Telephony, Switching & Optical-Fiber Communications – Comprehensive Study Notes

Telephony: Scope & Core Ideas

Telephony = electronic transmission of Voice, Video, Data across distance
Light-based systems ≈100× faster signal velocity than electricity ➔ justification for optical & IP telephony
Internet (IP) Telephony: digitised voice sent via Internet Protocol; eliminates need for dedicated voice circuits

Historical Evolution

1876 G. Bell patents telephone ➔ vibration → electrical conversion
1878 1st telephone exchange (Connecticut) ➔ operator-based switching
1915 Cross-bar switch introduced (electro-mechanical)
1960 Tone-pad (DTMF) dialling supersedes rotary pulse dialling
1968 Stored-Program-Controlled (SPC) digital switch ➔ software call control
1972 First fully Digital Exchange (UK) ➔ foundation for modern PSTN/VoIP

Transduction: Converting Sound ↔ Electricity

Transducer = device performing energy conversion
Telephone set basic block:
- Microphone (Tx) – carbon granule, diaphragm, DC bias, induction coil
- Receiver (Rx) – permanent magnet + coil, magnetic diaphragm
- Transmission line (2-wire loop) carries varying current reflecting audio envelope

Bell-Type Carbon Transmitter Construction

Diaphragm (thin Al alloy) + moving front electrode
Carbon chamber w/ granules; resistance varies with pressure ➔ amplitude modulation of DC current
Induction coil (step-up) couples varying primary current to secondary for long-loop drive

Receiver Construction & Operation

Horseshoe permanent magnet, wound coil terminals to loop
Magnetic diaphragm adjacent to poles; varying current alters field strength, diaphragm vibrates → sound
Perforated cover protects & acoustically couples diaphragm

Telephone Systems

Local Battery (LB): each set has battery & hand-generator • rugged, quick deploy • limited subs • ideal field/mobile
Common Battery (CB): central office supplies DC & signalling • higher capacity, planning, skill, permanent networks
Common-Battery Signalling (CBS): CB voice power + LB/CB hybrid signalling scheme
- Comparison overview:
- Mobility: LB > CB
- Capacity, robustness, technician demand: CB > LB

Public Switched Telephone Network (PSTN)

Global circuit-switched grid using copper, fibre, microwave, satellite, submarine cables
Requires dedicated voice channel per call (circuit switching)
Interconnect fabric: Central Offices (CO), Switchboards; evolved into electronic & digital switches
Migration path: PSTN ⇄ IP via VoIP gateways, SIP proxies

Modern Bangladeshi Telecom Structure

Layers: ANS (Access Network Service – 6 mobile operators), ICX (Inter-Connection Exchange), IGW (International Gateway), IIG (International Internet Gateway), ISP ➔ international carriers
Domestic off-net call route: ANS ➔ ICX ➔ ANS
International call route: ANS ➔ ICX ➔ IGW ➔ foreign carrier

IP Telephony vs VoIP

IP Telephony = umbrella for any comms using IP (voice, video, IM)
VoIP = specific subset delivering voice over IP
All VoIP ⊂ IP Telephony; not vice-versa
Architecture: SIP devices ↔ Proxy ↔ Call Server ↔ Gateways ↔ PSTN

Switching Fundamentals

Switching = selecting a path among many to connect endpoints
Switching System = organised set of elements (matrix, control, signalling) enabling connection between N inlets & M outlets
Four basic call types: Local, Outgoing, Incoming, Transit
Busy Hour traffic measured in Erlangs (E)
- $E = \frac{\text{Traffic Time (Hrs)}}{1\,\text{Hr}}$
Congestion arises when offered traffic > capacity ➔ queue growth

Classification of Switching Systems

Manual → Automatic
Automatic → Electromechanical (Strowger SxS, Cross-bar) & Electronic (SPC)
- Space Division: physical paths (cross-points)
- Time Division: time-slot interchange (TSI)
- Combinational: space–time–space (STS) or time–space–time (TST)

Dialling & Signalling

Pulse Dialling (rotary): 10 pps make/break loop; inter-digit gap distinguishes digits
DTMF (Touch-Tone): dual-frequency pairs (e.g., 852 Hz + 1477 Hz for digit 9) ➔ faster (<100 ms/digit)
Subscriber tones:
- Dial Tone (33/50/400 Hz steady)
- Ring-Tone (double ring pattern)
- Busy-Tone (400 Hz 0.75 s ON/OFF)
- Number Unobtainable (400 Hz steady)

Digital Transmission Concepts

Bandwidth = channel capacity in bits/s (bps); e.g., $10\,\text{Mbps}=10\times10^6\,\text{bps}$
Multiplexing Families:
- Analog FDM: each user distinct frequency slot ➔ possible crosstalk
- Digital TDM: full bandwidth but divided in time slots; synchronous vs statistical (asynchronous)
- Optical WDM: multi-wavelength light beams combined via MUX, separated via DEMUX
- Analogy: prism dispersion/combination of white light

Modulation/Keying Schemes

Analog: AM, FM, PM
Digital: ASK, FSK, PSK, APSK, QAM
Modulation = superimposing baseband on carrier

Switching Techniques for Data Networks

Circuit Switching: dedicated path, phases = setup ➔ data ➔ teardown; e.g., PSTN
Message Switching: store-and-forward whole message; queued; higher utilisation; ex. e-mail
Packet Switching: message → packets; each routed individually (datagram) or via virtual circuits; ex. Internet

