Telecommunication Networks

Telecommunication

Telecommunication is communication at a distance: the transmission, emission or reception of signs, signals, writing, images, sounds or data of any kind by wire, radio, optical or other electromagnetic systems (ITU definition). A telecom system has terminals, transmission links and switching (exchanges) that connect any user to any other user on demand.

Evolution of telecommunication with switching

The growth of telecom is closely tied to the way calls were switched:

1. Telegraph era (1837–1876) – Morse's electric telegraph (1837/1844). Messages were carried by hand or by store-and-forward at telegraph offices (message switching by operators).
2. Telephone and manual switching (1876–1890s) – Bell's telephone (1876). With many telephones, point-to-point wiring needs $n(n-1)/2$ lines, so the manual exchange (first at New Haven, 1878) was introduced. Operators connected calls with cords and jacks on a switchboard; local-battery and later central-battery (CB) systems.
3. Automatic electromechanical switching
   • Strowger step-by-step (1889–1892) – Almon B. Strowger invented a two-motion selector driven directly by dial pulses; first automatic exchange at La Porte, USA (1892). Rotary dial (1896) came with it.
   • Crossbar switching (1920s–1950s) – crossbar switch with common control (markers, registers); faster, less wear, more flexible routing.
4. Electronic switching – Stored Program Control (SPC), 1965 – Bell's No. 1 ESS: switching controlled by a computer program. Easy to add services (abbreviated dialling, call forwarding), centralised and later distributed control. Switching fabric was still space-division (reed relays).
5. Digital time-division switching (1970s) – with PCM, speech is digitised and switched using time slots (TSI, T-S-T networks): E10 (France, 1970), No. 4 ESS (1976), and later EWSD, 5ESS, AXE. Smaller, cheaper, works with digital transmission (integrated digital network) and common channel signalling (SS7).
6. ISDN and packet switching (1980s) – end-to-end digital voice and data; X.25 and later frame relay, ATM.
7. Mobile and IP era (1990s–now) – mobile switching centres (GSM), softswitches, IMS and all-IP networks (VoIP, 4G/5G core), where calls are carried as packets and switching is done by routers and software.

Each step reduced manual work, increased speed and capacity, and added new services.

Definition (ITU, Geneva 1992)

According to the Constitution of the International Telecommunication Union (Geneva, 1992), telecommunication is any transmission, emission or reception of signs, signals, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems.

Telecommunication as an evolutionary process

Telecom did not appear suddenly; each technology was built on the earlier one: telegraph (1837) → telephone (1876) → manual exchange (1878) → Strowger automatic exchange (1892) → wireless/radio (Marconi, 1895–1901) → crossbar → electronic SPC (1965) → digital switching and PCM → satellites, optical fibre → mobile, Internet and IP networks. Nepal also followed this path, step by step.

Historical background in Nepal since 1913 AD

Key phases

• Early phase (1913–1955): telephones only for the Rana government and palace; manual magneto exchanges; HF radio for long distance.
• Public service (1955–1975): automatic exchange in Kathmandu (1962), telex and trunk links; telecom handled by a government department/board.
• Corporation phase (1975–2004): NTC built microwave backbone, satellite earth station, digital exchanges, and started GSM (1999).
• Liberalisation (1997 onward): Telecommunication Act 2053 and NTA opened the sector to private operators (Ncell, UTL, ISPs), and Nepal Telecom became a public company (2004).
• Broadband era (2010–now): fibre backbone, 3G/4G, FTTH and widespread mobile Internet.

(Dates of early events are as commonly given in Nepal Telecom's history; small differences exist between sources.)

Telecommunication evolved gradually: each new system grew out of the limits of the previous one, moving from visual and electrical signalling to voice, then to automatic switching, radio, digital and finally packet/IP and mobile networks.

Major milestones

Main trends

• Manual → automatic: operators replaced by Strowger, crossbar and then computer-controlled (SPC) exchanges.
• Analogue → digital: PCM, TDM and digital switching gave better quality and integration of voice and data.
• Copper → fibre and radio: higher bandwidth and lower loss; satellites and microwave for long distance.
• Fixed → mobile: cellular networks from 1G to 5G.
• Circuit → packet: Internet, VoIP and all-IP networks.

Each milestone increased capacity and quality and lowered cost, which is why telecom is called an evolutionary process.

Telephone exchange

A telephone exchange (switching office or central office) is the system that connects the line of a calling subscriber to that of the called subscriber, or to a trunk leading to another exchange, for the duration of a call and releases it afterwards. It avoids wiring every telephone to every other ($n(n-1)/2$ links). Its functions include detecting a call, receiving the dialled number, finding a free path, ringing the called party, supervising the call, charging and clearing.

Switching hierarchy

Exchanges are arranged in a hierarchy so that any two subscribers can be connected with a small number of trunks. The classic (AT&T/North American) five-level hierarchy is:

          Class 1: Regional centre
          /                 \
   Class 2: Sectional    Class 2: Sectional
   centre                centre
       |                     |
   Class 3: Primary      Class 3: Primary
   centre                centre
       |                     |
   Class 4: Toll centre  Class 4: Toll centre
     /     \                 |
 Class 5   Class 5        Class 5
 end office end office    end office
   |  |       |              |
 subscribers subscribers  subscribers

 Final (hierarchical) route: ─── (vertical links)
 High-usage trunks: direct links between busy
 offices at any level (shortcuts)

Routing: a call first tries a high-usage trunk (direct route) between the two exchanges; if all are busy it overflows upward to the next level, and finally uses the final route through the hierarchy. ITU-T uses similar levels: local exchange, primary, secondary, tertiary and quaternary (international) centres.

Advantages of DTMF over pulse (rotary) dialling

DTMF (Dual Tone Multi-Frequency) dialling sends each digit as two audio tones played together, one from a low group and one from a high group, instead of loop-break pulses.

Other advantages: shorter call set-up time so exchange registers are held less time (fewer receivers needed), and support for supplementary services.

