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Chapter
6
Telecommunications
and
Networks
Overview of Communication
Communications is the transmission of a signal by
way of a medium from a sender to a receiver.
Telecommunications
Electronic transmission of signals for
communications
Telephone
Radio
Television
Lessens barriers of time and distance
Telecommunications
Data communications
Specialized subset of telecommunications
Includes the electronic collection, processing, and
distribution of data
Telecommunications media
Anything that carries an electronic signal and interfaces
between a sending device and a receiving device
Elements of a Telecommunications
System
[Figure 6.3]
Analog and Digital Signals
Analog
Continuous fluctuations over time between high and low
voltage
Digital
A discrete voltage state
Analog and Digital Signals
Analog
signal
Fig. 6.4
Digital
signal
Analog Waves
Frequency
The number of times per second a continuous wave
completes one cycle
Amplitude
The difference of the extremes from the midpoint in a
cycle
Basic Modulation Techniques
Amplitude modulation (AM)
Converts digital data to analog signals using a single
frequency carrier signal
High-amplitude wave denotes a binary 1
Low-amplitude wave denotes a binary 0
Basic Modulation Techniques
Frequency modulation (FM)
Uses a constant amplitude carrier signal and two
frequencies to distinguish between 1 and 0
Phase modulation
Uses a phase shift at transition points in the carrier
frequency to represent 1 or 0
Communications Speed
Bits per second (bps)
Number of bits that can be transferred in one second
Bandwidth
Range of signal frequencies that can be sent over a
medium at the same time
Broadband
Bandwidth of 256,000 to 1 million bps
Types of
Communications Media
Twisted wire cable
Coaxial cable
Fiber-optic cable
Microwave transmission
Cellular transmission
Infrared transmission
Twisted Wire Cable
A cable consisting of pairs of twisted wires
The twist helps the signal from “bleeding” into
the next pair
Coaxial Cable
Inner conductor wire surrounded by insulation,
called the dielectric
Dielectric is surrounded by a conductive shield,
which is in turn covered by a layer of
nonconductive insulation, called the jacket
Fiber-Optic Cable
Consists of many extremely thin strands of glass
or plastic bound together in a sheathing
Transmits signals with light beams
Wireless Transmission
Microwave: Terrestrial
Microwave: Satellite
Cellular
PCS
Telecommunications Devices
Modems
Fax modems
Special-purpose modems
Multiplexers
Communications processors
How a Modem Works
Multiplexers
Allow several telecommunication signals to be
transmitted over a single communications
medium concurrently
Time division multiplexer (TDM)
Multiple incoming signals are sliced into small time
intervals
Frequency division multiplexer(FDM)
Incoming signals are placed on different frequency ranges
Time Division and Frequency
Division Multiplexers
[Figure 6.14]
Communications Processors
Called front-end processors because they are in
front of and before the main system CPU
Incoming and
outgoing jobs
[Figure 6.15]
Main
system
Front-end
processor
Telecommunications
Carriers and Services
Common carriers
Long-distance telephone companies
Value-added carriers
Companies that have developed private
telecommunications systems and offer their services for a
fee
Telecommunications
Carriers and Services
Switched line
Standard telephone line that uses switching equipment to
allow one transmission device to be connected to others
Dedicated line
Provides a constant connection between two points
Also called a leased line
Telecommunications
Carriers and Services
Private branch exchange (PBX)
Manages both voice and data transfer within a building
and to outside lines
Wide area telephone services (WATS)
A billing method for heavy users of voiceband media
Phone and dialing services
Additional services from the common carriers (i.e. ANI)
Telecommunications
Carriers and Services
Integrated services digital network (ISDN)
Technology that uses existing common-carrier lines to
simultaneously transmit voice, video, and image data in
digital form
Digital subscriber line (DSL)
Businesses/homes that have a DSL over existing phone
wires can use a special modem that transmits up to 768
Kbps
Telecommunications Act
of 1996
Completely rewrites the 62-year-old
Communications Act
Goal: To promote competition and eliminate
monopolies for a wide range of communication
services
May cause profound changes in U.S.
