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SIMS-201

SIMS-201. The Telephone System. Wired and Wireless. Overview. Chapter 13 The Telephone System: Wired and Wireless Analog Telephone system Digital telephone system Cellular telephone system. Introduction.

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SIMS-201

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  1. SIMS-201 The Telephone System Wired and Wireless

  2. Overview Chapter 13 • The Telephone System: Wired and Wireless • Analog Telephone system • Digital telephone system • Cellular telephone system

  3. Introduction • Now that we have learned what information is, how to represent it, and how to convert it from analog to digital form, we can now learn the techniques and systems used to transmit this information • The oldest system used to transmit information globally is considered to be the telephone system • We will specifically learn about the analog and digital telephone system, as well as the cellular telephone system

  4. The Analog Telephone System • The analog system was the first telephone system established worldwide. Currently, telephone systems in a lot of countries are still completely analog. In time however, these systems will become redundant as the world switches to digital telephony. • The components of a telephone system include: • Microphone • Receiver • Transmission System • Switching and signaling system

  5. Telephone system components Signaling and switching system Transmission system receiver microphone

  6. Microphone • Converts the vibrations in the air into an electrical signal • Receiver • Converts the received electrical signal into sound waves (the reverse action of a microphone) e.g.: loudspeaker • Transmission system • Conveys the information representing the audio signal from the microphone to the receiver • Signaling and switching system • Determines and makes appropriate connections among the pieces of the transmission system to create a path from the transmitter to the receiver

  7. The Digital Telephone System • While the description of the analog telephone system provides an accurate overview of the principles of current telephone systems, it is a fact that most telephone calls today are really digital telephone calls • In a digital telephone system, the two ends of the call are analog, and the middle section is digital. Conversions from analog to digital (A/D), and back to analog (D/A), are made in such a way that it is essentially impossible for human ear to determine that there was any conversion at all • Although the analog telephone system is gradually being converted to digital, the input and output of the system still remains analog because the eventual use is for humans that are only able to process analog information

  8. At present, most telephone calls are analog from the telephone at home to the first switching office, so the A/D and D/A conversion is made at this office • In the future, as telephone systems become all digital, this conversion from A/D and from D/A will be made within the telephone set at home • The A/D conversion process was explained in the previous lectures- The voice signal- an analog waveform was sampled at a sampling frequency, and quantized to a number of levels. These values were then assigned binary codes to complete the conversion process from analog to digital • The D/A process was also explained briefly. The bits were decoded into their quantized values, and a waveform similar to the original analog waveform was obtained

  9. For voice, we recall that the standard sampling frequency is 8000Hz • The standard number of quantization levels for audio signals is 256, requiring 8 bits • So, the bit rate for a digital telephone call is: 8,000x8=64,000 bits per second (64 Kbps) • This is the bit rate that would reach the central office if the A/D conversion was being done inside the telephone at home • Since many calls arrive at the central office, they can all be combined, and switched to another center to be routed to the destination • Combining many channels and sending them simultaneously through a single transmission line is called multiplexing. We will learn more about this in a later chapter 1/s=Hz bits

  10. One advantage of digital transmission, is that after digitization, all types of information are in the form of bits, so a single system, such as a telephone system can be used to carry telephone calls, internet data or any other data at a suitable bit rate

  11. The Cellular Telephone System • The cellular telephone system is different from the previous systems that we discussed, because the major transmission medium is air instead of wires (between the mobile unit and the base station) as in the analog and digital telephone systems • In a cellular system, the signal from a mobile unit (cell phone) to a base station is transmitted by radio waves through the air, instead of through metallic wires • However, the signal from the base station is sent to a mobile switching center and possibly to a telephone central office through electrical wires where it is switched to the appropriate destination • The antenna at the base station converts the radio waves to electrical signals and circuits in the base station send the signal to the appropriate mobile switching center

  12. The Cellular Concept Base station

  13. An example cellular telephone system

  14. How cellular telephone systems work • The area (a city, or a part of town) is divided into a number of cells (typically 2 to 10 miles in size, but can be smaller for more crowded areas) and a base station is positioned within each cell • If a user (mobile phone) is within a particular cell, the call is handled by the corresponding base station within that cell • The base station transmits the signal to the mobile switching center (also called MTSO), which switches the signal to another base station, or to a Public Switched Telephone Network (PSTN), depending on the destination of the call: whether another mobile unit or a regular telephone • As a user moves from one cell to another, the call is “handed over” to the base station of the other cell-This is called hand-off • The handover is (hopefully) transparent to the user

  15. MTSO Mobile Telephone Switching Office PSTN Public Switched Telephone Network

  16. The mobile unit and the base station in a cell communicate at a certain frequency • The signal from the mobile unit arrives at the antenna of the base station and is converted into an electrical signal Base station antenna (3 sector) 1/3rd of cell is covered by each sector of antenna

  17. Every cell uses a different set of frequencies • So how does the phone “know” what frequency to be on? • A Cell-Site Controller handles this process • When a cell phone is turned on, it registers with the network and guards a control frequency • When a call is placed, the phone requests that a frequency (really two) be assigned • When a call is received, the call is set up over the control channel (find the right phone, tell it what frequencies to use, connect the call)

  18. Comments for next class • Finish chapter 13 • Generations of cellular systems • Satellite telephones Chapter 16 • Radio-Frequency and Satellite Systems -Satellite systems -GPS

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