Keypad and frequencies

            1209  1336  1477  1633 Hz
   697 Hz    1     2     3     A
   770 Hz    4     5     6     B
   852 Hz    7     8     9     C
   941 Hz    *     0     #     D

Example: pressing 5 sends 770 Hz + 1336 Hz together.

Design features of a DTMF telephone set

1. Frequencies inside the voice band (697–1633 Hz) so tones pass through the normal speech channel and filters.
2. Two groups of four tones (2-out-of-8 code): exactly one tone from each group. Receiver checks that one and only one tone of each group is present – a strong check against errors.
3. Non-harmonic frequencies: no frequency is a harmonic of another, and sum/difference (intermodulation) products of any two tones do not fall on another DTMF frequency. Spacing is about a constant ratio (~10% in low group, ~10% in high group), so harmonics of speech or of a tone are not mistaken for a digit.
4. Protection against talk-off (speech imitation): speech rarely contains two pure tones from both groups with steady amplitude; receiver also requires a minimum tone duration (about 40 ms) and a minimum pause (about 40 ms) between digits.
5. Frequency tolerance: generator accuracy within about ±1.5%; receiver accepts ±1.5% and rejects beyond ±3.5%.
6. Level and twist: each tone sent at about −6 to −8 dBm; high-group tone sent slightly louder (about 2 dB, "pre-emphasis/twist") to make up for greater line loss at high frequencies; receiver accepts a limited twist (e.g. up to 4–8 dB).
7. Signal-to-noise and guard: the energy of the two tones must be much larger than total energy in other frequencies (guard action), so music and speech are rejected.
8. Electronic generation: a crystal oscillator and divider IC generate tones accurately; the speech circuit is muted during tone sending so that no voice is mixed with tones, and loop current powers the set.
9. Compatibility: many sets have a switch for pulse/tone mode so they work with old exchanges.

These features give fast, reliable signalling with very low probability of false digit detection.

Switching hierarchy

Exchanges are arranged in levels so that a call between any two subscribers passes through as few links as possible and traffic can overflow to higher levels when direct routes are busy.

          Class 1: Regional centre
          /                 \
   Class 2: Sectional    Class 2: Sectional
   centre                centre
       |                     |
   Class 3: Primary      Class 3: Primary
   centre                centre
       |                     |
   Class 4: Toll centre  Class 4: Toll centre
     /     \                 |
 Class 5   Class 5        Class 5
 end office end office    end office
   |  |       |              |
 subscribers subscribers  subscribers

 Final (hierarchical) route: ─── (vertical links)
 High-usage trunks: direct links between busy
 offices at any level (shortcuts)

• Class 5 – End office (local exchange): subscribers are connected through local loops.
• Class 4 – Toll centre / tandem: joins end offices and gives access to long-distance network.
• Class 3 – Primary centre, Class 2 – Sectional centre, Class 1 – Regional centre: higher levels of the long-distance network; regional centres are fully meshed.
• Routing rule: first try a direct high-usage trunk; if busy, overflow upward; the last choice is the final route along the hierarchy.

Evolution of telecommunication up to the telephone

1. Ancient signalling: drums, smoke and fire signals, messengers and carrier pigeons – line of sight or physical transport.
2. Optical (semaphore) telegraph, 1790s: Claude Chappe's towers with movable arms in France sent coded messages from tower to tower.
3. Electric telegraph, 1837–1844: Cooke and Wheatstone (UK) and Samuel Morse (USA) sent messages as electric pulses over wires using Morse code; the first Washington–Baltimore line (1844). Telegraph networks and undersea cables (1866 transatlantic) followed.
4. Telephone, 1876: Alexander Graham Bell patented the telephone, converting voice to a varying electric current with a transmitter and back to sound in a receiver. Edison's carbon microphone improved it. This led directly to telephone exchanges (1878) and the switching systems above.

Exchange and its types

A telecommunication exchange (switching centre) is a node of the network where subscriber lines and trunks terminate and where connections are set up between them on demand, supervised during the call and released at the end.

Types

• By technology:
  • Manual exchange – operator connects calls with cords on a switchboard (local battery or central battery).
  • Automatic electromechanical – Strowger (step-by-step) and crossbar exchanges.
  • Electronic SPC exchange – computer-controlled; space-division (analogue) or time-division (digital) switching.
  • Softswitch / IP exchange – call control in software, media carried as packets.
• By function/position in network:
  • Local (end) exchange, tandem exchange (joins local exchanges in a city), trunk/toll (transit) exchange, international gateway exchange, and private exchanges (PBX/PABX) for organisations.

Essential features and major equipment

 Subscriber
 lines
   │
  MDF ─> Subscriber ─> Switching ─> Trunk ─> other
         line units    network     units    exchanges
             │            │          │
             └──── Control (processor) ┘
                   + signalling units
                    O&M terminal, billing, power plant

1. Main Distribution Frame (MDF): terminates outside cables; cross-connects lines to equipment; protection against lightning/over-voltage.
2. Subscriber line interface (BORSCHT): Battery feed, Over-voltage protection, Ringing, Supervision (on/off-hook), Codec, Hybrid (2W/4W), Testing.
3. Switching network (fabric): space or time switches that connect inlets to outlets.
4. Control subsystem: processor(s) with stored program that do call processing, routing and number translation.
5. Signalling equipment: dial pulse/DTMF receivers, tone generators (dial, busy, ringback), inter-exchange signalling (CAS, SS7).
6. Trunk interfaces: connect PCM/E1 trunks to other exchanges.
7. Charging/billing equipment and call records.
8. Operation and maintenance (O&M): terminals, alarms, test equipment.
9. Power plant: −48 V DC battery with rectifiers and generator for uninterrupted service.
10. Synchronisation (clock) in digital exchanges.

Telecommunication has moved from sending simple on/off electrical pulses over wires to sending terabits of data over glass fibre and radio. Each stage was made possible by a key technical breakthrough.

