Modes of Data Communication and Computer Networks

Modes of Data Communication and Computer Networks



Data; consists of raw, unprocessed facts which when processed give information which is used for decision making. Used interchangeably with information.

Communication is “Exchange of meanings between individuals through a shared system of symbols”

·         Data communication is the transmission of electronic data over some medium.

·         Data communicated may consist of voice, sound, text, video, graphics etc

·         The systems that enable the transmission of data are often called data communication networks.

Modes of communication, in general;

(1) Written

·         Memos

·         Letters

·         Posters/Post-its/Stick-ems

(2) Face -to- Face

(3) Nonverbal

(4) Verbal (telephone)

(5) Electronic


Basic Elements of a Communication System

The following are the basic requirements for working of a communication system.

·         The sender (source) who creates the message to be transmitted

·         A medium that carries the message

·         The receiver (sink) who receives the message


Sender or receivers - These devices are the data sources and destinations in a network (i.e., end nodes where data originates or is received).



·         Personal computers

·         Terminals

·         Workstations

·         Computers

·         Point of sale cash registers

·         Automatic teller machines


Transmission media - Also called communication channel

Is the pathway over which information can be conveyed. The channel could be a physical device e.g. a wire or air as with radiated energy source such as a Radio that bears no obvious physical presence

Electronic media transmit electronic or light signals and consist of different media.  Transmission media may be bounded or unbounded.


Bounded Media

·         Twisted pair wire

·         Co-axial cable

·         Fiber optic cables

·         Wave guides

Unbounded Media (air or a vacuum)

·         AM and FM radio

·         TV broadcasting

·         Satellite communication

·         Microwave radio

·         Infrared signals


Modes of data communication

Communication channels have a direction associated with it;

·         Simplex channel: a channel whose direction of transmission is unchanging. Example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back. Also commercial television

·         Half-duplex channel: is a single physical channel, in which the direction may be reversed, i.e. messages may flow in two directions but never at the same time. Example, a telephone call, also in most data processing applications

·         Full-duplex channel: allows simultaneous message exchange in both directions. It consists of two simplex channels, a forward and reverse channel.

o   Most terminals are configured to work in this mode.

o   This type of transmission requires more software and hardware controls on both ends.

Most digital messages are transmitted in packets because it is not economical or practical to send all bits of a message simultaneously


Two physical messages transmission;


·         The data is sent one bit at a time.

·         It uses a single conductor to provide communication between devices.

·         Standard telephone lines can be used to transmit data serially.

·         Transmitting data in this mode is more complex than parallel transmission


·         Input/output ports can transmit data bit by bit or send the entire byte in a single parallel operation employing 8 lines, one for each bit.

·         A byte is placed on the output port of a device and a single pulse transfers the data to the receiving device.


What is a computer network?

An interconnected (i.e. capable of exchanging messages) collection of autonomous (i.e. do not control one another) computers

Two aspects

·         Hardware: physically connects machines to one another (allows signals to be sent)

·         Protocols: specify the services the network provides

o   Protocols make the hardware usable by programmers and application software


Building blocks of networks

·         Terminals, Workstations, Computers, and other devices (end nodes)

·         Transmission Media (for transmitting data and control signals)

·         Network electronics devices (intermediate devices for routing data from source to destination)

·         Software to control data transmission

·         Network Architecture Standards (Standards to ensure interoperability between different equipment made by different vendors)


Data communication & computer networks

Terminals and Workstations

·         Personal computers

·         Terminals

·         Workstations

·         Computers/savers

·         Point of sale cash registers (scanners)

·         Automatic teller machines (card readers)

·         Printers/plotters


Transmission Media - Bounded and Unbounded media


Network Electronics devices - Devices for;

·         Routing or switching data from source to destination or

·         Providing the interface between different transmission media or different communication protocols


·         Bridges

·         Concentrators

·         Routers

·         Front End Processors

·         Private Branch Exchange (PBX)

·         Switches

·         Hubs

·         Multiplexers

·         Gateways

·         Modems

Software to control data transmission

·         Computers send and receive data across communication links by use of data communication software.

