Telecommunications

Objectives:

Determine the principles of sound techniques.Explain the transmission of sound.Define the function of telephone transmitters.Explain the function of telephone receivers.Identify the components and functions of the telephone.

History

•Australia First telegraph began working in 1854

•1870 international system were established

•Telex machines were a descendant from telegraphy

•Telex was to the first major communication system to employ digital encoding techniques

•1876 Alexander Bell demonstrated the first telephone

• 1880 a point to point telephone link was installed in Melbourne

•1912 the first automatic telephone exchange was working

•1956 a prototype subscriber trunk dialling facility was introduced

SegregationSegregation of telecommunications cabling from power cabling is important for the general safety of customers, cabling providers and carrier staff.

A communication cable and electrical cable without separation suffer simultaneous sheath and insulation penetration from nails, screws, drills, saws and sharp objects. The result could be a dangerous voltage appearing on the communications cable.

A communication cable touches a hazardous service, such as pipes containing corrosive or flammable liquid or gas, or when there are temperatures higher than 60 degrees Celsius, causing damaging to a communication cable.

A communication cable is pulled across other surfaces causing sheath burning due to the heating effect of friction. This may cause the sheath and insulation to melt and compromise the service. Simultaneous burning of communication cables and power cables is the greatest concern

Industry regulators•1989 Telecommunications Act deregulated the telecommunications sector, enabling licensed providers to undertake cabling work formerly undertaken by Telecom

•Australian Telecommunication Authority , Austel, was established.•Cabling licences were established, Base and restricted.

•1997 the Australian Communications Authority ,ACA, was established and was responsible for regulating telecommunications and radio.

•2005 The ACA merged with Australian Broadcasting Authority to become the Australian Communication and Media Authority, ACMA.•See page 446 Hampson for The 10 CPRs

Principle of sound

• Sound is the vibration of matter as perceived by the sense of hearing.

• Sound is characterised by the generic properties of waves:-

• Frequency• Wavelength• Period amplitude• Intensity• Speed • Direction

Principle of sound• A telecommunications system must convert sound , an acoustic energy, into an

electrical signal.

• Noise is any unwanted signal existing with the wanted signal

• Distortion is any unwanted change in the shape of the information signal

• Attenuation is the reduction in amplitude and intensity of a signal

• Audible frequencies for human hearing is between 20Hz and 20kHz

• Pitch. The most noticeable difference between various tonal sounds is that some sound higher or lower than others.

• The intensity of sound relates to the energy of the sound waveform.

• The decibel (dB), is a logarithmic unit of measurement that expresses the magnitude of a physical quantity relative to a specific or implied reference level.

Sound waves•The frequency and intensity of the sound determine the limits of audibility

•The internationally agreed voice band frequency is from 300Hz to 3400Hz .

•Resonance is the tendency of a system to oscillate at maximum amplitude at a specific frequency – resonance frequency

•Rarefaction moves backwards

•Compression moves forward

•Velocity of sound is 330 metres per seconds in air and 1490 m/s seawater

Basic function of a telephone

Basic function of a telephone

The telephone has 4 functions

1. Incoming ring

2. Incoming voice

3. Outgoing voice

4. Outgoing signalling

Transmitters

The function of a transmitter is to convert acoustical energy generated from speech into an electrical energy.

A telephone transmitter should have a directional response pattern that is unidirectional.

Telephone transmitter typesCarbon TransmitterElectret TransmitterDynamic TransmitterPiezo-electric Transmitter

Transmitters

Telephone Receivers

The telephone receiver is a transducer that converts electrical energy into acoustic energy (or sound).

Telephone receiver types

Magnetic diaphragm receiverRocking armature receiverDynamic receiverPiezo-electric receiverloudspeaker

Telephone Circuits

Telephone CircuitsThe dialling circuit – is responsible for sending dialling signals to the local exchange (note 48V dc)

Incoming signalling circuit – is known as the telephone ringer, Ring signal – is the incoming ac signal , typically around 90v rms at 25Hz, from the exchange

Answer circuit – to answer the call and stop the ringing

Release circuit – placing the handpiece onto the cradle (hanging up)

Outgoing signalling circuit – generates the number of the called party.

Transmitter and Receiver circuit – consists of a transmitter and receiver connected in series to provide transmitting and receiving features of a telephone.

Earthing

Earthing

Equipotential bonding – minimises risk associated with voltage differences between the accessible metallic parts of the electrical equipment and accessible metallic parts not associated with the electrical equipment Clause 20.11 As/ACIF S009:2006

Communications earth terminal (CET) –must be installed in a convenient and readily accessible location. Clause 20.11.3 AS/ACIF S009: 2006

Communications earth system (CES) – is a system of earthing using common elements to provide earthing facilities for electrical and communications equipment within a premise, includes protective and functional earths. Clause 20.12 AS/ACIF S009:2006

EarthingTelecommunications reference conductor system ( TRC) – exclusive to telecommunications services, primary purpose is to provide a low –noise earth reference. Clause 20.13 AS/ACIF S009:2006 Cables for TRC must violet.

Telecommunications functions earth electrode (TFEE) – is the earth stake for the telecommunications system, must comply AS/NZS 3000:2007

TFEE – a number of items suitable for use as a TFEE see Clause 20.13..8.2 AS/ACIF S009:2006

Surge suppression – The effects may range from minor noise to disruption of the service, note avoid using telephones during thunderstorms. Devices for protection must connect to a compliant earth reference through a green/yellow cable Clause 20.20 AS/ACIF S009:2006

Customer Switching Systems

Customer Switching Systems(CSS)

Customer access equipment (CAE) – these follow the public switched telephone network (PSTN) systems. See page 527

CAE – is a multiple-port equipment installed in a customer’s premises.CAE capabilities – provides access to the PSTN,

capable of switching, storage, processing, conversion, integration, line isolation/coupling or multiplexing analogue or digital voice.

Several types of CAE include PABX, key systems, line isolators and more

Private Automatic Branch ExchangePABX

• PABX is a telephone exchange that serves a business or office.

• Is installed at the customer’s premises

• Is connected to the PSTN via trunk lines

• Makes connection between the internal telephones

Private Automatic Branch ExchangePABX

The PABX perform 3 mains duties 1. Establishing connections between the telephone sets of two

users 2. Maintaining such connections as long as the users require

them3. Providing information for accounting purposes

KEY SYSTEMSAre typically small private telephone systems

The difference between a PABX and a key system is, the user of a key system can manually select an outgoing line, while a PABX selects the outgoing line automatically.

Comprises of telephones (stations) and a main equipment.

Note:- The PABX, key phone systems and hybrid phone systems are Customer Switching Systems (CSS) but are part of the Customer Access Equipment (CAE) area.

CSSTesting

End to end continuity testing is simple and effective for detecting wiring errors and short or open circuits.

Measuring the resistance of a conductor and comparing with the manufacturer's speciation's.

Measuring the resistance of the cable insulation by connecting an insulation resistance tester to the isolated cable

Tools Effset, continuity tester, insulation tester and a Field telephone

Hazards

Printed Boards

Static electricity –discharging static electricity into printed board may cause harm to electronic components.

Safe Handling1. Ensure power is off2. Bring body to same potential as the equipment. Use a anti

static strap, anti static floor mats.3. Handled PBAs by the edges 4. Do not touch any component with fingers5. Keep PBAs out of sunlight and ultraviolet light.