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Data Communication
Computer Studies Higher Grade
Grades 11 + 12
Data Communication
Definition
 the transfer of
 electronic representation of information
 such as data, sound, graphics or video
 from one point to another
 by means of an electronic transmission
system also known as a network
Three Vital Components
Sender
 Receiver
 Communication Medium
 e.g. A computer on either end of a
network cable


Computers usually act as senders and
receivers
Method of Data Transmission

Simplex mode


Half-Duplex mode


One direction only
Both directions but only one direction at a
time
Full-Duplex mode

Both directions simultaneously
Type of Physical Connection

Serial Communication
Data sent one bit at a time along a wire
 Very slooooow


Parallel Communication
Number of bits sent at one time along many
wires.
 Usually one byte at a time
 Faster but not effective over long distances

Type of Physical Connection (cont.)

Serial
10100101

Parallel
1
1
0
0
1
1
0
0
0
0
1
1
0
0
1
1
Timing is everything

Two major types of
timing


Asynchronous
Synchronous
Asynchronous:
a timeless masterpiece
No clock is present to time sends of data
 Each piece of data includes a start and
stop bit. Inefficient!
 Data transmitted in irregular bursts, not
in steady streams
 Simple and cheap to implement
 Low speeds and efficiency

Synchronous:
time for improvement
Data transmitted in blocks
 Clock regulates transmission
 Data stream is continuous
 Data sent at regular intervals
 Ideal for high speed transmissions
 More expensive than asynchronous
transmission

Computer Networks
Definition
 a number of computing devices called
nodes
 that are linked by some kind of physical
medium
 in order to share information
 electronically according to a specific set
of rules
Types of Area Networks

LAN


MAN


Metropolitan Area Network
WAN/GAN


Local Area Network
Wide (Global) Area Network
PAN

Personal Area Network
Are you a MAN or
a mouse?
Networks… good…
Sharing Resources
 Sharing Software
 Sharing Data
 Security
 Back-ups
 Easy Maintenance and Upgrades

Network Administrators:
such hard-working folk














Creating new users
Creating groups of users
Removing users
User directory management
Setting rights and policies
Administration of print queues
Installation of server software
Viruses
Backups
System monitoring
Upgrading
Power failure management
Documentation
Training New Users
Transmission Problems

Crosstalk


Attenuation


Loss of signal strength during transmission usually
due to distance
Electromagnetic Interference


Interference caused by crossing electromagnetic
fields of two wires
EM waves from other sources interfering with
signals.
Eavesdropping

Signal detection
Three Types of Transmission Media

Cables
 Unbounded or
Wireless Media
 Bluetooth
Cables Cables Cables
Three Important Cables
 Twisted Pair
UTP
 STP

Coaxial
 Fibre Optical

Twisted Pair:
the evil twins
Copper wires good conductors
 UTP – Unshielded Twisted Pair

Sets of twisted pairs with simple plastic
encasement as insulation
 RJ45 Connector


STP – Shielded Twisted Pair
Insulated cable
 Bundled pairs wrapped in foil shielding
 Apple talk or IBM connectors

Twisted Pair (cont.):
pics
Twisted Pair (cont.):
more pics
Twisted Pair (cont.)

Advantages





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Inexpensive, UTP cheapest cable available
Easy to install, manage and reconfigure
STP more expensive and harder to install
Basic Technology
Standards are mature and stable
Disadvantages



High rate of attenuation
Data rates 1Mbps to 1Gbps at distances up to
100m
Susceptible to interference and signal capture
Coaxial Cable:
back in the old days…
Two conductors sharing a common axis
 Inner copper wire insulated in plastic
foam
 Outer conductor surrounds plastic
 Acts as shield from interference
 Tough plastic outer insulation

Coaxial Cable (cont.)

