By M. Magomelo
Computer Science Dept
Communication is the transfer of information
from one participant to another during a
conversation. The information is transmitted via
Telecommunications is the sending of
information in any form (e.g. voice, data, text and
images) from one place to another using
electronic or light –emitting media and
Data communication is a more specific term
that describes the transmitting and receiving of
data over communication links between one or
more computer systems.
THE OBJECTIVES OF COMMUNICATION
To offer more timely interchange of information
and data among users.
To reduce the effort and cost required to collect
and transmit business data and information.
To support better performance of tasks and
improved management control over an
organization especially with several remote
Various transmission modes are available for
transmitting signals from a sending device to a
Network devices use three transmission modes
(methods) to exchange data, or “talk” to each
other, as follows:
Simplex permits transmission in one direction
only e.g. broadcasting of radio and TV.
Half – duplex permits transmission in either
direction but in only one direction at a time e.g.
conversation with walkie – talkies.
Full – duplex permits transmission in both
directions simultaneously e.g. a phone
conversation the listener can interrupt the
speaker at any time.
• Is the transmission of a signal that takes a continuous
state e.g. speech.
• Analogue signals are continuous sine waves, which
send a continuous 5-volt signal on a channel, but the
signal will vary continuously from +5 to –5 volts.
• The number of cycles per second is the frequency of the
signal and is expressed in hertz (Hz).
• In digital data only a limited number of discrete values
or levels for the variable are possible e.g. 2 levels 0 and
1 for binary data. The 0 and 1 can be used to
represent the “Off” and “On” state of electrical pulses.
• A cycle consists of 2 pulses. The number of pulses per
second is a unit called a baud rate.
• Is the transmission of characters one at a time at irregular
• It uses start and stop bits to mark the beginning and the end
• Characters are transmitted independently with their own
start and stop bits.
• Is the transmission of data in blocks at fixed rates. A block is a
prepared set of characters that are transmitted together as a
• It is more efficient when transmitting large quantities of data.
• Synchronization of sender and receiver is achieved by means
of special synchronization characters, which align clocks at
each terminal so that they function simultaneously.
This is an arrangement of telecommunication
equipment, media, including the software, where
a sender transmits a message to a receiver over a
channel consisting of some type of medium.
End user terminal
• These include such devices as the VDU, terminal, and end-user workstations used
for inputting and outputting data.
• These support data transmission and reception between terminals and computers.
• Devices such as modems, multiplexors, and front-end processors perform a variety of
control and support functions e.g. a modem converts digital data to analogue and
Telecommunication channel and media
• Are used to carry data and information from one point/place to another e.g. copper
wires, coaxial cable, fibber optic cables, micro–wave systems and telecommunication
• Carry out the information processing assignments e.g. a mainframe computer can act
as a network server or a host computer in a large network assisted by a mini–
computer serving as a front-end processor.
• Consist of programs that control telecommunication activities and manage the
functions of telecommunication network. Examples are network operating system
(Novell Network, Unix, Windows NT etc)
A network is any collection of independent computers
that communicate with one another over a shared
The purpose of networking:
• To share resources like expensive peripherals such as
printers, optical disks and scanners.
• For communication with between offices e.g.
electronic mail (e-mail).
• To use/share multi-user software.
• To access other computer systems such as
Computer Network types can be classified by:
• Size or distance covered
• Structure or topology
CLASSIFICATION BY SIZE
Networks can be divided into Local Area
Networks (LANs), Metropolitan Area Networks
(MANs) and Wide Area Networks (WANs).
The connection of two or more networks is called
The worldwide Internet is a well-known example
of an internetwork.
LANS (LOCAL AREA NETWORKS)
LANs are networks usually confined to a geographic
area, such as a single building or a college campus.
LANs can be small, linking as few as three computers,
but often link hundreds of computers used by
thousands of people.
do not exceed tens of kilometres
tend to use one type of transmission media
entirely contained within the same building or the same flow
or the same floor of a multi-storey building
is organisation owned.
Provides high internal data rates (about 10Mbps to 100Mbps)
compared to WANs
LAN technologies include
Network Workstation – are usually microcomputers or terminals
which are used to input and output data in the network:
File servers - these are computer systems connected to the network
that controls access to and manages one or more hard disks to allow
workstations to share disk space, programs and data.
