Communications and Networks

 

Communications, sometimes called data communications, refers to the transmission of data and information over a communications channel, such as a telephone line or between computers.  Telecommunications describes ant type of long-distance communications including television signals.  Teleprocessing refers to the use of computers and communications equipment to access files located elsewhere.

 

Advances in software and hardware now allow people to easily transmit voice and data communications around the world.  The following applications rely on communications technology:

·         Electronic mail

·         Voice mail

·         Teleconferencing or video conferencing

·         Facsimile or FAX

·         Groupware (workgroup technology)

·         Telecommuting

·         Electronic data interchange (EDI)

ü       Lower transaction costs

ü       Reduced time to transmit documents

ü       Reduced data entry errors because data need not be reentered at the receiving end

ü       Reduced paper flow

·         Global positioning systems (GPS)

ü       Satellites can determine the location of GPS equipment on Earth up to 100 feet.

ü       Used to track and navigate vehicles

·         Bulletin board systems (BBS)

·         Online services or information services

 

Transmission media are the materials used to establish communications channels (links).  There are two types of transmission media: CABLES and WIRELESS.

 

CABLES:

1.)                  Twisted pair

2.)                  Coaxial

3.)                  Broadband

4.)                  Fiber optics

 WIRELESS:

1.)                  Microwaves (radio waves)

2.)                  Satellite (higher frequency microwave signals)

--uplink/downlink

 

Wireless offers design flexibility and portability, but slower transmission than wired connections.

 

Types of Signals:

                DIGITAL - individual electrical pulses represent the bits grouped together to form characters

                ANALOG - a continuous electrical wave

 

Transition Modes:

                ASYCHRONOUS - bytes are transmitted at irregular intervals (like when data is entered) - SLOW

                SYCHRONOUS - large block of data are transmitted at regular intervals - EXPENSIVE/FASTER

 

Transmission Direction:

                SIMPLEX - data flows in one direction only - used when sending device never needs a response

                HALF-DUPLEX - data flows in both directions, but only one direction at a time

                FULL-DUPLEX - data can be sent in both directions at the same time

 

Transmission Rate

                BANDWIDTH - range of frequencies that a channel can carry

                BITS PER SECOND - (bps) number of bits that can be transmitted in one second

                BAUD RATE - number of times per second that the signal being transmitted changes

 

Communications Equipment:

·         Modems - converts the digital signals to an analog signal

·         Multiplexors-

·         Front-end Processors -

·         Network Interface Cards - fits in an expansion slot and attaches the device to the network

 

Communications Software:

·         Dialing software

·         File transfer software

·         Terminal emulation software

·         Data encryption

 

PROTOCOL is a set of rules and procedures for exchanging information between computers.  Protocol defines how the communications link is established, how information is transmitted, and how errors are detected and corrected.  By using the same protocol, different types of computers can communicate with one another.

 

Networks:

LAN - A Local Area Network is a privately owned communications network that covers a limited geographic area such as a school computer lab, an office, a building or group of buildings.

·         Hardware Resource Sharing (printers, etc.)

·         Software Resource Sharing (site license)

·         Information Resource Sharing

ü        Server - a computer dedicated to handling the communications needs of the other computers in the network

File-server: the server sends the entire file and the requesting computer performs the processing

Client-server: As much processing as possible is done on the server before the data is transmitted.

Network Operating System runs on the server computer in addition to the client operating system provide the following capabilities:

1.)                  Administration (clean-up and backups)

2.)                  File Management

3.)                  Printer Management

4.)                  Security

WAN - A Wide Area Network is geographic in scope and uses telephone lines, microwaves, satellites, or a combination of communications channels.  Public WAN include common carriers like telephone companies.

 

The Internet is a worldwide network of computer networks.  It is the largest and best known WAN.  It has 30 million users worldwide in over 100 countries with over 21,000 connected networks and 2 million connected computers.  It was originally started in 1969 as a way to link government researchers at four universities.  Since 1988, the Internet has doubled in size every year.  Some of the services it provides are:

·         Electronic mail

·         File transfer

·         Database research

·         Remote logon

·         Discussion and news groups

·         Games

 

Topology:

·         Star Network

·         Bus Network

·         Ring Network