Advantages

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
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Simple to install
Higher bandwidths
Resists interference better than UTP
Sturdy
Cheaper than fibre optic
Disadvantages


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Moderately high attenuation (lower than TP)
More expensive than TP
Bulkier than TP
Coaxial Cable (cont.):
pics
Coaxial Cable (cont.):
more pics
Fibre Optic:
shedding some light on the subject
Light signals transmitted
 Light conducting glass or plastic core
 Core surrounding by more glass called
cladding
 Outer layer is tough plastic sheath
 Sheath protects against extreme
temperatures, bending, stretching and
breaking

Fibre optic (cont.):
the good the bad and… some pics

Advantages (the good)

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Immune to interference and eavesdropping
Low attenuation rates
High speeds (100Mbps to 2Gbps)
Reliable and secure
Disadvantages (the bad)



Expensive
Complex installation
Connections require precision manufacturing
Fibre Optic (cont.):
pics
Fibre Optic (cont.):
more pics
Fibre Optic (cont.):
a moving pic
Data transmission via light medium
Learn This Table
Media
Cost
Installation
Data Rate
Attenuation
EMI
UTP
Very
low
Easy
1 – 100
Mbps
High
(100m)
High
STP
Low
Fairly Easy
1 – 100
Mbps
High
(100m)
Little
less than
UTP
Coax
Lowish
Fairly Easy
1 Mbps –
1 Gbps
Moderate
(few 100m)
Less
than
UTP
Fibre
Optic
High
Difficult
10Mbps –
2 Gbps
Low
(few km)
Immune
Unbounded or Wireless Media:
becoming airbourne

Three Types
Microwave
 Light Transmission
 Radio Waves

Each of the three types has it’s own subcategories with different forms of
transmission
Microwave:
now we’re really cooking

Terrestrial System

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Repeaters situated on land
Repeat signals over large distances
Often used to link separate buildings where cable
isn’t possible
Line of sight antennas
Satellite System

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
Orbiting satellites
Fixed position above earth
Satellite dishes on earth send and receive signals
from and to other dishes by reflecting signals off the
orbiting satellite
Microwave (cont.):
terrestrial vs. satellite
Type Of System
Advantages
Less
Terrestrial
expensive
Useful where
cabling not
possible
High data rates
Disadvantages
Requires
license
Complex installation
No line of sight
EMI
Rain and fog cause
attenuation
Expensive
Rain
High
Satellite
data rates
Earth antenna
can be stationary
and fog cause
attenuation
Latency over long
distances
EMI and eavesdropping
Light Transmission:
walk towards the light

Infrared Transmission System

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


Infrared light used to transmit signals from node to
node
Restricted to line of sight in a single room
Infrared transmitter-receiver installed
Infrared beam similar to remote control
Laser Transmission System



Narrow infrared beam divided into pulses in order
to carry data
Beam received and translated into bits
Can replace microwave over very small distances
Light Transmission (cont.):
I’ll shoot you with my “laser”
Type of System
Advantages
Cost
Infrared Transmission
System
effective
Medium speeds
Good where cables aren’t
Immune to eavesdropping
Disadvantages
Short
distances only
Reflective sources
problematic
Atmospheric conditions
and obstacles may cause
attenuation
Short
High
Laser Transmission
System
speeds of data
transmission possible
Immune to EMI and
eaves-dropping
distances
Atmospheric conditions
and alignment cause
attenuation
Radiation
Cannot reflect off
surfaces like infrared
Radio Waves

Radio Wave Transmission System
Frequencies between 10KHz and 1 GHz
 Line of sight antennas


Cellular Radio Transmission System
Users cell phone connected to cellular site
 Connected via fibre optic to telephone
network
 From telephone network data is transmitted
via satellite, cable or microwave