It also controls all the operations/activities of the network.
Print servers – these are computers that control access to and
manages the printer resources attached to the network
Communication servers – these are computers that provide and
manages communications devices (modems and multipexors) in the
network. They are usually microcomputers dedicated to handling
Network Interface Card (NIC)
Each component in the network is attached to the network through
the use of an NIC. These cards/adapters provide the necessary
translation of signals to and from devices in the network.
METROPOLITAN AREA NETWORKS
Covers an entire city i.e. 5 – 50 kms
Uses different hardware and transmission media.
Mainly used for interconnecting private LANs.
Supports data rates of 100 to 1000 Mbps.
Normally owned by someone else: an
independent or government service provider.
WANS (WIDE AREA NETWORKS)
Wide area networking combines multiple LANs that are
This is accomplished by connecting the different LANs
using services such as dedicated leased phone lines, dial-up
phone lines, satellite links, and data packet carrier services.
Wide area networking can be as complex as hundreds of
branch offices globally linked using special routing
protocols and filters to minimize the expense of sending
data sent over vast distances.
WAN links are connected by switches. The switches relay
information through the WAN.
Routers determine the most appropriate path through the
internetwork (communication subnet) for the required data
It supports data rates of 28.8 Kbps to 2 Gbps.
WHAT IS AN INTERNETWORK?
An internetwork is a collection
of individual networks,
connected by intermediate
networking devices, that
functions as a single large
Figure 1-1 illustrates some
different kinds of network
technologies that can be
interconnected by routers and
other networking devices to
create an internetwork.
Figure 1-1: Different Network
Technologies Can Be Connected to
Create an Internetwork
NETWORK CONNECTION DEVICES
Its primary purpose is to enable the network to expand beyond
the distance limitations of the transmission medium.
A repeater regenerates a weak signal from one port onto the
other ports to which it is connected.
It does not filter or interpret anything; instead it merely
regenerates a signal, passing all network traffic in all directions.
A repeater merely passes along bits of data, even if a data frame
Hubs are used to connect together two or more network segments
of any media type. In larger designs, signal quality begins to
deteriorate as segments exceed their maximum length.
Hubs provide the signal amplification required to allow a
segment to be extended a greater distance.
A hub takes any incoming signal and repeats it out to all ports.
NETWORK CONNECTION DEVICES
A bridge is used to connect similar networks.
Bridges perform the same functions as repeaters but are a bit
When a data packet/frame arrives, the bridge checks the frame’s
destination address (MAC) and forwards the frame to the
segment that contains the destination address, if it cannot find
the destination address in its routing table it forwards the frame
to all segments except the source segment.
A router is hardware that helps Local Area Networks (LANs)
and Wide Area Networks (WANs) achieve interoperability and
connectivity, and can link LANs that have different network
topologies (such as Ethernet and Token Ring).
Routers match packet headers to a LAN segment and choose the
best path for the packet, optimizing network performance.
NETWORK CONNECTION DEVICES
These are used to interconnect two or more dissimilar or
incompatible networks i.e. networks with different
architectures (different hardware and software).
Gateways normally change the representation of data
before passing it on while repeaters, bridges and routers do
not change the data in any way. A gateway can change
ASCII coding system to EDCDIC and vice versa.
A gateway translates between different transport protocols
or data formats (for example, IPX and IP) and is generally
added to a network primarily for its translation ability.
NETWORK CONNECTION DEVICES
A firewall is a combination of hardware and software that
provides a security barrier between networks.
Firewalls are generally set up to protect a particular network
from attack (or unauthorized access) by outside invaders.
All traffic to or from the protected network must go through the
User A might have the authority to connect to network 2 whilst
User B might not. When someone tries to connect network 2, the
firewall checks the authority of the user and if the user does not
have any authority, the firewall prevents the connection.
CLASSIFICATION BY PHYSICAL
A physical network topology defines the wiring or
layout for a network. This specifies how the elements
in the network are connected to each other.
Topology defines the physical configuration of
A node is an active device connected to the network,
such as a computer or a printer.
A node can also be a piece of networking equipment
such as a hub, switch or a router.
A star network is a network in which the
nodes are connected to a central component.
