Chapter Eight:The Telephone System

Introduction• The public switched telephone system is the largest and

most important communication system in the world

• Public refers to the idea that anyone can connect to it; switched indicates that anyone can connect to anyone else

• Though originally designed for voice communications, telephone networks have been adapted to serve data communications, facsimile, and video

Public Switched Telephone Network• The topology of a local calling area (local access and

transport area, or LATA) is indicated in the figure below

LATA Topology• Each subscriber is connected via a local loop• Each local loop is connected to a central office• Central offices are connected to one another via trunk lines• If too many users connect at the same time, call blocking will occur• Tandem offices connect central offices without having direct

connecting to individual telephones• Long-distance calls used to be routed through toll stations• A flat network usually lets the system find a direct route from one

area of the country to another• Each long-distance carrier has a point-of-presence (POP) to the local

telephone system

Hierarchical Switched Network

Nonhierarchical Long-Distance Network

The Local Loop• Ordinary telephone systems are often referred to as POTS

(plain old telephone service)

• Normally, each subscriber is connected to the central office by a single twisted pair of wires

• The wires are twisted to reduce crosstalk

• Future developments include the inclusion of fiber-optic connections direct to the subscriber for greater bandwidth

Signals on the Local Loop• A phone not in use is referred to as on the hook• The central office maintains a voltage of about +48 volts

across the line• A telephone on the hook appears as an open to the central

office• When the telephone is in use, a current flows in the loop• The presence of this current signals the central office to make

a line available (seizing the line)• A telephone off the hook drops a voltage between 5 and 10

volts across it

Local Loop

Dialing• Dialing can be accomplished in two ways:

– Pulse dialing uses interruptions in the current loop to dial a number– Dual-tone multifrequency dialing (DTMF) uses two tones for

dialing. Also known as touch dialing

The Central Office Switch• Early telephone switchboards

were manually operated and used patch cords

• The first automatic telephone switch was the Strowger step-by-step switch

• The crossbar switch superseded the Strowger switch

• The crosspoint switch allows the connection of any incoming line to any outgoing line

The Subscriber Line Interface Card• The local loop connects to the central office by means of a

subscriber line interface card (SLIC or line card)• The functions of the card are:

– Battery supply– Overvoltage protection– Ringing– Supervision (monitoring hook status)– Coding– Hybrid– Testing

The Telephone Instrument• Ordinary telephones use carbon

microphones and magnetic earphones (transmitters and receivers)

• The carbon microphone needs DC bias current to operate

• Carbon microphones are reliable and simple but have poor audio quality

• Modern telephones often use electret condenser microphones to achieve better quality

Simple Telephone System• A single twisted-pair line is

required to carry both sides of the conversation, thus providing full-duplex communication

Signals and Noise in the Telephone System

• Again, two of the most important characteristics of any communication system are bandwidth and signal-to-noise ratio

• The bandwidth of a voice-grade telephone system is about 3.2 kHz

• Noise in a system increases in relation to the distance from the source

• Repeaters are used to increase the signal level and improve the signal-to-noise ratio

Frequency-Division Multiplexing• Analog telephone signals can be combined (multiplexed)

using FDM• The channel can be a twisted-pair, a coaxial cable, a

microwave radio link, or satellite• In FDM, the spectrum is divided among a number of

information signals• In FDM telephony, the modulation is typically SSB or

SSBSC, and 4 kHz of spectrum is allocated to each conversation

FDM Hierarchy• The number of conversations transmitted using FDM

depends upon the total bandwidth available

• The channels are grouped according to a hierarchical structure:– Group - lowest level

– Supergroup - 5 groups

– Mastergroup - 10 supergroups

– Jumbogroup - 6 mastergroups

– Superjumbogroup - 3 jumbogroups

FDM Hierarchy

Generation of a Group

Digital Transmission

• Over the past 30 years, telephone systems have gradually been converted to digital technology

• PCM is a typical scheme for digital transmission along telephone lines

Time-Division Multiplexing• DS-1 lines can be used for voice or data transmission using

TDM techniques

Telephone-Network Signaling• Local-loop signals such as DTMF tones, dial tones, busy

signals, and ringback signals are called in-channel signals because they use the same channel as the voice, only at different times

• These signals are AC and and in the same range as voice signals and are also referred to as in-band signals

• DC signals and currents such as on/off-hook signals are referred to as out-of-band because DC is not part of the same frequency range as a voice signal

Integrated Services Digital Network (ISDN)

• ISDN is designed to allow voice and data to be transmitted along the same lines

• ISDN provides a way to standardize data and voice communications without resorting to modems

• ISDN connects at a primary access point with a data rate of 1.544 Mb/s

• One of these channels is the D (data) channel and is used for setting up and monitoring calls

• The other 23 channels are called B (bearer) channels and can be used for voice or data

ISDN Access

Asymmetrical Digital Subscriber Line (ADSL)

• ADSL lines use the frequencies above the voice range for high-speed data while leaving the use of the local loop for analog telephony intact

• Typical ADSL uses include Internet access and interactive television