Radio Waves:
radio vs. cell
Type of System
Advantages
Disadvantages
Directional
Radio Wave Transmission
System
Equipment
unnecessary
Globally accessible
Stations can be
stationary
Transceivers are cheap
low to moderate
speeds possible
Susceptible to eavesdropping and EMI
Susceptible to eavesspeeds of data
dropping and EMI
transmission possible
Atmospheric conditions
Can be implemented over
may cause attenuation
long distances
High
Cellular Radio
Transmission System
Only
Wireless Transmission Media:
comparison
Media
Cost
Installation
Data Rate
Attenuation
EMI
Radio
Medium
Easy
1 – 10 Mbps
High
(25m)
High
Microwave
Medium
Fairly
Complex
1 – 10 Mbps
Depends on
conditions
High
Satellite
High
Extremely
Difficult
1 – 10 Mbps
Depends on
conditions
High
Fairly
Simple
100 Kbps –
16 Mbps
Depends on
quality of
light
Fairly
Immune
Infrared Medium
Bluetooth:
one bluetooth, many blueteeth?
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Short-range technology
Connects devices within 10 meters, 100 meters with
power boost
720 Kbps
Transmits in unlicensed 2.4 GHz band
Connects devices in a PAN
Relatively low cost
Can transmit through physical barriers
Line of sight not required
Printers connecting to cell-phones
Cell-phones connecting to fridges
Cat connected to your hamster etc.
Connectivity Devices:
hardware needed in networks


Network Interface Cards (Adapters)
Hubs
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Passive Hubs
Active Hubs
Intelligent (Switched Hubs)
Repeaters
Bridges
Modems
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Optical Modem
Short haul Modem
Fax Modem
V.90 – 56 Kbps modem
Network Interface Cards:
nic nacs

Card or adapter slots
into motherboard
 Jacks for appropriate
connectors situated
at the back
Hubs:
active, passive and intelligent

Provide a central point of connection between
two or more parts (nodes) of a network
 Connects computers, printers and other
devices
 Organises cables and transmits incoming
signals to other parts of the network
 Different Types

See next slide for details
Hubs (cont.):
the “next slide” referred to previously

Passive
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Active
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No amplification or regeneration
All nodes connected to the hub receive every signal sent from
all other nodes
Amplification and regeneration
All nodes connected to the hub receive every signal sent from
all other nodes
Intelligent (switched)
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Used in large networks to reduce traffic
Data packets examined for source and destination
Data sent on shortest path to destination node ONLY
Handles network broadcasts
Amplifies and regenerates signals
Hubs (cont.):
finally some more pics
Hubs (cont.):
real hubs are never tidy
Repeaters:
repeaters, repeaters, repeaters
Deals with attenuation
 Signals weaken over long distances
 Repeaters re-amplify signals
 Two types

Repeaters
 Regenerators
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Does not reduce network traffic!
Bridge:
not the kind cars use to cross water

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Bridges extend maximum distance of a
network by connecting separate network
segments
They selectively pass messages from one
segment to another
Determine source and destination
Used to divide over-loaded networks
Reduces network traffic by preventing data
from traveling to segments that are not
supposed to receive the data
Bridges:
diagram
Modems

Telephone lines are used over long distances
 Modems convert digital signals from computers into
analogue signals that can be transferred via telephone
lines
 The received data is then converted back from
analogue to digital signals by the receiving modem
 Modems can be internally slotted into the motherboard
or connected externally via the comm port
 Modems are connected via a telephone cable
 Once the modem makes a connection the
communications software takes control
Modems (cont.):
various types

Optical Modems
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
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Short Haul Modems
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Used to connect different offices in the same building
Distances involved are less than 30km
Transmit electrical signals
Fax Modem
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Data transfer via fibre optic cables
Modem converts signals to and from digital signals into pulses of
light
Can answer or dial other modems
Functionality extended by communication software e.g. Fax
V.90
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Full-duplex
Data rates are 56 Kbps downstream and 33.6 upstream
Common everyday modem
Modems (cont.):
a diagram
Some Terminology
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Bits per second (bps)
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Baud
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The speed or rate at which data is sent
If a device sends data at 24000bps that is the rate at which it
will be received
Data is received at the rate it is sent
Signal events per second
Number of times per second a data transmission channel
changes state (i.e. from sending to receiving or vice versa)
Bandwidth


Capacity of a communication medium to handle data
Usually measured in megabits per second
LAN Configurations

