Chapter 2: Fundamentals of Data
Communications

Learning Objectives
• Explain the concepts of bit rate, baud rate, frequency, and bandwidth
• Distinguish between digital and analog signals
• Describe the purpose of a modem and define three types of modems
• Define the data codes used in data communications
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Learning Objectives
• Describe how a telephone works
• Describe the structure of the telephone network
• Describe the telephone numbering system in the United States
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Introduction
• Data is transmitted from one point to another in data communications
• Data must be in an acceptable format
• Various forms of signals are used to transmit the data
• Telephone system is examined in this chapter
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Signal Representation
• Signal – electrical charges or pulses of light
• Bit rate
– Bit – smallest unit of information
– Binary notation (1 or 0) or (on or off)
– Data codes (ways to represent characters)
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Signal Representation
• Bit rate
– Byte (a character)
– Bit rate (speed at which data is transmitted)
– Bits per second (used to describe bit rate)
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Figure 2-1. Frequency spectrum.
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Signal Representation
• Baud Rate
– Baud (change in signal)
– Baud rate (number of changes per second)
– Voltage (+ and – charges)
– Current (flow of electric charge)
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Chapter 2: Fundamentals of Data Communications 9

Signal Representation
• Baud Rate
– Alternating current (AC) (electrical outlets)
– Direct current (DC) (batteries)
– Signals made up of more than 1 bit
• Dibit (signal made up of 2 bits)
• Tribit (signal made up of 3 bits)
• Used for transmission of large files
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Signal Representation
• Frequency
– Number of times the current goes through a cycle
– Hertz (Hz) (measure of frequency)
– Kilohertz (kHz) (1,000)
– Megahertz (MHz) (1,000,000)
– Gigahertz (GHz) (1,000,000,000)
– Audio frequencies (lowest frequencies)
– See Table 2-1 for list of frequencies
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Signal Representation
• Bandwidth
– Range of frequencies
– BW = High frequency - Low frequency
– Example:
• 2,500 Hz – 200 Hz = 2,300 Hz BW
– Guardbands (prevents frequencies from interfering with each other)
– Wider bandwidth means more data can be transmitted
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Digital Signals Versus Analog
Signals
• Digital data
– Discrete items (text, integers)
• Analog data
– Continuous data (light, voice)
• Frequency modulation
– Signal is changed based on the number of waves
• Process control computers
– Process continuous (analog) data
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Digital Signals Versus Analog
Signals
• Modems
– Convert digital to analog and analog to digital
– MOdulator/DEModulator
– Internal
• Called a “card”, PC cards for laptops
– External
• Sits on the desk next to the computer
• Connected by a cable
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Digital Signals Versus Analog
Signals
• Modems
– Modem speeds
• 9600 to 56,000 bps
• Table 2-2 shows modem costs
– Data compression
• Transmits fewer characters, resulting in faster transmission speeds
• Example: V.42bis (32 chars. to 11 chars.)
• Maximum transmission rate is called the throughput
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Digital Signals Versus Analog
Signals
• Modems
– Always used in pairs
– Internet Service Providers
• Provide connection to users
• Has a permanent Internet connection
– On-line services
• America OnLine, Microsoft Network, Yahoo
• Connection to Internet & other services
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Digital Signals Versus Analog
Signals
• Modems
– Bulletin Board System – for leaving messages
– Cable Modem
• Maximum speeds of 40 Mbps (compare to 56Kbps)
• Higher speeds but at a higher cost than ISP
– Modem eliminator (null modem)
• 50 feet or less
• 1,200 to 38,400 bps
– Short-haul modem
• Up to 20 miles
• 1,200 to 9,600 bps depending on distance
– Fax modem
• Sends fax through a computer
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Data Codes
• General
Description
– Data codes represent characters
– Rules for converting bits to characters
– ASCII, EBCDIC, Unicode
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Data Codes
• American Standard Code for Information
Interchange (ASCII)
– Standard ASCII – 7 bit code
– Extended ASCII – 8 bit code
– Special ASCII characters for data communications – Table 2-4
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Data Codes
• Extended Binary-Coded Decimal
Interchange Code (EBCDIC)
– 8-bit code
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Data Codes
• UNICODE
– 16-bit code
– Supports up to 65,536 characters
– Used for languages such as Chinese,
Japanese, Korean
– Includes mathematical, geometric and other symbols
– Only used with OS that can display the codes
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Telecommunications – Voice
Communications
• Introduction
– Good quality transmission medium
– Telephone network components
• The Telephone
– Converts human voice to electrical signal
– Converters (does the conversion)
– Transmitter and Receiver (both are converters)
– Diaphragm vibrates to convert sound to electricity
• Telephone network supports many features
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Telecommunications – Voice
Communications
• The Telephone Network
– End office (aka central office, exchange office)
– Local loop (connects home to end office)
– Local exchange carrier (provides dial tone)
– T-Carrier Circuits (1.5Mbps rate)
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Telecommunications – Voice
Communications
–Local call (made within local end offices)
• Intraoffice call (same prefix)
• Interoffice call (different prefix)
–Tandem office (handles calls within an area code, unit calls)
–Toll offices (handles calls to a different area code)
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Telecommunications – Voice
Communications
• The Telephone Network
– Telephone lines
• Voice-grade circuits
• Multiplexing used to combine many calls on a single line (uses guardbands to separate calls)
– Used by cable TV as well
– Telephone signals
• Dial tone, ringing, busy, etc.
• Table 2-5 shows types of signals
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Telecommunications – Voice
Communications
• Telephone Numbering Systems
– Direct distance dialing (DDD)
– Area codes (expansion in 1995)
– Exchange codes (prefix in phone number)
– Country codes (identifies other country)
– 800 service (WATS, toll free)
• 888, 877, 866 are now also toll free
– 900 service (caller charged)
– Dial around services (10-10-xxx numbers)
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Communications Trailblazer
• John T. Chambers, Cisco Systems
– Cisco sells routers & switches
– Cisco is successful even when the economy is on a downturn
– Chambers has received many awards
– Previously at Wang labs and IBM
– Has open communication with employees
– Promotes tremendous loyalty among employees
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Spotlight On
• Wireless Technology in Schools
– Wireless technology now in grade and high schools as well as universities
– Uses
• In classroom to do research
• Accessing courses on the Web
• PDAs to access e-mail, Internet, other users, to take notes
– Higher Ed – Classroom on a cart
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Summary
• Signals transmit information
• Data codes convert information
• Transmission speed
• Transmission capacity
• Digital versus analog data
• ASCII code, EBCDIC code, UNICODE
• Telephone network
• Quality of telephone lines
• Telephone numbering system
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