Saturday, December 20, 2014
DATA COMMUNICATION
Definitions
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).
Examples
·
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;
SERIAL
·
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
PARALLEL
·
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
Includes:
·
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.
Workstations
·
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
Switch/Hub
·
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
Repeaters
·
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
Bridges
·
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.
Routers
·
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
TOPOLOGY
·
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
0 comments for "Modes of Data Communication and Computer Networks"