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Telecommunications
and
Networks
Chapter
6
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Overview of Communication
Communications is the transmission of a signal by way of a medium from a sender to a receiver.
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Telecommunications
Electronic transmission of signals for communicationsTelephoneRadioTelevision
Lessens barriers of time and distance
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Telecommunications
Data communicationsSpecialized subset of telecommunicationsIncludes the electronic collection, processing, and
distribution of data
Telecommunications mediaAnything that carries an electronic signal and interfaces
between a sending device and a receiving device
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Elements of a Telecommunications System
[Figure 6.3]
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Analog and Digital Signals
AnalogContinuous fluctuations over time between high and low
voltage
DigitalA discrete voltage state
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Analog and Digital Signals
Fig. 6.4
Analog signal
Digital signal
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Analog Waves
FrequencyThe number of times per second a continuous wave
completes one cycle
AmplitudeThe difference of the extremes from the midpoint in a
cycle
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Basic Modulation Techniques
Amplitude modulation (AM)Converts digital data to analog signals using a single
frequency carrier signalHigh-amplitude wave denotes a binary 1Low-amplitude wave denotes a binary 0
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Basic Modulation Techniques
Frequency modulation (FM)Uses a constant amplitude carrier signal and two
frequencies to distinguish between 1 and 0
Phase modulationUses a phase shift at transition points in the carrier
frequency to represent 1 or 0
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Communications Speed
Bits per second (bps)Number of bits that can be transferred in one second
BandwidthRange of signal frequencies that can be sent over a
medium at the same time
BroadbandBandwidth of 256,000 to 1 million bps
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Types of Communications Media
Twisted wire cableCoaxial cableFiber-optic cableMicrowave transmissionCellular transmissionInfrared transmission
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Twisted Wire Cable
A cable consisting of pairs of twisted wires
The twist helps the signal from “bleeding” into the next pair
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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
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Fiber-Optic Cable
Consists of many extremely thin strands of glass or plastic bound together in a sheathing
Transmits signals with light beams
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Wireless Transmission
Microwave: TerrestrialMicrowave: SatelliteCellular
PCS
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Telecommunications Devices
ModemsFax modemsSpecial-purpose modemsMultiplexersCommunications processors
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How a Modem Works
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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
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Time Division and Frequency Division Multiplexers
[Figure 6.14]
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Communications Processors
Called front-end processors because they are in front of and before the main system CPU
[Figure 6.15]
Mainsystem
Front-endprocessor
Incoming andoutgoing jobs
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TelecommunicationsCarriers and Services
Common carriersLong-distance telephone companies
Value-added carriersCompanies that have developed private
telecommunications systems and offer their services for a fee
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Telecommunications Carriers and Services
Switched lineStandard telephone line that uses switching equipment to
allow one transmission device to be connected to others
Dedicated lineProvides a constant connection between two points Also called a leased line
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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 servicesAdditional services from the common carriers (i.e. ANI)
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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
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Telecommunications Actof 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
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Global Telecommunications
One of the largest and fastest growing sectors of the global economies
Criticality of implementing and managing global networksIncrease of international acquisitions and mergersNeed to compete on a global basis
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Computer Network
Includes communications media, devices, and software needed to connect two or more computer systems and/or devices
Benefits of a NetworkCan turn a company into an agile, powerful and creative
organization, giving it long-term competitive advantageCan be used to share hardware, programs, and databases
across the organization
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Basic Data Processing Strategies
Centralized processingAll processing occurs in a single location
Decentralized processingProcessing devices are placed at various remote locations
Distributed processingComputers at remote locations are connected to each other
via telecommunications devices
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Network Concepts and Considerations
Network topologyLogical models that describe how networks are structured
or configured
Star networkA network that has a central hub or computer system
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[Figure 6.17]
Four Basic Typesof Network Topology
(a) ring (b) bus
(d) star(c) hierarchical
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Types of Networks
Local area networks (LAN)Connect computer systems and devices within the same
geographical area
Regional networksTie regional areas together via telecommunications
systems
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Types of Networks
Wide area networks (WAN)Tie large geographic regions together using microwave and
satellite transmission or telephone lines
Private WANsComputer 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 networksLink system together between countries
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Fig 6.20
“Dumb”terminal
Terminal-to-Host
Applications and databases reside on the same host computer
User interacts with the application using a “dumb” terminal
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LANs and WANs
[Figure 6.18] [Figure 6.19]
A Typical LAN in a BUS Topology A Wide Area Network
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Fig 6.21
File Server
Applications and databases reside on the same host computer
File server transfers data and programs to PCs on the network, which do the processing
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Fig 6.22
Client/Server
Applications and databases reside on specialized host computers
Servers do most or all of the processing and transmit the results to the client
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The Client/Server Approach
Advantages Cost potentialImproved performanceIncreased security
DisadvantagesIncreased cost potentialLoss of controlComplex multivendor environment
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Functions of Communications Software
Error checkingMessage formattingCommunications logsData security and privacyTranslation capabilities for networks
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Communications Software
Network operating systemsSystems software that controls the computer systems and
devices on a network and allows them to communicate with each other
Network management softwareSoftware tools and utilities available for managing
networks
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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
SNAIBM’s communications protocol
EthernetA popular protocol often used with local area networks
Other protocols
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Bridges, Routers,and Gateways
BridgeConnects two or more networks
RouterFeatures more sophisticated addressing software than bridges
GatewayLinks LANs or networks that employ different higher-level
protocols
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Bridges, Routers, and Gateways
[Figure 6.24]
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Coordinating Communications and Networks
PollingA 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 passingA technique in which an electronic “token” is passed from
one device to another
ContentionA procedure in which a device checks to see if a message
is currently being transmitted
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Packet switchingCommunications process that divides electronic messages
into small segments (data packets)
Frame relayUses wideband communications media and high speed
switching devices
Voice-over frame relayMoves voice traffic onto frame relay to bypass the public
telephone network
Data Transfer Modes
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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 high-
performance, mission-critical LAN applications
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Telecommunications Applications
Links between PCs and mainframesFacilitates the upload or download of data
Voice storage and forwardingSometimes called voice mailAllows users to leave, receive, and store verbal messages
for and from people around the world
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Telecommunications Applications
Electronic mailUsers connected to a network can send messages to others
by computer
Electronic document distributionInvolves transporting documents over communications
lines and networks
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Telecommunications Applications
Electronic software distributionInvolves 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
TelecommutingWorking at home on a PC and communicating with the
central office or other workers via networks
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Telecommunications Applications
TeleconferencingAllows groups to have conferences in one connected voice
phone call
VideoconferencingAn extension of teleconferencing that combines voice,
video, and audio transmission
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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]
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Telecommunications Applications
Public network servicesGive PC users access to
vast databases and other services, usually for an initial fee plus usage fees [Figure 6.28]
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Telecommunications Applications
Specialized information systemsInclude legal, patent, and technical information
Regional information systemsInclude local electronic bulletin boards and electronic mail
facilities that offer information regarding local club, school, and government activities
Distance learningExtends the classroom by using telecommunications
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Network Design Considerations
ReliabilitySecurityResponse timePrivacy issuesVendor services and supportCompany support
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Communications Audits
Purpose: To make sure the communications system is operating as intended
Types of communications audits:Walk-thoroughSoftware checkNetwork checkPolicy and proceduresProgram and database check