Transcript Blake`s Slides Chapter 8 File

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 stepby-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 highspeed data while
leaving the use of the
local loop for analog
telephony intact
• Typical ADSL uses
include Internet access
and interactive
television