Evolution

1. Telegraphy (1837–1870s): Morse's electric telegraph and Morse code; undersea telegraph cables (transatlantic 1866). First fast long-distance communication, but only text and only between offices.
2. Telephony (1876 onward): Bell's telephone carried voice; manual exchanges (1878), then Strowger automatic exchanges (1892) and crossbar systems.
3. Wireless telegraphy and radio (1895–1920s): Marconi's spark transmitter; transatlantic radio (1901); vacuum-tube amplifiers and oscillators led to AM broadcasting and radiotelephony.
4. Multiplexing and long-haul (1920s–1950s): FDM carrier systems on open wire and coaxial cable; microwave line-of-sight relays (1940s–50s) carried thousands of calls and TV.
5. Digital transmission (1960s): PCM and T1/E1 TDM systems; Telstar and later geostationary satellites (Intelsat, 1965) for intercontinental links.
6. Electronic and digital switching (1965–1980s): SPC exchanges, digital TDM exchanges, common channel signalling (SS7), ISDN.
7. Optical fibre (1970s–now): low-loss fibre (1970), semiconductor lasers, SDH/SONET, erbium-doped fibre amplifiers (EDFA, ~1990) and DWDM, giving terabit capacity and undersea fibre cables.
8. Wireless/mobile (1979–now): 1G analogue cellular, 2G GSM digital (1991), 3G, 4G LTE, 5G; Wi-Fi, and Internet over mobile.

Key technological breakthroughs

• Morse code and electromagnetic relay – practical electrical signalling.
• Telephone transmitter/receiver and carbon microphone – voice over wires.
• Strowger switch – automatic switching without operators.
• Vacuum tube (1906) and transistor (1947) – amplification, then cheap reliable electronics; later integrated circuits and microprocessors enabling SPC and digital systems.
• PCM and sampling theory (Nyquist, Shannon) – digital voice, TDM and error-free regeneration.
• Satellites (1962–65) – global coverage.
• Laser (1960) and low-loss fibre (1970) – huge bandwidth with very low loss; EDFA and WDM multiplied capacity.
• Cellular concept (frequency reuse, handover) – mobile telephony for millions.
• Packet switching and TCP/IP – the Internet and all-IP networks.

These breakthroughs together took telecom from a few words per minute to billions of bits per second.

Definitions (Electronic Communications Act, Norway, 2003)

• Electronic communication: the transmission or reception of sound, text, images or other data by means of electromagnetic signals in free space or in cables (electrical, magnetic, radio or optical signals). In short, any exchange of information carried by electromagnetic signals.
• Electronic communication network: a system consisting mainly of cables and associated infrastructure, switches and other equipment (including passive elements) that is used to transmit sound, images, data or other information by means of electromagnetic signals in free space or in cables, between defined termination points. It covers fixed, mobile, broadcasting and Internet networks.

(The wording above follows the meaning of the Act's definitions section; the Act is technology-neutral.)

Main components of telecommunication networks

Every telecom network, whatever its type, is built from these components:

By network type

• PSTN (fixed telephone): telephone sets, local loop, local/tandem/toll exchanges, trunks, SS7.
• Mobile (GSM/LTE): mobile station, BTS/BSC (or eNodeB), MSC/core network, HLR/VLR, transmission backhaul.
• Data/Internet: hosts, switches, routers, modems, servers, links.
• Broadcast: studio, transmitters, antennas, receivers.
• Satellite: earth stations, satellite transponders, VSAT terminals.

Role and need of an exchange

Without an exchange, $n$ telephones would need $n(n-1)/2$ separate lines (1000 phones → 499,500 lines). An exchange (central office or branch office) connects all subscriber lines to one point and joins any two of them only when needed.

Roles/functions:

• Detect a call request (off-hook) and give dial tone.
• Receive and store the dialled digits.
• Find the called line or route to another exchange through trunks.
• Test whether the called line is busy; ring the called party and give ringback/busy tone.
• Set up the speech path, supervise the call, charge for it, and release it at the end.
• Provide shared use of costly trunks and services (operator, emergency, supplementary services).
• A branch office/PBX does the same for an organisation's internal phones, sharing a few external lines.

Rotary dial operation

  finger wheel with 10 holes (1..9, 0)
  finger stop ─┐
               v        Off-normal contacts
  Wind clockwise  ─────> spring is wound
  Release ─────> wheel returns at fixed speed
                (governor) and cam opens the
                pulsing contact n times
  Loop current:
  ──┐  ┌──┐  ┌──┐  ┌──────   (digit 3)
    └──┘  └──┘  └──┘
    break make

1. The subscriber puts a finger in the hole of a digit and turns the wheel clockwise to the finger stop, winding a spring.
2. On release, the spring returns the wheel; a centrifugal governor keeps the return speed constant (10 pulses per second).
3. A cam opens and closes the impulsing contacts in the subscriber loop, giving one break (loop open) per digit unit: digit 1 = 1 break, digit 9 = 9 breaks, digit 0 = 10 breaks. Break:make ratio is about 2:1 (66.7 ms : 33.3 ms).
4. Off-normal contacts short the receiver during dialling to avoid clicks and keep the loop steady.
5. The time taken to rotate the dial for the next digit gives the inter-digit pause (at least ~200–300 ms) that the exchange uses to separate digits.

Drawbacks

• Slow: digit 0 takes about 1 s plus rotation time.
• Mechanical wear of governor and contacts changes pulse speed/ratio, causing wrong numbers.
• Only 10 signals; no * and # for services.
• Pulses cannot pass end-to-end over a call (no IVR use).
• Pulse distortion on long lines; needs DC loop signalling, not suitable for electronic systems.
• Uncomfortable for users; more misdialling.

Each digit is sent as that many loop breaks (0 = 10 breaks) at 10 pulses per second (period 100 ms, break 66.7 ms, make 33.3 ms), with an inter-digit pause between digits.