·         Communication software instructs computer systems and devices as to how exactly data is to be transferred from one place to another

·         The procedure of data transformation in the form of software is commonly known as protocol

The data transmission software or protocols perform the following functions:

·         Data sequencing: A long message to be transmitted is broken into smaller packets of fixed size for error free data transmission

·         Data Routing: It is the process of finding the most efficient route between source and destination before sending the data

·         Flow control: All machines are not equally efficient in terms of speed. Hence the flow control regulates the process of sending data between fast sender and slow receiver

·         Error Control: Error detecting and recovering is the one of the main functions of communication software. It ensures that data are transmitted without any error


Network Architecture Standards

·         Interface: the point of interaction between two devices such as a printer and a PC

·         Interconnection standards:

o   Specify methods of interfacing two devices, making it unnecessary for vendors to know the insides of each other’s equipment provided the specifications at the boundary are met

What is a networking hardware?

Includes all computers, peripherals, interface cards and other equipment needed to perform data-processing and communications within the network


File Servers

·         Stands at the heart of most networks

·         It is a very fast computer with a large amount of RAM and storage space, along with a fast network interface card.

·         The network operating system software resides on this computer, along with any software applications and data files that need to be shared

·         Controls the communication of information between the nodes on a network. For example, it may be asked to send a word processor program to one workstation, receive a database file from another workstation, and store an e-mail message during the same time period.



·         All of the user computers connected to a network are called workstations.

·         A typical workstation is a computer that is configured with a network interface card, networking software, and the appropriate cables.

·         Workstations do not necessarily need floppy disk drives because files can be saved on the file server.

·         Almost any computer can serve as a network workstation.


Network Interface Cards (NICs)

·         The NIC provides the physical connection between the network and the computer workstation.

·         Most NICs are internal, with the card fitting into an expansion slot inside the computer.

·         Network interface cards are a major factor in determining the speed and performance of a network.

·         Includes the Ethernet cards, Local Talk connectors, and Token Ring cards



·         A concentrator is a device that provides a central connection point for cables from workstations, servers, and peripherals.

·         Most switches are active, that is they electrically amplify the signal as it moves from one device to another.

·         Switches memorize addressing of computers and send the information to the correct location directly.

·         Switches are:

o   Usually configured with 8, 12, or 24 RJ-45 ports 

o   Sold with specialized software for port management



·         Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater.

·         The repeater electrically amplifies the signal it receives and rebroadcasts it.

·         They are used when the total length of your network cable exceeds the standards set for the type of cable being used.

·         Can be a separate unit or incorporated in a concentrator



·         A device that allows you to segment a large network into two smaller, more efficient networks

·         Monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location

·         Can inspect each message and, if necessary, broadcast it on the other side of the network

·         The bridge manages the traffic to maintain optimum performance on both sides of the network.



·         Translates information from one network to another; it is similar to a super-intelligent bridge

·         Select the best path to route a message, based on the destination address and origin

·         Can direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along back roads and shortcuts

·         Know the addresses of computers, bridges, and other routers on the network

·         "Listen" to the entire network to determine which sections are busiest -- they can then redirect data around those sections until they clear up


Types of networks

1)     Local area network (LAN)

·         A network that is confined to a relatively small area such as a writing lab, school, or building

·         Rarely are LAN computers more than a mile apart

·         LAN configuration has one computer designated as the file server which; (1) Stores all of the software that controls the network and (2) the software that can be shared by the computers attached to the network

·         On most LANs, cables are used to connect the network interface cards in each computer


2)     Wide area network (WANS)

·         WANs connect larger geographic areas, such as Arusha, Tanzania or the world

·         Dedicated transoceanic cabling or satellite uplinks may be used to connect this type of network

·         Using a WAN, schools in Arusha can communicate with places like London in a matter of minutes, without paying enormous phone bills

·         A WAN is complicated; it uses multiplexers en-route to connect local and metropolitan networks to global communications networks like the Internet.


Why networks?

·         Resource Sharing:

o   One printer (or other special hardware) can be shared by many machines instead of requiring each machine have its own printer.

o   Other expensive resources include plotters, colour laser printers, terminals, storage devices, special machine architectures, etc.