communications that can lead to a complete
renovation in the way information is delivered to
consumers
Global Telecommunications
One of the largest and fastest growing sectors of
the global economies
Criticality of implementing and managing global
networks
Increase of international acquisitions and mergers
Need to compete on a global basis
Computer Network
Includes communications media, devices, and
software needed to connect two or more computer
systems and/or devices
Benefits of a Network
Can turn a company into an agile, powerful and creative
organization, giving it long-term competitive advantage
Can be used to share hardware, programs, and databases
across the organization
Basic
Data Processing Strategies
Centralized processing
All processing occurs in a single location
Decentralized processing
Processing devices are placed at various remote locations
Distributed processing
Computers at remote locations are connected to each other
via telecommunications devices
Network Concepts
and Considerations
Network topology
Logical models that describe how networks are structured
or configured
Star network
A network that has a central hub or computer system
Four Basic Types
of Network Topology
(a) ring
(b) bus
(c) hierarchical
(d) star
[Figure 6.17]
Types of Networks
Local area networks (LAN)
Connect computer systems and devices within the same
geographical area
Regional networks
Tie regional areas together via telecommunications
systems
Types of Networks
Wide area networks (WAN)
Tie large geographic regions together using microwave
and satellite transmission or telephone lines
Private WANs
Computer equipment owned by users and data
communications equipment provided by a common carrier
Value-added networks (VAN)
Offer services in addition to those offered by a traditional
network (e.g., CompuServe)
International networks
Link system together between countries
Terminal-to-Host
“Dumb”
terminal
Fig 6.20
Applications and databases reside on the same
host computer
User interacts with the application using a
“dumb” terminal
LANs and WANs
[Figure 6.18]
A Typical LAN in a BUS Topology
[Figure 6.19]
A Wide Area Network
File Server
Fig 6.21
 Applications and databases reside on the same host
computer
 File server transfers data and programs to PCs on the
network, which do the processing
Client/Server
Fig 6.22
 Applications and databases reside on specialized host
computers
 Servers do most or all of the processing and transmit
the results to the client
The Client/Server Approach
Advantages
 Cost potential
Improved performance
Increased security
Disadvantages
Increased cost potential
Loss of control
Complex multivendor environment
Functions of
Communications Software
Error checking
Message formatting
Communications logs
Data security and privacy
Translation capabilities for networks
Communications Software
Network operating systems
Systems software that controls the computer systems and
devices on a network and allows them to communicate
with each other
Network management software
Software tools and utilities available for managing
networks
Communications Protocols
 Open systems interconnection (OSI)
 A protocol (or standard) endorsed by the International Standards
Committee
 TCP/IP
 Developed by the U.S. government to link its defense research
agencies
 SNA
 IBM’s communications protocol
 Ethernet
 A popular protocol often used with local area networks
 Other protocols
Bridges, Routers,
and Gateways
Bridge
Connects two or more networks
Router
Features more sophisticated addressing software than bridges
Gateway
Links LANs or networks that employ different higher-level
protocols
Bridges, Routers,
and Gateways
[Figure 6.24]
Coordinating Communications and
Networks
Polling
A procedure in which the main computer checks each
device one at a time to see if there are any messages
waiting to be communicated to other network devices
Token passing
A technique in which an electronic “token” is passed from
one device to another
Contention
A procedure in which a device checks to see if a message
is currently being transmitted
Data Transfer Modes
Packet switching
Communications process that divides electronic messages
into small segments (data packets)
Frame relay
Uses wideband communications media and high speed
switching devices
Voice-over frame relay
Moves voice traffic onto frame relay to bypass the public
telephone network
Data Transfer Modes
Asynchronous transfer mode (ATM)
A high-speed network technology that provides transport
for voice, video, text, and other multimedia data
Fiber distributed data interface (FDDI)
Designed specifically to serve the needs of highperformance, mission-critical LAN applications
Telecommunications Applications
Links between PCs and mainframes
Facilitates the upload or download of data
Voice storage and forwarding
Sometimes called voice mail
Allows users to leave, receive, and store verbal messages
for and from people around the world
Telecommunications Applications
Electronic mail
Users connected to a network can send messages to others
by computer
Electronic document distribution
Involves transporting documents over communications
lines and networks
Telecommunications Applications
Electronic software distribution
Involves installing software on a file server for users to
share by signing onto the network and downloading the
software onto their computers over the network
Telecommuting
Working at home on a PC and communicating with the
central office or other workers via networks
Telecommunications Applications
Teleconferencing
Allows groups to have conferences in one connected voice
phone call
Videoconferencing
An extension of teleconferencing that combines voice,
video, and audio transmission
Telecommunications Applications
 Electronic data
interchange (EDI)
 Uses network systems and
follows standards and
procedures that allow output
from one system to be
processed directly as input to
other systems, without human
intervention
 [Figure 6.27]
Telecommunications Applications
 Public network
services
 Give PC users access to
vast databases and other
services, usually for an
initial fee plus usage fees
[Figure 6.28]
Telecommunications Applications
Specialized information systems
Include legal, patent, and technical information
Regional information systems
Include local electronic bulletin boards and electronic mail
facilities that offer information regarding local club,
school, and government activities
Distance learning
Extends the classroom by using telecommunications
Network Design Considerations
Reliability
Security
Response time
Privacy issues
Vendor services and support
Company support
Communications Audits
Purpose:
To make sure the communications
system is operating as intended
Types of communications audits:
Walk-thorough
Software check
Network check
Policy and procedures
Program and database check