Telecommunications

and

Networks

Chapter

6

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 communicationsTelephoneRadioTelevision

Lessens barriers of time and distance

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

Elements of a Telecommunications System

[Figure 6.3]

Analog and Digital Signals

AnalogContinuous fluctuations over time between high and low

voltage

DigitalA discrete voltage state

Analog and Digital Signals

Fig. 6.4

Analog signal

Digital signal

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

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

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

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

Types of Communications Media

Twisted wire cableCoaxial cableFiber-optic cableMicrowave transmissionCellular transmissionInfrared 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: TerrestrialMicrowave: SatelliteCellular

PCS

Telecommunications Devices

ModemsFax modemsSpecial-purpose modemsMultiplexersCommunications 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

[Figure 6.15]

Mainsystem

Front-endprocessor

Incoming andoutgoing jobs

TelecommunicationsCarriers and Services

Common carriersLong-distance telephone companies

Value-added carriersCompanies that have developed private

telecommunications systems and offer their services for a fee

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

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)

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 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

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

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

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

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

[Figure 6.17]

Four Basic Typesof Network Topology

(a) ring (b) bus

(d) star(c) hierarchical

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

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

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

LANs and WANs

[Figure 6.18] [Figure 6.19]

A Typical LAN in a BUS Topology A Wide Area Network

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

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

The Client/Server Approach

Advantages Cost potentialImproved performanceIncreased security

DisadvantagesIncreased cost potentialLoss of controlComplex multivendor environment

Functions of Communications Software

Error checkingMessage formattingCommunications logsData security and privacyTranslation capabilities for networks

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

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

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

Bridges, Routers, and Gateways

[Figure 6.24]

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

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

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

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

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

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

Telecommunications Applications

TeleconferencingAllows groups to have conferences in one connected voice

phone call

VideoconferencingAn 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 servicesGive PC users access to

vast databases and other services, usually for an initial fee plus usage fees [Figure 6.28]

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

Network Design Considerations

ReliabilitySecurityResponse timePrivacy issuesVendor services and supportCompany support

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