This central component may be a switching
device like a PABX (Private Automatic
Branch Exchange), a computer with a
switching or polling capability or just a
wiring centre that is a common termination
point for the nodes called a hub.
A PABX is a telephone switching system
configured for communication in a private
telephone network and with access to the
public telephone system. It provides
A hub is a component that serves as a
common termination point for multiple nodes
and that can relay (store and forward) signals
along the appropriate paths.
STAR TOPOLOGY (CONT)
Advantages of star networks
Easy to add/remove nodes
Because each node has its own link to the central node, the
star is more reliable.
Easy to troubleshoot and isolate problems.
All traffic between 2 nodes passes through central node. If
the central node breaks down the whole network is down.
Number of ports of the central component limits the
number of connected nodes
Uses a trunk or backbone to which all of the computers on the
Systems connect to this backbone using T connectors or taps.
In a bus network the messages are broadcasted and travel in both
directions and do not go through the individual nodes, but every node
can hear each message as it goes past.
When this signal reaches an end of the bus, a terminator absorbs it,
to keep it form travelling back again along the bus line, possibly
interfering with other signals /messages already on the line.
BUS TOPOLOGY (CONT)
When a node breaks down the network does not
Little cable is used relative to other topologies
Does not use any specialized network
Diagnosis /troubleshooting can be difficult.
Network disruption when computers are added
A break in the cable will prevent all systems
from accessing the network.
TREE TOPOLOGY /
A tree topology is a hybrid
physical topology that combines
features of star and bus
topologies. The nodes are
connected in a tree line form.
If the nub goes down, then the
nodes connected to it are no
longer networked but other
parts of the network keep on
A ring network has nodes
connected in a circular way.
Each node has 2 neighboring
nodes. Data/messages flow only
in one direction.
A message is forwarded in one
direction until it reaches the
destination with intermediate
nodes acting as relay (store and
The destination node copies the
message and passes the
message again to the ring.
This message them continue to
circulate around the ring to the
RING TOPOLOGY (CONT)
Collisions of packets are rare
Little cabling needed compared to star
Each node acts as a relay unit
Cable faults are easily located, making
Adding or removing nodes disrupts the network.
If one of the nodes breaks down, the whole
network will break down.
This uses direct (point-to-point)communications lines
to connect some or all of the computers in the network
to each other.
A fully connected mesh network with n(n-1)/2 devices
Each network devices must have (n - 1) I/O ports.
MESH TOPOLOGY (CONT)
No traffic problems
Robust - provides redundant paths between
Privacy & security
Simple fault identification and isolation.
Hard to manage and inflexible
Expensive to setup.
Perform a variety of support functions between
the terminals and computers in a
Is a device that converts digital signals from a
computer transmission terminal at one end of the
telecommunication link into analogue
frequencies, which can be transmitted over
ordinary telephone lines.
Modems at the other end of the communication
link convert the transmitted data back into
digital form and the whole process is called
modulation and demodulation respectively.
An analogue signal is a signal that takes a
continuous state e.g. speech generated signal.
A digital signal is any signal that takes a discrete or
non-continuous state e.g. all signals from computer are
digital in nature.
Note: Modem is an acronym for
A carrier wave of high frequency is used to carry digital data. Some
characteristics of the carrier wave are changed in order to carry the
Modulation is therefore a technique where you change some
characteristic of a carrier signal for the purpose of transmitting
Concisely we say modulation is conversion of digital to
analogue signals, and demodulation, analogue to digital
Frequency Modulation (FM)
In FM the frequency of the
carrier wave is adjusted so that
it represents the binary 1and
0. A high frequency for a 1 and
a low frequency for a zero.
Amplitude Modulation (AM)
In AM the amplitude of the
carrier wave is modified so
that it represent the binary 1
and 0 by using 2 different
amplitudes. High amplitude
for a 1 and low amplitude for a
Phase Modulation (PM)
In PM the signals is adjusted
by a fixed amount so that
binary 0 or 1 can correspond to
different phase shifts.
is a device that allows a single communication channel
to carry simultaneous data transmissions from many
Typically a multiplier merges the transmission of
several terminals at one end of a communication
channel while a similar unit separates the individual
transmissions at the receiving end (demultiplexor).