Loop current (high = make/closed, low = break):
         4 pulses        10 pulses (digit 0)  1 pulse
 ───┐ ┌┐ ┌┐ ┌┐ ┌──────┐ ┌┐ ┌┐ ... ┌┐ ┌──────┐ ┌──────
    └─┘└─┘└─┘└─┘  IDP └─┘└─┘└ ... ┘└─┘  IDP └─┘
     digit 4             digit 0           digit 1
 One pulse = 100 ms: break 66.7 ms + make 33.3 ms
 IDP = inter-digit pause (~200 ms or more)

The total time = (dial rotation time for every digit) + (pulse time for every digit) + (inter-digit gaps). The 33% duty ratio only fixes how each pulse period is split into make (33%) and break (67%); it does not change the pulse period.

Digits and pulses (number 0119144414630, 13 digits; digit 0 gives 10 pulses):

Total pulses $= 58$.

Nominal rate (10 pps): pulse period $T = 1/10 = 100$ ms (make 33.3 ms, break 66.7 ms).

$$\begin{aligned} t &= 13 \times 600 + 58 \times 100 + 12 \times 200 \\ &= 7800 + 5800 + 2400 \\ &= 16000\ \text{ms} = 16.0\ \text{s} \end{aligned}$$

Worst case (slowest rate, 10% tolerance → 9 pps): $T = 1/9 = 111.1$ ms.

$$\begin{aligned} t_{max} &= 7800 + 58 \times 111.1 + 2400 \\ &= 7800 + 6444.4 + 2400 = 16644.4\ \text{ms} \end{aligned}$$

(At the fastest rate, 11 pps, $T = 90.9$ ms and $t_{min} = 15472.7$ ms.)

Answer: about 16.0 s at the nominal 10 pps; up to 16.64 s in the worst case (9 pps). (12 inter-digit gaps are counted, between the 13 digits.)

In a Strowger (step-by-step) exchange, a calling subscriber must first be connected to a free first group selector that will receive the first dialled digit. Two design approaches are used for this subscriber access:

1. Uniselector (line switch / pre-selector) approach

 Sub A ─> [Uniselector A] ─┐
 Sub B ─> [Uniselector B] ─┼─> free 1st group selector
 Sub C ─> [Uniselector C] ─┘    (outlets of uniselectors)

• Every subscriber has his own uniselector (a single-motion rotary switch) at the exchange.
• When the subscriber lifts the handset, his uniselector rotates and hunts over its outlets until it finds a free first selector, then stops and gives dial tone.
• Fast access (no searching for the caller), simple control.
• Costly: one switch per subscriber, though most subscribers are idle most of the time; poor equipment utilisation.

2. Line finder approach

 Sub A ─┐
 Sub B ─┼─ bank of ─> [Line finder 1] ─> 1st selector
 Sub C ─┘  contacts   [Line finder 2] ─> 1st selector
 (100-200 lines)      (few finders shared)

• A small number of line finders (two-motion switches, each permanently paired with a first selector) is shared by a group of 100–200 subscribers.
• When a subscriber goes off-hook, an allotter assigns a free line finder, which searches its bank (vertical then rotary) to find the calling line and connects it to the first selector.
• Fewer switches are needed because the number of finders is set by traffic, not by the number of subscribers – cheaper for low-traffic lines.
• Slightly slower (search time) and needs an allotter/control circuit.

Classification of switching systems

            Switching systems
           /                 \
      Manual              Automatic
                       /            \
          Electromechanical      Electronic (SPC)
            /        \             /         \
      Strowger    Crossbar    Space       Time division
   (step-by-step)            division      (digital)
                         (centralised or distributed
                                control)

• Manual: operator connects calls (local battery, central battery boards).
• Electromechanical automatic:
  • Strowger (step-by-step) – direct progressive control by dial pulses.
  • Crossbar – common control with markers and registers.
• Electronic (Stored Program Control): a computer controls switching.
  • Space-division – separate physical path per call (reed relays, crosspoints).
  • Time-division – shared paths with time slots; analogue PAM or digital PCM (time and T-S-T switches).
  • Control can be centralised or distributed.
• By information transfer: circuit switching, message switching and packet switching.

Functions of a digital switching system

1. Attending/supervision: continuously scan lines and trunks to detect off-hook (call request) and on-hook.
2. Information receiving: give dial tone and receive dialled digits (pulse or DTMF).
3. Information processing: analyse digits, translate the number and decide routing and charging.
4. Busy testing: check whether the called line or trunk is free.
5. Interconnection: set up a path through the time/space switch network (assign time slots).
6. Alerting: ring the called party and send ringback tone to caller.
7. Supervision during the call: watch for answer and clear-down; start/stop charging.
8. Information sending: signal to other exchanges (CAS/SS7) for outgoing calls.
9. Disconnection/release of the path at the end of call.
10. Charging, maintenance and administration: billing records, fault detection, traffic statistics, subscriber data changes, and supplementary services (call forwarding, waiting, conference).

Definition (ITU, Geneva 1992)

Telecommunication is any transmission, emission or reception of signs, signals, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems (Constitution of the ITU, Geneva 1992).

Classification of switching systems

• Manual – operator-controlled switchboards.
• Automatic
  • Electromechanical: Strowger (step-by-step) and crossbar.
  • Electronic / Stored Program Control: space-division and time-division (digital) switching, with centralised or distributed control.
• By transfer mode: circuit, message and packet switching.

Historical background in Nepal since 1913

• 1913: first telephone service started in Kathmandu for the Rana government.
• 1935: manual telephone exchange (about 100 lines) in Kathmandu.
• 1950s: HF radio links; telephone opened to the public; telecom department formed.
• 1962: first automatic exchange (about 1,000 lines) in Kathmandu.
• 1975: Nepal Telecommunications Corporation (NTC) established.
• 1982: satellite earth station at Sanga for international calls.
• 1997–98: Telecommunication Act 2053 and Nepal Telecommunications Authority (regulator).
• 1999: GSM mobile service by NTC; 2004: NTC became Nepal Doorsanchar Company Ltd (Nepal Telecom); 2005: private GSM (Mero Mobile, later Ncell).
• 2010s: 3G, 4G (2017), optical fibre backbone and FTTH.

