.35 dB/km, 1550 nm ≈ 0.2 dB/km, fusion splice ≈ 0.05–0.1 dB, good connector ≈ 0.3 dB.

An OTDR sends a light pulse and plots backscatter against distance, measuring fibre loss, splice and connector loss and locating a break; a visible fault locator only confirms continuity.

Cellular reuse: identical frequencies are reused in cells spaced far enough apart; cluster size N = i² + ij + j² (valid values 1, 3, 4, 7, 9, 12…), and the reuse factor is 1/N.

Co-channel interference is reduced by increasing N, sectoring a site into 3 × 120° sectors, or splitting congested cells into smaller ones.

GSM: 900/1800 MHz in India, 200 kHz carriers, TDMA with 8 slots per 4.615 ms frame, GMSK modulation, 33 kbps gross rate and 13 kbps speech codec.

GSM nodes: MS (with SIM/IMSI) ↔ BTS ↔ BSC ↔ MSC; the MSC consults HLR and VLR for authentication and location, the AuC holds keys and the EIR holds the IMEI white/black list.

Multiple access: FDMA (frequency slices, 1G), TDMA (time slots, 2G), CDMA (orthogonal codes, 3G UMTS uses 5 MHz WCDMA carriers), OFDMA (4G LTE downlink and 5G NR).

Generations: 1G analogue voice; 2G digital voice plus SMS; 2.5G GPRS/EDGE packet data; 3G mobile data from 384 kbps; 4G LTE all-IP; 5G NR high bandwidth and low latency.

Handoff transfers a call between cells - hard handoff breaks before making, soft handoff (CDMA) makes before breaking; roaming lets a subscriber use a visited network through HLR/VLR exchange.

Antenna gain is referenced to an isotropic radiator (dBi) or a half-wave dipole (dBd), where 0 dBd = 2.15 dBi; a half-wave dipole has 2.15 dBi gain and about 73 Ω feed-point impedance.

Short-range wireless: Wi-Fi IEEE 802.11 at 2.4/5 GHz, Bluetooth IEEE 802.15.1 at 2.4 GHz (about 10 m), Zigbee 802.15.4 low-rate mesh, NFC at 13.56 MHz.

OSI has 7 layers (physical, data link, network, transport, session, presentation, application); TCP/IP collapses them to 4 (network access, internet, transport, application). PDUs: bit, frame, packet, segment, data.

IPv4 is 32-bit dotted decimal; IPv6 is 128-bit hexadecimal. Private IPv4 ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.

A /24 prefix = 255.255.255.0 = 256 addresses with 254 usable hosts; /25 gives 128 addresses per subnet; usable hosts = 2^h − 2 where h is the host-bit count.

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

TCP is connection-oriented (SYN / SYN-ACK / ACK handshake, sequencing, retransmission, flow control); UDP is connectionless and used for DNS queries, VoIP, streaming and SNMP.

ARP maps an IPv4 address to a MAC address by broadcast request and unicast reply; a MAC address is 48 bits written as six hex octets; DHCP uses DORA (Discover, Offer, Request, Ack).

Access technologies: ADSL/VDSL over the copper pair, cable (DOCSIS) over coax, FTTH with GPON (2.488 Gbps down / 1.244 Gbps up), VSAT, and 4G/5G fixed wireless.

Performance terms: bandwidth = theoretical capacity; throughput = achieved rate; latency = round-trip delay; jitter = variation in delay (ruins voice quality); packet loss forces retransmission and stalling.

ITU IMT-2000 broadband targets: 2 Mbps stationary, 384 kbps pedestrian and 144 kbps vehicular - the benchmark that originally defined mobile broadband.

UTP categories: Cat5e 100 MHz / 1 Gbps to 100 m; Cat6 250 MHz / 1 Gbps to 100 m and 10 Gbps to about 55 m; Cat6a 500 MHz / 10 Gbps to 100 m. Maximum channel is 90 m permanent link plus 10 m of patch cords.

T568B order: white-orange, orange, white-green, blue, white-blue, green, white-brown, brown. Straight-through = same order both ends (PC to switch); crossover swaps pairs 2 and 3 (pins 1-3, 2-6).

Optical connectors: SC (square push-pull), LC (small form-factor duplex clip), ST (bayonet), FC (threaded); polish types PC, UPC (blue body) and APC (green, 8° angled, better return loss).

Splicing: fusion by electric arc gives 0.05–0.1 dB and is permanent; mechanical gel-filled splices give 0.2–0.5 dB and are a quick field fix. Clean and cleave before splicing.

Telecom power plant: −48 V DC nominal, VRLA battery strings (24 cells of 2 V), SMPS rectifiers on float and boost charge, DG set for long outages. Fully charged specific gravity is 1.215–1.240.