TYPES OF MULTIPLEXORS
a) Frequency division Multiplexor
This divides a high speed channels into multiple slow speed channels
The bandwidth of the high speed channel is divided into some non-
overlapping frequency bands.
b) The division multiplexor (TDM)
divides the time each terminal can use a high-speed channel into
very short time slots or time frames.
Each user gets periodically the entire bandwidth of a communication
channels for that time slot. The multiplexing is done bit, byte for byte
or sometimes block for block.
c) Statistical time division multiplexor (STDM)
Instead of giving all terminals equal time slots, this multiplexor
dynamically allocates time slots only to active terminals.
Time slots are allocated based on the traffic demand on individual
There are no longer empty slots as in conventional TDM, then
information has to be transmitted with some identification to
which/what channels the time slots belong.
With TDM it is likely that a high-speed line is underutilized. A
concentrator overcomes this deficiency by gathering the bits from
each slow terminal or a group of slow terminals and holding them
in its buffer store until they are sufficient to justify forward
Thus it accepts bits at a slow rate and then transmits them in a
high – speed burst occupying a period of time division
4) Front End Processors (FEP)
In communications, a computer that is located between
communications lines and a main (host) computer and used to
relieve the host of tasks related to communications.
A front-end processor is dedicated entirely to handling
transmitted information, including error detection and control;
receipt, transmission, and possibly encoding of messages; and
management of the lines running to and from other devices.
The FEP relieves the host computer so that it can concentrate on
its information processing functions.
5) Backend processors
a computer system connected to the mainframe
or host computer which links the host to the
Its main purpose is to retrieve data from the
NETWORK OPERATING SYSTEM
Programs /software that control the computer
systems and devices on a network and allow
them to communicate with each other.
They also allow you to;
Install each network device
Install application software on the network
To diagnose network problems
Manage file, print and communication servers
WAN SWITCHING TECHNOLOGIES
WAN services are provided using the following
primary switching technologies
1. Circuit switching (CS)
Provide a dedicated communication path between two
stations and offers bandwidth that cannot be infringed
upon by other users.
An end-to-end path is established before
communication can occur.
The path is a connected sequence of links between
Circuit switching has the following major phases:
I. circuit establishment – a station to station circuit
II. Data transfer – data is now sent over the
III. Circuit disconnect – connection termination.
WAN SWITCHING TECHNOLOGIES
Advantages Circuit switching (CS)
dedicated transmission channel with guaranteed data rate.
Disadvantages Circuit switching (CS)
channel reservation for duration of connection even if no
data are being transferred is an inefficient media use
long delays in call setup.
designed for voice traffic (analog).
2. Message Switching (MS)
No dedicated path is established between the two stations for an entire
Messages are sent in their entirety with source and destination
There is often no real limit on the message / block size.
A message switching node accepts complete messages from originators,
stores the messages, examines the address in the headers of the
messages and then forwards / routes the each message to the next
switching node or destination when circuits become available.
Mostly used for the E-mail system.
more devices can share network bandwidth
reduced traffic congestion
one message can be sent to many destinations through broadcast
often costly – must have large storage devices to hold potentially long
not compatible with most real time applications
3. Packet switching (PS)
PS differs from MS in that long messages are
broken down into smaller and manageable units
called packets, which are then sent to the
destination independently using the fastest
Packets may travel in different routes to the
A Packet Assembler / Dissembler (PAD) is used
to break data into packets at sender and
reassemble them at receiver.
No call set-up is required.
Fast and suitable for interactive applications.
TELECOMMUNICATION MEDIA/ LINKS/
These are the means by which data and other forms of
communication are transmitted between the sending
and receiving devices.
It is any medium through which data is transmitted
from source to destination.
Factors to consider when choosing the transmission
i. Transmission rate to be implemented on the line.
ii. Line capacity or bandwidth. Bandwidth is the range of
usable frequencies that a medium can accommodate.
iii. Transmission distances involved – this determine
attenuation of a signal along the cable. Attenuation is the
loss of signal power as the signal moves along the
iv. Cost of the medium and ease of installation
v. Resistance to environmental conditions like EMI (Electrical
LINKS/ CHANNEL (CONT)
Channel Transmission Impairments
All transmission media suffer the following major
problems, attenuation, noise, and distortion
Attenuation – is the loss of power as a signal propagates
through a medium.