Major drawbacks of manual telephone exchanges:

• Slow call set-up: operator must answer, ask the number and plug cords.
• Lack of privacy: the operator can hear and know who is calling whom.
• Human errors: wrong connections, delays, language and accent problems.
• High running cost: many operators needed round the clock.
• Limited capacity: a switchboard can handle only a few hundred lines; large cities need many boards.
• Operator dependence: service quality varies with operator mood and fatigue; possible favouritism.
• No advanced services and poor 24-hour service in small exchanges.

Working principle

The Strowger (step-by-step) system uses two-motion selectors that move directly in step with the dial pulses sent by the subscriber (direct, progressive control).

     Bank: 10 levels x 10 contacts
   level 10 o o o o o o o o o o
     ...
   level 2  o o o o o o o o o o
   level 1  o o o o o o o o o o
            ^
     wipers │ vertical step (one per pulse)
            └─> rotary step (hunt or per pulse)

 Caller ─> Uniselector/line finder ─> 1st group
 selector ─> 2nd group selector ─> final selector
 ─> called line

1. The subscriber lifts the handset; a uniselector or line finder connects him to a free first group selector, which sends dial tone.
2. Dialling the first digit sends $n$ pulses; the vertical magnet raises the wiper shaft $n$ steps to level $n$.
3. In a group selector the wipers then rotate automatically across that level, hunting for a free outlet (trunk to the next selector). Each digit thus selects a group of lines.
4. The final selector (connector) uses the last two digits: the tens digit gives vertical steps and the units digit gives rotary steps, reaching the exact called line.
5. The final selector tests the line; if free it rings it, otherwise returns busy tone. On clearing, the release magnet returns all switches to normal.

Why it was the beginning of automatic switching

• It was the first system (patented 1891; first exchange at La Porte, 1892) in which calls were connected without a human operator, using only the pulses from the subscriber's dial.
• It introduced the rotary dial, numbered subscriber lines and selection by digits – the basis of all later numbering schemes.
• It introduced selectors arranged in stages (grouping, hunting, final selection) – the idea of a multistage switching network still used today.
• It gave privacy, 24-hour service and reduced operating cost, which led to world-wide replacement of manual exchanges.

Why digital switching is preferred over manual switching

Digital (SPC, time-division) exchanges are also cheaper per line, easy to expand by software and support automatic billing.

Evolution of telecommunication in Nepal

• 1913 AD: first telephone line in Kathmandu for the Rana rulers.
• 1935: manual exchange of about 100 lines in Kathmandu.
• 1950s: HF radio links; public telephone service begins.
• 1962: first automatic exchange (about 1,000 lines) in Kathmandu.
• 1975: Nepal Telecommunications Corporation (NTC) formed.
• 1982: satellite earth station at Sanga for international service.
• 1980s–90s: digital exchanges and microwave backbone across the country.
• 1997–98: Telecommunication Act 2053; Nepal Telecommunications Authority set up; private operators allowed.
• 1999: GSM mobile by NTC; 2004: Nepal Doorsanchar Company Ltd (Nepal Telecom); 2005: private GSM (now Ncell).
• 2010 onward: 3G, 4G LTE (2017), national optical fibre backbone, FTTH broadband and mobile Internet reaching most of the country.

Telecommunication in Nepal began in 1913 AD, when the first telephone line was set up in Kathmandu for the Rana administration. A manual exchange of about 100 lines followed in 1935. In the 1950s HF radio links joined district headquarters and other countries, and the telephone service was opened to the public.

The first automatic exchange (about 1,000 lines) came to Kathmandu in 1962, followed by telex and trunk links. Nepal Telecommunications Corporation (NTC) was established in 1975 and built the Sanga satellite earth station (1982), a microwave backbone and digital exchanges in the 1980s–90s.

The Telecommunication Act 2053 (1997) created the Nepal Telecommunications Authority (1998) as regulator and opened the sector to private operators. NTC launched GSM mobile (1999) and became Nepal Doorsanchar Company Ltd (Nepal Telecom) in 2004; private GSM (Mero Mobile, now Ncell) started in 2005, with UTL and ISPs adding competition.

Since 2010, 3G, 4G LTE (2017), a national optical fibre backbone and FTTH have spread broadband, and mobile phones now reach most of the population.

Telephone exchange

A telephone exchange is the switching centre where subscriber lines and trunks end, and where a temporary path is set up between a calling and a called subscriber on demand, supervised and then released. It removes the need for direct lines between every pair of users.

Major types

1. Manual exchange – an operator connects calls with cords and jacks.
   • Local battery (magneto): each telephone has its own battery; caller turns a magneto to signal.
   • Central battery (CB): battery at the exchange; lifting the handset lights a lamp at the switchboard.
2. Strowger (step-by-step) exchange – electromechanical; two-motion selectors step directly with dial pulses (direct control).
3. Crossbar exchange – electromechanical crossbar switches with common control (registers, markers); faster and more reliable than Strowger.
4. Electronic (SPC) exchange – a stored-program computer controls the switching network; offers many services.
   • Space-division – separate metallic/reed-relay path for each call.
   • Time-division (digital) – PCM speech switched in time slots (e.g. E10, EWSD, 5ESS, AXE); used in modern networks.
5. Exchanges by network role: local (end) exchange, tandem exchange, toll/trunk exchange, international gateway, and private branch exchange (PBX/PABX) in offices.

Switching hierarchy

          Class 1: Regional centre
          /                 \
   Class 2: Sectional    Class 2: Sectional
   centre                centre
       |                     |
   Class 3: Primary      Class 3: Primary
   centre                centre
       |                     |
   Class 4: Toll centre  Class 4: Toll centre
     /     \                 |
 Class 5   Class 5        Class 5
 end office end office    end office
   |  |       |              |
 subscribers subscribers  subscribers

 Final (hierarchical) route: ─── (vertical links)
 High-usage trunks: direct links between busy
 offices at any level (shortcuts)

• Class 5 end offices serve subscribers; Class 4 toll centres connect end offices to the long-distance network; Class 3, 2, 1 (primary, sectional, regional centres) form the higher levels.
• Calls first use high-usage (direct) trunks; on overflow they move up the hierarchy and finally use the final route.

