Chapter Eight: The Telephone System

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 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

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