Noise – noise is unwanted signals from sources other than the
signal. It is sometimes referred to as circuit interference.
Distortion – means that the signals are deformed a more or
less different signal as it propagates through the medium.
There are 2 basic categories of transmission media:
guided and unguided.
1. Guided transmission media uses a cabling
system that guides the data signals along a
specific path. The data signals are bound by the
cabling system. Guided media is also known as
bound media. "Cabling" is meant in a generic
sense, and is not meant to be interpreted as
copper wire cabling only.
2. Unguided transmission media consists of a
means for the data signals to travel but nothing
to guide them along a specific path. The data
signals are not bound to a cabling media and are
therefore often called unbound media.
GUIDED TRANSMISSION MEDIA
There 4 basic types of guided media:
1. OPEN WIRE
Open wire is traditionally used
to describe the electrical wire
strung along power poles.
There is a single wire strung
No shielding or protection from
noise interference is used.
We are going to extend the
traditional definition of open
wire to include any data signal
path without shielding or
protection from noise
This can include multi
conductor cables or single
This medium is susceptible to a
large degree of noise and
interference and consequently
is not acceptable for data
transmission except for short
distances under 20 ft.
2. TWISTED PAIR WIRE
These are ordinary telephone wires consisting of 2 insulated
copper wires twisted together in a helical from to reduce cross
talk or electro-magnetic interference from similar pairs close by.
Widely used in established communication networks through out
the world for both voice and data transmission.
Twisted pair is extensively used in home and office telephone
systems and many LANS.
It has big attenuation, which limits the possible distances,
for larger distances amplifiers are needed.
Very much affected by noise
Very sensitive to electromagnetic interference.
TWISTED PAIR WIRE(CONT)
Cheap to install and repair
Easy to terminate
Very sensitive to noise
Only effective for short distances
Data can be easily tapped off (data security is
TWISTED PAIR WIRE(CONT)
STP or shielded twisted pair is used
with the traditional Token Ring
cabling or ICS - IBM Cabling System.
It requires a custom connector. IBM
STP (shielded twisted pair) has a
characteristic impedance of 150 ohms.
Has a thin layer of cover
More extensive EMI than STP cables
Cross talk between UTP pairs limits
max cables length
Very cheap (Costs the Least)
Care must be taken to avoid electrical
noisy devices e.g. electrical motors
Flexible to handle
A typical impedance for UTP is 100
ohm for Ethernet 10BaseT cable.
UTP or unshielded twisted pair cable
is used on Ethernet 10BaseT and can
also be used with Token Ring. It uses
the RJ line of connectors (RJ45, RJ11,
3. COAXIAL CABLES
It consists of a stiff copper
wire as the core surrounded
by an insulating material.
A cyclical conductor often as a
branded mesh encases the
The other conductor is
covered in a protective plastic
This installation minimizes
interference and distortion of
signals the cable carries.
Group of coaxial cables can be
banded together in a big cable
for easy of installation.
Coaxial cable is used
extensively in television,
radio, network and data
They can also be used to
connect or interconnect
computer and peripheral
Characteristics of coaxial cable
Higher bandwidth and data rates (high – speed transmission)
More expensive than twisted pair
Can be easily tapped
Low error data rates
Good noise immunity
Easy to install
Reasonably high bandwidth
Low data error rates
Data security is low (they can be easily tapped)
4. FIBER OPTIC CABLES
Data is transmitted in the form of
It uses cables consisting of one or
more hair – thin filaments of glass
fiber wrapped in a protective jacket.
Have very high data transmission
They are light in weight and not
affected by electromagnetic radiation
They are not affected by lightning or
Light sources are Light Emitting
Diodes (LED) and Injection Laser
Diodes (ILD) and there is a
photodiode to detect the light rays.
FIBER OPTIC CABLES(CONT)
Advantages of Fiber Optic
Very high transmission capacities (bandwidth)
Smaller and lighter than copper wire
Immune to cross talk /EMI
Suitable in hostile environments (noisy environments)
Less susceptible to spying (can not be easily tapped)
Longer distances than copper wire
Faster transmission rate
Disadvantages of Fiber Optic
Repairing and installing is quite difficult and needs specialist
personnel and equipment.