Marconi's induction coil radio transmitter

Marconi's transmitter (1895–1901) was a spark-gap transmitter used for wireless telegraphy (Morse code).

 Battery ─┬─ Morse key ─┐
          │             v
          │      ┌─────────────┐   spark   ┌── Aerial
          │      │ Induction   │──┤gap├────┤
          └──────│ coil (Ruhm- │   (balls) │
                 │ korff)      │───────────┴── Earth
                 └─────────────┘

1. A battery and a Morse key feed the primary of an induction (Ruhmkorff) coil. An interrupter (vibrator) makes and breaks the primary current rapidly.
2. Each interruption induces a very high voltage (tens of kV) in the secondary winding.
3. The secondary is connected across a spark gap between two metal balls; one side goes to an elevated aerial (antenna) and the other to earth.
4. When the voltage breaks down the gap, a spark discharges the aerial–earth capacitance, producing damped high-frequency oscillations in the aerial, which radiate electromagnetic waves.
5. Pressing the key for short or long times sends sparks in dots and dashes, i.e. Morse code.
6. At the receiver, a coherer (metal filings tube) became conductive when waves arrived, operating a relay and a Morse inker; a tapper reset it.

Marconi's use of a tall grounded aerial greatly increased range, leading to the first transatlantic signal in 1901.

Roles and operations of an exchange

An exchange connects a calling subscriber to any called subscriber (local or distant) on demand, using shared equipment, and releases the connection after the call. It avoids $n(n-1)/2$ direct links among $n$ users and allows costly trunks to be shared.

 Subscribers                 Other exchanges
  ──┐   ┌─────────────────────────┐   ┌──
  ──┼MDF┤ line units ─ switching ─┤ trunks
  ──┘   │   network ─ trunk units │   └──
        │        │                │
        │  control + signalling   │
        └─────────────────────────┘

Basic operations in setting up a call (switching functions):

1. Attending (call detection): the exchange continuously scans lines; when the caller lifts the handset (loop closes), a call request is detected.
2. Information receiving: a free register/digit receiver is connected and dial tone is sent; the dialled digits (pulse or DTMF) are received and stored.
3. Information processing: the digits are analysed to identify the called line or route (local, trunk, international) and the charging rate.
4. Busy testing: the called line (or outgoing trunk) is tested to see if it is free.
5. Interconnection: a path is set up through the switching network between the caller and the called line/trunk.
6. Alerting: ringing current is sent to the called phone and ringback tone to the caller; if busy, busy tone is sent.
7. Supervision: when the called party answers, ringing stops, the speech path is completed and charging (metering) starts; the exchange watches for clear-down.
8. Information sending: for calls to other exchanges, the number is signalled to the next exchange (CAS or SS7).
9. Disconnection: when either party hangs up, the path is released, charging stops and equipment returns to idle.

Other roles: charging and billing, routing traffic over alternative routes when busy, maintenance and fault detection, traffic recording, providing supplementary services (call forwarding, waiting, conference), emergency and operator services, and power feeding (−48 V) to subscriber telephones.

Demerits of manual switching

• Slow call set-up and long waiting for the operator.
• No privacy; operator can listen to calls.
• Human errors, misunderstanding of numbers/language.
• High labour cost; operators needed 24 hours.
• Limited capacity; unsuitable for large cities.
• Service depends on operator efficiency, fatigue and behaviour.

Marconi's induction-coil radio transmitter

Marconi's transmitter (1895–1901) was a spark-gap transmitter that sent Morse code as bursts of electromagnetic waves – the first practical wireless telegraphy.

 Battery ─ Morse key ─> Induction ─> spark gap ─┬─ Aerial
                        coil (HV)               └─ Earth

1. Pressing the Morse key connects the battery to the primary of an induction (Ruhmkorff) coil; an interrupter breaks the current many times a second.
2. The secondary produces a very high voltage across a spark gap connected between an elevated aerial and earth.
3. Each spark discharges the aerial capacitance, setting up damped high-frequency oscillations that radiate as electromagnetic waves.
4. Short and long key presses give dots and dashes. At the receiver a coherer detects the waves and operates a relay/inker.
5. The tall grounded aerial gave long range – transatlantic in 1901.

Mobile vs nomadic

Important features of Nepal Telecommunication Act 2053 (1997)

• Establishes the Nepal Telecommunications Authority (NTA) as an autonomous regulator to make telecom services reliable, affordable and competitive.
• Licensing: no one may operate telecom service without a licence from NTA; NTA sets licence conditions, fees and renewal.
• Frequency management: radio frequency allocation through the government's Radio Frequency Policy Determination Committee; use of frequencies needs a licence.
• Tariff and interconnection: NTA approves tariffs and interconnection terms and settles disputes between operators.
• Rural Telecommunication Development Fund to extend service to rural and remote areas (universal service).
• Quality and consumer protection: standards for service quality, type approval of equipment.
• Government powers: in emergencies or for national security the government may control or suspend services; provision for lawful interception.
• Offences and penalties: fines/imprisonment for operating without licence, damaging telecom equipment, illegal interception, etc.; appeal against NTA decisions to the Appellate (now High) Court.
• Allows private sector participation and competition, ending the government monopoly.

The subscriber loop (line plant or outside plant) is the network of cables and wires that connects each subscriber's telephone to the local exchange. It is usually a twisted copper pair per subscriber (two wires forming a "loop"), and it is the most expensive part of a local network because it is not shared.