Earthing: separate pits for equipment, battery and lightning; target earth resistance below 1 Ω at an exchange and below 5 Ω at a small site, measured with a three-point earth tester.

Antenna and feeder checks: VSWR below 1.5:1, feeder sweep with a site master, azimuth and tilt per plan, connectors waterproofed with self-amalgamating tape.

Availability = MTBF ÷ (MTBF + MTTR). Example: MTBF 1000 h and MTTR 10 h gives 99.0% availability; cutting MTTR raises uptime faster than chasing MTBF.

Maintenance split: preventive (scheduled inspection, cleaning, battery discharge test, torque checks) versus corrective/breakdown repair - log every fault with time stamp and root cause.

ARPU = total revenue ÷ average number of subscribers in the period; churn rate = subscribers lost ÷ subscribers at the start of the period × 100%.

SLA metrics for enterprise circuits: uptime %, latency, jitter, packet loss, MTTR and helpdesk response time; service credits or penalties apply when targets are missed.

Customer onboarding requires valid KYC documents and a completed Customer Application Form (CAF), with verification before activation under DoT subscriber verification norms.

Mobile number portability lets a subscriber keep the same number when changing operator or circle; the donor operator releases the number after the porting window and clearing-house validation.

Complaint flow: ticket ID at first-level support → second level/NOC → appellate authority and the TRAI grievance portal; log every interaction with time and agent ID.

TRAI TCCCPR 2018 governs unsolicited commercial communication through DND registration, prior consent and penalties; subscribers can report spam SMS or calls to their operator.

Voice quality is scored on the MOS scale of 1 (bad) to 5 (excellent), with 4 or above being toll quality; quality falls when one-way latency exceeds 150 ms, jitter 30 ms or loss 1%.

Billing basics: the CDR (call detail record) drives postpaid bills, prepaid deducts from balance in real time, and telecom service attracts 18% GST in India.

Hierarchy of control: eliminate → substitute → engineering control → administrative control → PPE. PPE is the last line of defence, never the first.

PPE for site and tower work: industrial helmet with chin strap, full-body harness with double lanyard, safety shoes, gloves, goggles, high-visibility vest and lanyarded tool bag.

Fall protection: 100% tie-off means at least one lanyard is always attached, anchored above shoulder level; never climb alone or during lightning, heavy rain or wind above the maker's limit.

Electrical safety five rules: isolate, secure against reconnection by lockout/tagout, verify absence of voltage, earth and short-circuit, and cover adjacent live parts.

RF exposure: never stand in the main beam of a live antenna; respect the operator's calculated safe distance and switch off or reduce power before working in front of an antenna.

Laser safety: fibre systems are Class 1/1M, but an open connector emits invisible infrared (1310/1550 nm) that can burn the retina - never look into a fibre end; use an optical power meter.

Fire classes A (solid), B (liquid), C (gas), D (metal) and electrical; use CO2 or dry powder on electrical fires, never water. PASS = Pull, Aim, Squeeze, Sweep.

Battery room hazards: lead-acid cells release hydrogen while charging - no smoking or sparks, forced ventilation, acid-resistant gloves and apron, eyewash, and neutralise spills with baking soda.

E-waste: batteries, PCBs and cable go to authorised recyclers under the E-Waste (Management) Rules; lead and cadmium are hazardous and must never be dumped in municipal bins.

Break-even quantity = fixed cost ÷ (selling price − variable cost per unit); margin of safety = (actual sales − break-even sales) ÷ actual sales.

A business plan states product or service, surveyed target market, competitor analysis, pricing, cost and revenue projection, funding requirement and identified risk.

Indian MSME support: free Udyam registration (Aadhaar + PAN), MUDRA loans (Shishu, Kishore, Tarun), PMEGP subsidy and Stand-Up India; telecom services additionally need DoT/ISP or TSP licensing and GST registration.

CV rules: one to two pages, reverse-chronological, quantified achievements, keywords matched to the role, no false claims; the cover letter names the post and the referrer.

STAR method for interviews: Situation, Task, Action, Result - keep the answer under two minutes and finish with a measurable outcome.

The 7 Cs of communication are clear, concise, concrete, correct, coherent, complete and courteous; active listening uses paraphrase and open questions instead of interrupting.

Time management tools: the Eisenhower matrix sorts tasks by urgency and importance, the Pareto rule says 80% of results come from 20% of effort, and Parkinson's law says work expands to fill available time.

Group discussion is scored on content, communication, listening and team behaviour; interrupting or dominating reduces the score regardless of content.

Business records for a small telecom venture: GST invoice with GSTIN and SAC code, cash book, bank reconciliation, and a separate current account for the enterprise.