Numbering Plans & Standards

National numbering: Open / Semi-Open / Closed
ITU-T E.164: $+\text{CC}\,\text{NSN}$ (max 15 digits) ensuring global uniqueness

Transmission Media Overview

Guided (wired) vs Unguided (wireless)
Guided classes & properties:
- Twisted Pair: CAT3 → CAT6; UTP vs STP; highest length ≈100 m; 10–10 000 Mbps
- Coaxial: RG-58 (Thinnet 10Base2), RG-59 (video), RG-6 (digital video), RG-8 (Thicknet 10Base5); connectors = BNC, F-type
- Optical Fiber: SMF/MMF; step-index vs graded-index; glass or plastic; bandwidth ≈ THz range
Unguided examples: Infrared, Radio, Microwave

Twisted Pair Detailed Points

2 insulated copper wires twisted → minimise crosstalk
Twist ratio ↑ ⇒ crosstalk ↓ but attenuation ↑
Shielding (STP) adds foil/braid; requires grounding; improves noise immunity
Category table highlights bandwidth: CAT5 ( $100\,\text{MHz},100\,\text{Mbps}$ ) ➔ CAT6 ( $250\,\text{MHz},10\,\text{Gbps}$ )

Coaxial Cable Detailed Points

Central conductor → dielectric → metallic braid → outer jacket
Self-shielded; supports TV distribution, long-distance telephony (10 000 voice channels), early Ethernet
Termination essential (impedance match) to prevent reflections

Optical Fiber Fundamentals

Core (5–10 µm SMF; 50/62.5 µm MMF) — high-n glass
Cladding — lower-n glass ➔ total internal reflection (TIR) containment
Buffer coating — mechanical protection (250–300 µm OD)
TIR Condition: $\theta<em>i \ge \theta</em>c = \arcsin\left(\frac{n<em>2}{n</em>1}\right)$

Fiber Classifications

By Mode: Single-Mode (SMF, laser launch, long-haul) vs Multi-Mode (MMF, LED launch, short-haul)
By Index Profile: Step-Index vs Graded-Index (parabolic n-profile reduces modal dispersion)
By Core Count: Single-core vs Multi-core (research area for spatial multiplexing)
By Material: Silica vs Plastic (POF)

Transmission Windows & Loss Figures

First window 800–900 nm, $\sim2\,\text{dB/km}$
O-band 1310 nm, $0.5\,\text{dB/km}$
C-band 1550 nm, $0.2\,\text{dB/km}$ ➔ preferred long-haul

Attenuation & Other Loss Mechanisms

Overall attenuation (dB) = Absorption + Scattering + Bending losses
- Intrinsic absorption (water, metal ions)
- Rayleigh scattering ∝ $\lambda^{-4}$
- Micro- & Macro-bending beyond min. bend radius
Non-linear scattering: Stimulated Brillouin (SBS), Stimulated Raman (SRS)

Optical Fiber Advantages / Disadvantages

Functional: enormous bandwidth, low loss, immunity to EMI, high security
Physical: light weight, small size, corrosion-free, dielectric (no sparks)
Drawbacks: high installation cost, fragility w/o sheath, point-to-point topology, need repeaters for very long spans

Safety Guidelines

Keep connectors clean/dry; leave dust caps on
Wear safety glasses, disposable aprons; wash hands before touching face/contacts
Dispose of fiber shards properly; avoid looking into active fibers

Termination, Splicing & Hardware

Splice types:
- Fusion: align ➔ electric arc melt; low insertion loss, high reliability; suited SMF
- Mechanical: alignment sleeve + index-matching gel; quick, reusable, higher loss
Tools: Fusion splicer, cleaver, stripper, OTDR, Optical Power Meter, VFL
Connectors: LC (1.25 mm latch), SC (2.5 mm push-pull), ST (bayonet), FC (screw), MPO/MTP (multi-fiber)
- Desirable traits: low loss, small form factor, ease of install, durability
Couplers: T-coupler, Star coupler, 3-port devices for splitting/combining optical power

Optical Network Equipment & Install

Optical Tx/Rx modules, Ethernet↔E1 converters, PCM MUX, 1×E1 MUX
Installation scenarios: small lab network, Tx/Rx link, MUX for voice + data
FTTx (FTTH/FTTB/FTTC): architecture, components (OLT, ONU/ONT, splitters), EPON vs GPON
Triple-Play delivery (voice, data, video) via IP over fiber

Key Formulas & Example Calculations

Ohm’s Law: $V = I \times R$
- Example 1: $I = 3.2\,\text{A},\; R = 50\,\Omega \Rightarrow V = 160\,\text{V}$
Resistance from V & I: $R = \frac{V}{I}$
- Example 2: $V = 8\,\text{V},\; I = 2\,\text{A} \Rightarrow R = 4\,\Omega$

Ethical, Practical & Real-World Implications

Migration from circuit to packet networks reduces cost but raises QoS/security challenges
Fiber-to-Home projects bridge digital divide but require skilled workforce & safety compliance
Number portability & E.164 compliance ensure fair competition & global reach

Assignments & Exam Focus

Describe switching system elements with diagrams
Compare Time Division vs Space Division switching
Differentiate Circuit vs Message switching
Transmission Media lab exercise & final exam (total 6 periods)

Reference Sources

Telephony Précis – School of Signals
Transmission Media & Switching Précis – School of Signals
Optical Fiber Communications, J.M. Senior, PHI
Fiber Optic Communication Systems, G. Keiser, McGraw-Hill
Multiple web resources (ITU-T, IEEE, vendor white-papers)