Structure

 Exchange
 ┌──────────┐
 │ Switch ─ │ MDF
 └──────┬───┘
        │ Main (feeder) cable, 600-2400 pairs
        v
   ┌──────────┐
   │ Cabinet/ │  primary cross-connection
   │ pillar   │  point (flexibility)
   └──┬───┬───┘
      │   │ Distribution (branch) cables, 50-200 pairs
      v   v
     [DP] [DP]  distribution points (pole/wall box)
      │ │  │
     drop wires (1-2 pairs)
      │ │  │
     subscribers' premises

1. Main Distribution Frame (MDF) at the exchange – outside cables end here and are cross-connected to the exchange line equipment; protection devices fitted.
2. Feeder (main/primary) cable – large multi-pair cable, usually underground in ducts, from MDF to cross-connect points.
3. Cross-connection cabinet (pillar) – lets any feeder pair be jumpered to any distribution pair, giving flexibility when demand changes.
4. Distribution (secondary) cable – smaller cables, underground or aerial, running along streets.
5. Distribution point (DP) – a terminal box on a pole or wall serving a few buildings.
6. Drop wire / service wire – from DP to the subscriber, ending at the subscriber's terminal/socket.

Electrical features

• Loop resistance limited (about 1200–1800 Ω) so that DC signalling and −48 V feeding work; loading coils used on long analogue loops.
• Carries DC power, ringing (75 V, 25 Hz), dial signals and speech in both directions on two wires (hybrid at each end).
• Modern loops are being replaced by fibre (FTTC/FTTH) and wireless access.

Technical structure of a telephone office

A telephone office (local exchange) has the switching and control equipment inside the building and the line (outside) plant outside it. Inside, all lines pass through the MDF.

          TELEPHONE OFFICE (EXCHANGE)
 ┌──────────────────────────────────────────┐
 │ Power   Control   Switching    Trunk     │─ trunks to
 │ plant   (SPC)  ─  network   ─  equipment │  other offices
 │ -48 V      │         │                   │
 │       Line equipment (BORSCHT)           │
 │                │                         │
 │        Main Distribution Frame (MDF)     │
 └────────────────┬─────────────────────────┘
       Feeder (main) cables in ducts
                  │
         ┌────────┴────────┐
     [Cabinet/PCP]     [Cabinet/PCP]
       │       │            │
  Distribution cables (aerial/underground)
     [DP]    [DP]         [DP]
     │ │      │            │
    Drop wires to subscribers' telephones

Inside equipment: MDF (termination, protection, cross-connection), subscriber line units, switching network, control processors, signalling units, trunk equipment, charging/billing, O&M terminals and the −48 V power plant with batteries.

Distribution plant

The distribution plant is the part of the outside plant that carries subscriber pairs from the exchange to individual premises. It is designed in a hierarchy so that cable sizes reduce as they approach subscribers:

1. Main/feeder cable: large cables (600–2400 pairs) leave the MDF through cable vaults and underground ducts to the serving areas.
2. Primary cross-connection point (PCP) or cabinet/pillar: a street cabinet where feeder pairs are jumpered to distribution pairs. It gives flexibility: spare feeder pairs can be used wherever demand grows, so fewer total pairs are needed.
3. Distribution cables: smaller cables (25–200 pairs), underground or aerial on poles, run along streets.
4. Distribution point (DP): a terminal box on a pole, building wall or inside a building, serving 10–20 subscribers.
5. Drop (service) wire: a one- or two-pair wire from the DP to the subscriber's premises, ending in a protector and socket.

Design considerations:

• Forecast demand (5–20 years) to size cables and leave spare pairs.
• Limit loop resistance and attenuation (gauge selection, loading coils on long loops) for good transmission and signalling.
• Use ducts and manholes for easy expansion and repair; aerial cables in rural areas for low cost.
• Protect against lightning, moisture (pressurised or jelly-filled cables) and physical damage.
• Today the same structure is used for fibre access (FTTC/FTTH) with splitters at cabinets and DPs.

Subscriber loop

The subscriber loop is the dedicated pair of wires (local loop) that connects a subscriber's telephone to the local exchange, forming a closed circuit through the telephone when it is off-hook.

Subscriber loop system and cable hierarchy

 Exchange MDF
      │
      │ Feeder / main cable (e.g. 1200 pairs)
      v
 ┌─────────────┐
 │ Cross-connect│ (cabinet / pillar)
 │ point (PCP) │
 └──┬───────┬──┘
    │       │  Distribution cables (e.g. 100 pairs)
    v       v
  [DP]     [DP]   Distribution points (10-20 pairs)
   │ │      │
   Drop wires (1-2 pairs)
   │ │      │
  Tel Tel  Tel   Subscribers

1. Main (feeder) cable: from the MDF of the exchange; large pair count; in ducts.
2. Cross-connection point (cabinet/pillar): jumpers connect any feeder pair to any distribution pair – flexibility and better use of pairs.
3. Distribution (branch) cables: smaller cables running through streets, underground or aerial.
4. Distribution point (DP): pole-mounted or wall-mounted terminal block.
5. Drop/service wire: individual pair from DP to the subscriber's socket.

Loop limits: loop resistance is kept below about 1200–1800 Ω and loss below about 7–8 dB at 800 Hz, so that −48 V feeding, ringing and dial signalling work; thicker conductors or loading coils are used on long loops. The loop carries DC feed, ringing, dialling and two-way speech on the same two wires.

Extending telephone service to a remote area needs careful planning of what services to give and of the fundamental technical plans that make the network work and remain manageable during failures and overloads. Assumed case: a hilly rural area of Nepal with scattered villages, no road access in some places and an unreliable power supply.

Major network services to consider

• Basic voice telephony (fixed and mobile) including public call offices.
• Data and Internet access (mobile broadband, Wi-Fi hotspots) for schools, health posts and local offices.
• Emergency services (police 100, ambulance 102) with priority.
• SMS and value-added services – mobile banking, e-governance, agriculture/market information.
• Leased lines for banks, government offices.

