IT101 Section 001

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IT-101Section 001
Introduction to Information Technology

• Lecture 10

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• Overview

• Chapter 13
• The Telephone System Wired and Wireless
• Analog Telephone system
• Digital telephone system
• Cellular telephone system

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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

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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

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Telephone system components
Signaling and switching system
Transmission system
receiver
microphone
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• 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

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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 to
  determine that they were made 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 able to
  process analog information

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• 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

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• For voice, remember 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
  8,000x864,000 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 about this
  in a later chapter

bits
1/sHz
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• 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

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The Cellular Telephone System

• The cellular telephone system is different from
  the previous systems that we learned, 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

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The Cellular Concept
Base station
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An example cellular telephone system
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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 (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

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MTSO Mobile Telephone Switching Office
PSTN Public Switched Telephone Network
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• 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
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• 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 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)

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• Comments for next class

• Finish chapter 13
• Generations of cellular systems
• Satellite telephones
• Chapter 16
• Radio-Frequency and Satellite Systems
• -Satellite systems
• -GPS