The Telephone System

1. The Telephone System Lecture 5

2. The Telephone System The modern telephone system draws from these Electrical Engineering subdisciplines: • Signal processing: Speech compression, noise reduction, A/D and D/A conversion.. • Communications and networking: transmission technologies, network architectures and protocols. • Digital and computer: configurable switching hardware. • Electromagnetics: microwave transmission hardware. • Solid state: miniaturization, integration of complex systems onto a single chip. • Power Electronics: extremely reliable power supplies. Lecture 5

3. Old Versus New • The early telephone system provided (what today is know as) POTS-”plain old telephone service”. • The only service provided by the early telephone system was voice transmission. • The modern telephone system provides voice transmission as well as a host of other services: • data transmission and video transmission • sophisticated billing and feature capabilities such as call waiting and call forwarding. Lecture 5

4. Speaker Mic. Switchboard Speaker Mic. Speaker Mic. Telephone Power Supply Telephone Central Office Telephone An Early Phone System Lecture 5

5. The Early Phone System • The major components of a telephone were a carbon microphone and a speaker made from an electromagnet and a paramagnetic diaphragm. • Telephones were connected to the central office by twisted-pair wires. • At the central office, calls were completed by a human operator at a switchboard-a physical connection between two telephones was made. Lecture 5

6. Earphone Earphone Battery Carbon Microphone Carbon Microphone Telephone Handset Telephone Handset Central Office An Early Phone Circuit Lecture 5

7. Electrical current flows in this circuit in a loop from the battery at the central office, through the components of the two telephones (the speaker and the microphone), and back into the battery. This circuit is a series connection of the components in the two telephones and the battery. All of the current that flows through the battery also flows through the components in the two telephones. The Phone Circuit Lecture 5

8. The microphone consists of loosely packed carbon granules in a box with a diaphragm on one side The electrical resistance of the carbon in the box is related to the displacement of the diaphragm-when the carbon granules are compressed, the resistance is reduced. Thus, the microphone converts changes in pressure to changes in resistance. The microphone is modeled electrically as a variable resistor. Microphone Lecture 5

9. The speaker was made from an electromagnet and a paramagnetic diaphragm. Changes in the current flowing through the electromagnet result in changes of the magnetic field strength, which in turn results in a change of the position of the diaphragm. Thus, the speaker converts changes in current to movement of a diaphragm which produces sound energy. The speaker is modeled electrically as an inductor. Speaker Lecture 5

10. Switchboard: the switchboard connects two telephones electrically. Battery: the battery provides the power necessary to create an electrical current flowing in the loop. Central Office Lecture 5

11. The Modern Telephone System • Fundamentally, the modern telephone systems appears much the same as the early system to handset users. • There are very significant differences: • Digital data, video, and other signals are transmitted along with speech. • Calls are routed automatically under software control. • Most transmission is digital. Lecture 5

12. PCM Encoder PCM Decoder Switching Network PCM Decoder PCM Encoder Analog Analog Digital A Modern Telephone Connection Lecture 5

13. time Analog Vs. Digital • An analog signal is a continuous-time signal: • A digital signal is a sequence of 1’s and 0’s: 1101001010011100100110001001110 Lecture 5

14. Why Digital? • Transmission over long distances degrades both analog and digital signals-digital signals can be “cleaned up”, allowing repeaters to be used without any signal distortion. • Can mix many types of information (phone, video, data, etc.) • Digital hardware is less expensive. • Digital data can be encrypted. Lecture 5

15. PCM-Pulse Code Modulation • A PCM encoder converts an analog signal into a digital signal with a particular format. • A PCM decoder converts a digital signal into an analog signal. • PCM is one form of quantization. • PCM is one form of analog-to-digital (A/D) conversion. Lecture 5

16. PCM Encoder A continuous signal is converted into a bit stream: 0000010100000000111111 Involves three operations: Sampling, Quantization, and Encoding Lecture 5

17. time Sampling Value of the signal is obtained at equally spaced points in time: Lecture 5

18. output voltage input voltage Quantizer • Each sample is quantized to one of a finite number of values. Quantizer input/output relationship: Lecture 5

19. Encoding • A pattern of bits is assigned to each possible output level of the quantizer. • n bits can represent 2n quantizer output levels. Lecture 5

20. PCM Decoder PCM decoder is one type of digital-to-analog (D/A) converter. 0000010100000000111111 Lecture 5

21. Telephone Network • A house or business is called a subscriber. • Typically, phone lines to houses or small businesses are analog twisted-pair wire connections. • Subscribers’ analog lines are connected to a Regional Terminal (RT) or to a Central Office (CO). • At the RT or CO, the analog signal is converted to a digital signal. Lecture 5

22. Subscriber RT Long-distance Network Subscriber CO Subscriber RT Subscriber Network Architecture Lecture 5