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Chapter 6

Information System Networksand

the Internet

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Communications Hardware Two main types of communications

hardware Communications channel – the link

over which data is transmitted Communications processors – provide

processing capabilities between the computer and the communications channel

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Communications Hardware Communications channel

characteristics Data is transmitted over the channel

as bits. Each bit is sent one after the other. Bits are grouped to form bytes that

represent characters using ASCII, EBCDIC, Unicode or some other code.

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Communications Hardware (cont’d.) The way in which bits are sent

determines two main characteristics of the channel Signal type Data rate

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Signal Type Bits can be sent either as

An Analog signal – data is transmitted by a wave pattern that varies continuously

A Digital signal – data is transmitted as a series of high and low pulses

The human voice is analog Telephones are analog Computers are digital

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Data Rate Data rate is measured as bits per

second (bps). Bandwidth also describes how

much data can be transmitted over a channel in a given period of time.

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Data Rate (cont’d.) Baud rate refers to the maximum number

of changes that can occur per second in the state of a communication channel (changes in a communication signal). The data transmition rate for a communication channel is equal to the baud rate for that channel times the number of bits that can be sent with each change in the state of that communication channel.

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Data Rate Broadband: refers to an analog

communications method that splits an analog communication channel in to multiple communication channels.

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Data Rate Broadband ISDN (B-ISDN): Refers to a

digital communication method that split a digital phone line in to multiple communication channel. Basic Rate ISDN (BRI) provide 2

channels each with 64000 capacity. Primary Rate ISDN (PRI): provide 23

channels with 64000 bps capacity. ISDN: Is an acronym for Integrated Services

Digital Network.

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Types of Communications Channel Media Wire cables – sometimes called

“copper cables” have been used since the 1800s. Two main forms

Twisted-pair wiring Coaxial cable

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Communications Channel Media (cont’d.) Twisted-pair wiring

Two insulated copper wires twisted together

Most telephone lines are twisted-pair Can also be used for data Data transmission rate is slow

compared to other media

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Communications Channel Media (cont’d.) Coaxial cable

Copper wire insulated with rubber and plastic.

Used with cable television systems. Data transmission rate is faster than

twisted-pair, and more expensive, but slower than other media.

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Communications Channel Media (cont’d.) Fiber-optic cables

Bundles of glass or plastic fibers. Each fiber is 1/2000 inch thick – about

the size of a human hair. Data is transmitted by a laser that

pulses light through the fiber. Data transmission rate is very fast.

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Communications Channel Media (cont’d.) Microwave (wireless)

Special types of radio signals sent from one microwave antenna to another.

Transmission is line-of-sight, i.e, one antenna must be able to “see” the other.

Two types Land based – antenna approximately

30 miles apart. Satellite – positioned 22,300 miles in

space – geosynchronous orbit.

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Communications Channel Media (cont’d.) Other wireless media

Infrared spectrum Very short distances Relatively slow

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Communications Processors

I. Channel interface devicesII. Communications control unitsIII. Protocols converter devicesIV. Communications security devices

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I. Channel Interface Devices Modems – convert digital signals to

analog signals at the origin and reverse the process at the destination of the signal. Digital to analog is MOdulation. Analog to digital is DEModulation.

Modems may be either internal or external to the computer.

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I. Channel Interface Devices (cont’d.) Modem data rates are typically 33 Kbps

to 56 Kbps. Cable modems (cable Adaptors) – are

devices to provide the interface between a computer and a cable system’s data transmission capability.

For ISDN or Digital Subscriber Line (DSL) digital modems (terminal adapters) is required.

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II. Communication Control Units Multiplexer – combines signals from

several slow-speed devices to transmit over a high speed channel.

Front-End Processor – is a computer that sits between the channel and the main computer (host computer) and is dedicated to performing communication functions.

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III. Communications Protocols Protocols, settings, standards, or

parameters are the rules that must be followed when computers communicate with one another to avoid errors.

Computers that wish to communicate and have different protocols must use a code converter and protocol converter to standardize their communication.

Code conversion (conversion among ASCII, EBCIDIC, & unicode)

Protocol conversion

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IV. Communications Security

A problem with data communications is the lack of security over a communications channel. One way of solving this problem is to use data encryption to convert the data to an unintelligible form for transmission and to return it to an intelligible form at the destination.

Decryption (decoding) requires a key. Asymmetric key cryptolography:

Public key (for encription) & private key (for decription)

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Communications Software Communications software is

required to control communications between computers. It receives data from communications processors connected to the channel and passes the data to other software in the computer for processing.

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Communications Software (cont’d.) Types of communications software

Multi-user processing environment Terminal emulators Telecommunications monitor

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Communications Software (cont’d.)

Client server processing environment requires Network Operationg System (NOS) which establishes and maintains the communication between the nodes (workstations and the file server)

File server software Client software (workstation software) – as

used in client/server computing Light client (Web browser) Heavy client (Traditional client programs that

where proprietary, expensive to maintain)

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Network Topologies Star network – each node is connected to

a central node. Hierarchical network – organized like a

family tree. Each computer communicate with other computers through a common node.

Bus network – each node is connected to a single communication channel.

Ring network – each node is connected to a common channel forming a loop.

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Types of Networks Local Area Network (LAN) Wide Area Network (WAN) Metropolitan Area Network (MAN) Internetwork – connected networks

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Local Area Networks Local Area Network Structure

Connection Topology Star, ring, Hierarchical, Bus, Hybrid

Communications channel wired and Wire less

Require Network Interface Cards (NIC) in each device on the network

Permits sharing of resources Protocol is Usually either Ethernet (carrier

sense multiple access with collision detection (CSM,CD)), Token Ring (token passing) or Polling.

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Wide Area Networks Connection Topology.

Star, Hierarchical, or hybrid Communications channel.

Any Type of media because of they use common carriers or telecommunications companies

(packet switching, circuit switching, and frame relay).

Permits sharing of resources. Can include a Virtual Private Network

(VPN).

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Internet It is a public international network Grew slowly at first and “exploded” in

1993 with the adoption of the Hyper Text Markup Language (HTML) standards for the World Wide Web (WWW).

Two protocols Transmission Control Protocol (TCP) Internet Protocol (IP)

Most users require a relationship with an Internet Service Provider (ISP).

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Intranets and Extranets An Intranet is an Internet like

network but access is restricted to within a business or organization. Firewalls are used to control access to

an Intranet. An Extranet permits controlled

outside access to an Intranet like network.

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Hardware and Software Requires the business have a web

site. The web software runs on a web

server which has special e-commerce software.