1. Chapter Eight: The Telephone System

2. 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

3. Public Switched Telephone Network
The topology of a local calling area (local access and transport area, or LATA) is indicated in the figure below
Insert fig. 8.1

4. 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

5. Hierarchical Switched Network

6. Nonhierarchical Long-Distance Network
Insert fig. 8.3

7. 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

8. 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

9. Local Loop

10. 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

11. 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

12. 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

13. 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

14. Simple Telephone System
A single twisted-pair line is required to carry both sides of the conversation, thus providing full-duplex communication

15. 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

16. 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

17. 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

18. FDM Hierarchy

19. Generation of a Group

20. 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

21. Time-Division Multiplexing
DS-1 lines can be used for voice or data transmission using TDM techniques

22. 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

23. 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 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

24. ISDN Access

25. 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