GRAP 3175Computer Applications for

Drafting

Unit VI

Data Communication

Data Communication Overview

• The process of sending data electronically from one point to another. Linking one computer to another.

• Direct-cable link or telecommunication (telephone / microwave)

The effects of Data Communication

• Our link to the world

• Necessary for CAD labs

Data Transmission

• Two forms of data transmission– Analog– Digital

Analog Data Transmission

• The transmission of data in a continuous wave form

• Telephone system

Digital Data Transmission

• The transmission of data using distinct on and off electrical states

• Computers

• Digital is faster and more efficient than analog communication

• Using the existing telephone systems to send digital transmission by modulation-demodulation

Modulation, Demodulation, and Modems

• Modulation -- process of converting a digital signal to analog

• Demodulation -- process of converting analog signal back to digital

• Modem -- modulation-demodulation

Three Types of Modems

• Acoustic modem (acoustic coupler)– Uses the telephone headset– Not used anymore

Three Types of Modems

• External direct-connect modem– External to the computer and connects directly to

the telephone– Less distorted and faster data transfer– Hayes Smartmodem - the standard (Hayes

compatible)– Connects to a serial interface on the computer– Allows phone to use line when modem is not

connected

Three Types of Modems

• Internal direct-connect modem– Board that fits into an empty slot– Convenient

• PCMCA Cards -- used in notebooks

Communication Channels

• Overview -- the medium, or pathway, through which data are transmitted between devices.

• Three basic types:– Wire cable– Microwave– Fiber optics

Wire Cable

• Twisted-pair– POTS -- Analog, up to 56k bps– ISDN -- Digital, up to 128k bps– ADSL -- Digital, up to 1.5 to 9M bps– Disadvantages -- analog, electrical interferences

• Coaxial -- Digital, up to 10M bps– Minimal interference

Fiber Optics

• Digital -- light impulses through clear flexible tubing

• 2.5 Billion bps

• Very high speeds and very reliable

• Optic backbone

Microwave or Radio Signals

• Signals transmitted through the atmosphere

• Line-of-sight -- Straight lines between transmitter towers and/or satellite

• 48M bps

• Reliable

• Expensive

Wireless Transceiver

• Between computers

• Between buildings

Channel Configurations

• Point-to-point -- computer or terminal is connected directly to another device

• Multipoint -- three or more devices are connected to one line.

Rate of Data Transmission

• Baud rate -- the number of times per second that a signal being transmitted changes. Often thought of as bits per second. Sometimes a signal carries more than one bit

• Modems speeds -- 300, 1200, 2400, 9600, 14.4, 28.8, 33.6, and 56k bps

• Direct connections between computer and output devices -- 9600+

Communication Channel Bandwidths

• Bandwith -- determines the rate or speed that data can be transmitted

• Three bandwiths:– Narrow-band -- 40 bps to 100 bps– Voice-band (POTS) -- 110 bps to 56k bps– Broad-band -- up to several million bps

• 57KB - 622M, T1, T3

Asynchronous & Synchronous Transmissions

• Asynchronous -- method that sends one character at a time

• Synchronous -- block of characters are transmitted at a time

• Protocol -- traffic rules and procedures. Must be the same on each end of the transmission.

Modes of Transmission

• Simplex -- data transmitted in only one direction. (Burglar alarm)

• Half-duplex -- allows a device to send and receive data, but not at the same time

• Full-duplex -- allows a device to receive and send data simultaneously.

Networks

Networks0verview

• When data-communication channels link several computers and other devices

• Each computer usually has its own processing capabilities

• WAN, MAN, LAN, & TAN

Network TopologiesOverview

• Each computer is called a node

• The arrangement is called the topology

• Star, Ring, Tree, Bus

Star Topology

• Several devices connected to one centralized computer

Ring Topology

• Consists of several devices connected to each other in a closed loop

• IBM Token-ring

Tree Topology

• Links computers in a hierarchical fashion and requires data to flow through branches

Bus Topology

• Each computer is connected to a single communication cable via an interface

• Every computer can communicate directly other computers or devices on the network

Wide Area NetworkWAN

• Two or more computers that are geographically dispersed but are linked by common communication carriers

• NSFnet, INTERnet, BITnet

• Compuserve, AOL, etc.

WANMethods to Move Data

• Circuit switching -- dedicated link. Real-time communication can take place. TELNET

• Message switching -- messages usually temporarily stored & then relayed to next node

Methods to Move Data

• Packet switching -- one of the most complicated methods. Messages (data) are divided into standard size packets or blocks. Used for large data transmission. FTP

• WWW -- world wide web, hypertext , links

Local Area NetworksLAN

• Two or more computers directly linked within a small area: room, building, cluster of buildings

• Stand-alone computers

• Gateway -- LANs to LANs or LANs to WANs

• Bridge -- connects rings or zones

LAN Components

• Two or more computers

• Server(s)

• Peripheral devices

• Cabling

• Interface card

• LAN software

LAN Servers

• Computer shared by network

• Types: file, print, communications

• LAN operating system

• Security

LAN Access Methods

• Token (Ring)– A token (string of bits) passed around network

• Ethernet (Bus)– Carrier Sense Multiple Access/Collision

Detection (CMSA/CD)– Device listens for clear channel

LAN Advantages

• Ability to share hardware, software, and data files

• Electronic mail

• Different computer architectures

• Internet/Intranet access

LAN Disadvantages

• Training

• Cost

• Security