·         Information Sharing

o   Electronic mail, or e-mail, has become ubiquitous for exchanging ideas quickly

·         Improve Reliability

o   Networks allow physical redundancy, ensuring continued service if a disaster strikes one location

·         Reduced Cost

o   One obtains more cost advantage by buying many PCs and workstations than a single mainframe machine

o   Provide access to needed resources from anywhere at anytime

o   Support collaborative group work independent of location

·         Scalability

o   Computer networks provide an effective mechanism to scale up and provide services to more users at more locations where needed and when needed

·         Manageability

o   Networks allow remote resources to be managed effectively (e.g.., remote control of telescopes or other resources)

·         Access to remote information through connection between a person and a remote database.

o   Financial institutions: people pay their bills, manage bank accounts, handle investments electronically.

o   Home shopping through on line catalogs of many companies.

o   Personalized on line newspapers.

o   On line job search and resume submission.

o   Access to the World Wide Web and digital libraries with information on any conceivable topic.


The internet

A global interconnected network of computers, world's largest computer network, the network of networks, scattered all over the world

With Internet

·         You can look at documents and images

·         View videos or listen to sound files

·         Publish, so that others can look at your information

·         Send messages through e-mail, as long as you know the e-mail address of the recipient

·         Transfer files between any two people or computers etc.


How Internet works

·         To access the Internet you need an Internet Service Provider or "ISP"

·         The ISP is connected to the Internet "backbone" which is the permanent cabling of the Internet

·         The backbone: copper wire, fibber optic cable, microwave, and even satellite connections between any two points

·         The standard for communicating on the Internet is called "TCP/IP" (Transmission Control Protocol/Internet Protocol


The internet backbone

·         TCP/IP enable the computer to know or figure out where all other computers are on the network, and thus send data by the quickest route

·         TCP/IP transfers information in small chunks called "packets."

·         Each packet includes the following information: the computer (or last few computers) the data came from, the computer to which it is headed, the data itself, and error-checking information

Local area network (LAN)

·         Are local networks consisting of a communication links, network operating system, microcomputers, servers and other shared hardware

·         Types of LAN’s

o   Client/Server LAN

o   Peer-to-Peer LAN

Client/Server LANs

Consist of requesting microcomputers called clients, and supplying devices that provide a service called servers


Types of servers

Server: Is a computer that manage shared devices

File server: computer that stores the programs and data files shared by users of a LAN

Database server: Computer in LAN that stores data. It doesn’t store programs unlike file server.

Print server: Computer in LAN that controls one or more printers

Peer-to-Peer LAN

One in which all microcomputers on the networks communicate directly with one another without relying on a server

·         Are less expensive

·         Work effectively for up to 25 computers



·         Is the logical layout or shape of the network

·         Physical topology, refers to the configuration of cables, computer and other peripherals

·         Main types of physical topologies

o   Bus

o   Star

o   Ring

o   Hybrid


Bus topology

·         Connects each computer (node) to a single segment trunk/linear cable/backbone cable

·         Signals travel from one end of the bus to the other

·         A terminator is required at each end to absorb the signal so that it doesn’t reflect back across the bus


Advantages of bus topology

·         Easy to implement (connect) and extend

·         Failure of one station doesn’t affect others

·         The least and cheapest topology to implement

·         Requires less cable length than a star topology


Disadvantage of bus

·         Difficult to identify the problem if the entire network shuts down

·         Limited to cable length and number of computers

·         Performance is affected with collisions


Star topology

·         Designed with each node (file server, workstations and peripherals) connected directly to a central hub/concentrator

·         Data in star network passes through the hub/concentrator before continuing to its Destination

·         Hub/switch/concentrator manages and controls all functions of the network.

·         It also acts as a repeater for the data flow


Advantages of star

·         Easy to add new stations or hub

·         Easy to monitor and troubleshoot

·         Easy to detect faults and to remove parts

·         No disruptions to the network when connecting or removing devices


Disadvantage of Star

·         Failure of the hub cripples the attached stations or nodes

·         Costs more than bus due to the hubs and more cables required than bus


Tree topology

·         Combines characteristics of linear bus and star topology

·         Consists of groups of star-configured workstations connected to a linear bus backbone cable


Advantages of tree

·         Supported by several hardware and software vendors

·         Point-to-point wiring for individual segments is possible


Disadvantages of the tree

·         Difficult to configure and wire than other topology

·         If the backbone line breaks, the entire segment goes down


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