Limited physical arc of cable. Bend it too much and it will break!
Difficult to splice
Optical transmission systems are unidirectional.
UNGUIDED TRANSMISSION MEDIA
Unguided transmission media is data signals
that flow through the air.
They are not guided or bound to a channel to
They are classified by the type of wave
There are three types of RF (radio frequency)
Line of Sight (LOS)
1. RF PROPAGATION
follows the curvature of
Ground waves have
carrier frequencies up to 2
AM radio is an example of
ground wave propagation.
bounces off of the Earth's
ionospheric layer in the upper
It is sometimes called double
It operates in the frequency
range of 30 - 85 MHz.
Because it depends on the
Earth's ionosphere, it changes
with the weather and time of
The signal bounces off of the
ionosphere and back to earth.
Ham radios operate in this
Line of sight
transmits exactly in the line
The receive station must be in
the view of the transmit
It is sometimes called space
waves or tropospheric
It is limited by the curvature
of the Earth for ground-based
stations (100 km, from
horizon to horizon).
Reflected waves can cause
Examples of line of sight
propagation are: FM radio,
microwave and satellite.
Microwave transmission is line of sight
The transmit station must be in visible
contact with the receive station.
Microwave antennas are usually
put/placed on top of buildings, towers,
hills and maintain peaks.
The antennas house radio transmitters
and receivers (transceivers) known as
This sets a limit on the distance
between stations depending on the local
Typically the line of sight due to the
Earth's curvature is only 50 km to the
Repeater stations must be placed so the
data signal can hop, skip and jump
across the country.
Microwaves operate at high operating
frequencies of 3 to 10 GHz.
This allows them to carry large
quantities of data due to their large
They require no right of way acquisition between towers.
They can carry high quantities of information due to their high
Low cost land purchase: each tower occupies only a small area.
High frequency/short wavelength signals require small antennae.
Attenuation by solid objects: birds, rain, snow and fog.
Reflected from flat surfaces like water and metal.
Diffracted (split) around solid objects.
Refracted by atmosphere, thus causing beam to be projected away
Satellites are transponders (units that receive on one frequency and retransmit on
another) that are set in geostationary orbits directly over the equator.
These geostationary orbits are 36,000 km from the Earth's surface.
At this point, the gravitational pull of the Earth and the centrifugal force of
Earth's rotation are balanced and cancel each other out.
Centrifugal force is the rotational force placed on the satellite that wants to fling it
out into space.
Quite similar to radio systems. Their difference with radio systems is that their
intermediate link stations are in orbit around the earth (geosynchronous
satellites) and can be as far as 30 000 km. Geosynchronous means the satellite
maintains a fixed position relative to the earth, and revolve at the same as the
earth under them.
The satellite overcomes the problem/weakness of the microwave for their demand
for line- of – sight position, since the horizon intervenes even between high points
at distance exceeding a few tens miles.
The uplink is the transmitter of data to the satellite.
The downlink is the receiver of data. Uplinks and downlinks are also called Earth
stations because they are located on the Earth.
The footprint is the "shadow" that the satellite can transmit to, the shadow being
the area that can receive the satellite's transmitted signal.
Advantages of satellites
Reliability of satellites is high
Band width reasonably large
Suitable for intercontinental communications
Disadvantages of Satellites
Huge delay – the long distance between terrestrial
stations via the satellite causes a significant delay of
about 240 ms.
It is expensive to launch a satellite
Big total attenuations because of the large distance
Data security is low. It is easy to intercept the
transmissions as it travels through the air.
Bad weather can severely affect the quality of satellite
ASSIGNMENT 4 DUE DATE: 24 – 10 - 2011
1. In a form of a table compare and contrast the use of
database with traditional filing system. 
2. Write short note on following
a. Print server 
b. File server 
3. Explain the role of a router in context of other
networking devices. 
ASSIGNMENT 5 DUE DATE: 31
– 10 - 2011
1. Explain ring topology. 
2. List the different types of guided media. 
3. Mention any three advantages and three
disadvantages of Internet Shopping. 
4. Explain any three ways in which viruses may be
spread in computer systems. 
5. Explain how you can increase security on an
information system against hacking.