Network plans to prepare (with reasons)

1. Traffic and demand forecast plan: estimate subscribers, calling rate and busy-hour traffic (Erlang) so that the right number of circuits and radio channels is provided (e.g. for 1% GoS).
2. Numbering plan: assign area codes and subscriber numbers that fit the national plan and leave room for growth.
3. Routing plan: decide how calls reach the parent exchange/core – primary route plus alternative routes for overflow and for link failures.
4. Switching plan: choose a remote switching unit or concentrator connected to a host exchange rather than a full exchange, to save cost.
5. Transmission plan: choose media suited to terrain: microwave radio, VSAT/satellite for very remote sites, optical fibre along roads, and wireless access (GSM/LTE base stations, WLL). Set loss and quality limits.
6. Signalling plan: compatible signalling (SS7/SIP) with the national network.
7. Synchronisation plan: clock distribution for digital links.
8. Charging/tariff plan: affordable rural tariffs; use of the Rural Telecommunications Development Fund as per NTA rules.
9. Power plan: solar panels with batteries and backup generators, since grid supply is weak.
10. Network management plan: remote monitoring (NMS), alarms, redundancy (ring topology, standby equipment), traffic control during overload (call gapping, priority to emergency calls), and maintenance teams and spares.
11. Environmental and site plan: tower sites, lightning protection, access roads, rights of way.

Argument: in remote areas traffic is low but cost per subscriber is high, so wireless access and satellite/microwave backhaul with shared switching is the most economical choice; redundancy and good network management are essential because repair trips are slow and the area may otherwise be isolated in emergencies.

DTMF (Dual Tone Multi-Frequency) telephone sets send each digit as the sum of two sine tones – one from a low group (697, 770, 852, 941 Hz) and one from a high group (1209, 1336, 1477, 1633 Hz).

            1209  1336  1477  1633 Hz
   697 Hz    1     2     3     A
   770 Hz    4     5     6     B
   852 Hz    7     8     9     C
   941 Hz    *     0     #     D

Example: key 8 = 852 Hz + 1336 Hz.

Advantages

1. Fast dialling: each digit takes about 50 ms of tone plus 50 ms pause (~100 ms) against up to ~1 s per digit for a rotary dial. Call set-up time and register holding time in the exchange are reduced.
2. End-to-end signalling: tones are in the voice band, so they pass through the connected call – used for IVR, telebanking, voicemail, remote control.
3. More signals: 16 combinations including * and # (and A–D) for supplementary services.
4. Reliability: electronic generation with crystal accuracy; no mechanical governor that wears out or drifts.
5. Error checking: exactly one tone from each group must be present (2-of-8 code), so most faults are detected.
6. Convenient: push buttons are easy, fewer misdialled numbers; suitable for redial and memory features.
7. Suited to electronic/digital exchanges – no DC pulse counting relays.

Disadvantages

1. Needs DTMF receivers at the exchange (digital filters/DSP), adding cost; old Strowger/crossbar exchanges cannot accept it without converters.
2. Talk-off: speech, music or noise may imitate a DTMF pair and cause false digits, so receivers need guard circuits.
3. Talk-down/level problems: on long lines the high-group tone is attenuated more (twist), and noise or echo can stop detection.
4. Needs power and electronics in the set; more complex than a simple dial.
5. Tones are audible and can be recorded or overheard (e.g. PINs) – a security issue.
6. Compatibility: sets often need a pulse/tone switch in areas with old exchanges.

Overall, the advantages far outweigh the disadvantages, so DTMF has replaced pulse dialling in modern networks.

Talk-off

Talk-off is the false detection of a DTMF digit by the exchange's DTMF receiver when speech, music or noise on the line happens to contain energy at a low-group and a high-group DTMF frequency at the same time with enough level and duration. The receiver then wrongly "hears" a digit, which can disturb a call in progress or a call being set up (e.g. IVR menus). Its opposite, talk-down, is when speech or noise stops a real tone from being detected.

Design features to protect against talk-off

1. Choice of frequencies

• Eight tones in two groups: low 697, 770, 852, 941 Hz; high 1209, 1336, 1477, 1633 Hz.
• Each digit uses exactly one tone from each group (2-out-of-8 code). Speech rarely produces two pure, steady tones from both groups simultaneously.
• Frequencies are not harmonically related: no tone is a multiple (2nd, 3rd harmonic) of another, so harmonics of voice or of one tone cannot create a valid pair.
• Intermodulation products (sum and difference frequencies) of any two tones do not fall on other DTMF frequencies.
• Spacing of about 10% between adjacent tones, matching filter selectivity.
• High-group tones lie above the strongest speech energy (speech formant energy is mostly below 1 kHz).

2. Receiver checks (guard features)

• Band-split filters: low and high groups are separated, limited and detected separately; the receiver checks that one and only one tone appears in each group.
• Minimum tone duration (about 40 ms) and minimum inter-digit pause (about 40 ms): short bursts in speech are ignored.
• Frequency tolerance: accept within ±1.5%, reject beyond ±3.5%.
• Signal-to-total-energy (guard) check: the two tones must contain most of the energy in the band; if much energy is present at other frequencies (as in speech), detection is blocked.
• Twist limits: the level difference between high and low tones must be within limits (e.g. high tone up to ~4 dB above or ~8 dB below the low tone).
• Minimum level threshold (e.g. −25 dBm) so weak background signals are ignored.
• Third harmonic/second harmonic checks in some receivers to reject speech.

3. Telephone set features

• Crystal-controlled oscillator gives accurate frequencies and steady amplitude.
• Microphone muted while a key is pressed, so voice is not mixed with tones.
• Proper sending level (about −6 to −8 dBm per tone) and pre-emphasis of the high group by ~2 dB to make up for line loss.
• Standard timing: tone ≥ 40–50 ms, pause ≥ 40–50 ms.

4. Network/exchange features

• The DTMF receiver is normally connected only during the dialling phase and disconnected after call set-up, so conversation cannot cause false digits (except for services that need end-to-end tones).

With these measures, the probability of talk-off becomes very small (standards such as Bellcore require only a few false detections over many hours of recorded speech) while genuine digits are detected reliably.