Baseband
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

LANS that are capable of carrying only one signal
at any given time
Entire bandwidth used for each and every signal
Broadband



Networks divide bandwidth into different
frequencies
This creates different channels or paths for data to
move along
This enables the network to connect many different
information paths at the same time using the same
connection medium.
Topologies
The physical layout or shape of the nodes
in a network determine its topology

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Star Topology
Bus Topology
Ring Topology
Mesh Topology
Cellular Topology
Composite Networks
Star Topology:
twinkle twinkle little star
Central device (usually a hub)
 Cables connect at central point
 Point-to-Point direct link from each
device to central point
 Signals travel from devices through
cables to the central point (or hub) where
it is sent to the rest of the network

Star Topology (cont.):
a diagram
Bus Topology:
the wheels on the bus go round and round

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One long cable called the backbone
Shorter cables stem from the longer cable
connecting to various devices
No central point
Backbone is terminated at both ends in order
to remove the signal from the wire after it has
passed to all devices
Signals move up and down the cable from one
node to the other
Bus Topology (cont.):
another diagram
Ring Topology:
ring-a-ring a rosey

Circular topology
 Closed loop
 Data signals or
packets move in one
direction around the
loop until they reach
the destination
computer
Ring Topology (cont.):
yet another diagram
Mesh Topology:
clean up this mesh at once!
Point-to-Point connections from one
device to every device on the network
 All devices on the network connected
directly to one another
 Each device needs an interface (or
port/jack) to every other device on the
network
 Large amounts of bandwidth is wasted

Mesh Topology (cont.):
more diagrams
Cellular Topology:
<your quirky saying goes here>
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Wireless point-to-point strategy
Divided into cells
Each cell is a portion of the entire network
Devices within a cells communicate via a
central station or hub
Hubs are interconnected to form a complete
network infrastructure
Not dependant on cables
Relies on location of wireless media hubs
Cellular Topology (cont.):
what? no diagram?
<INSERT DIAGRAM HERE>
Composite Networks
Many networks consisting of a
combination of the above-mentioned
topologies
 i.e. in-bred networks

TIP OF THE DAY: Never jump in front of a
moving bus
Comparison of Cable Topologies
Type
Advantages
Station
Star
failure not
problematic
Easy to expand
Different cables can
be used
Easy trouble-shooting
Disadvantages
Costly,
more cables
and a hub is needed
If hub fails all
connected computers
are affected
If
Cheaper,
Bus
less cable
and no hub
Simple and stable for
small networks
Easy to expand
one node fails the
other are affected
Traffic can slow
network considerably
Security risk
Trouble-shooting
difficult
Comparison (cont.)
Type
Advantages
Can
Ring
cover greater
distances
No computer can
monopolise the network
as all nodes are given
equal access
Disadvantages
Not
as reliable as star
because faulty cable or
node affects rest of the
network
Trouble-shooting
difficult
Expansion of the
network disrupts the
setup
LAN Architectures
LAN architectures are different flavours of
LANS. They include a specific type of
topology, protocol and cabling. Each
LAN architecture has it’s own set of rules
and ways of operating.
LAN Architectures (cont.)
Ethernet Technology
 Token Ring Technology
 ARCnet
 FDDI
 Frame Relay
 ATM

explanations to follow
Ethernet Technology
Most common architecture
 Developed by Bob Metcalfe
 802.3 standard (IEEE)
 802.3 standardizes the use of CSMA/CD
protocol (more about this later)

Token Ring
Second most commonly used
 Works with ring topologies
 Uses a token passing technique

More about this later
Some other architectures

ARCnet
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

Attached Resource Computer Network
Really really old
FDDI


Fiber Distributed Data Interface
Mainly used for fibre optics

Frame Relay
 ATM

Asynchronous Transfer Mode
Network Types

Computers in a LAN can be classified
as:
Servers – computers providing services
 Clients – computers requesting services
 Peers – computer requesting and providing
services


The network operating system plays a
crucial role in determining the roles of
the computers on the network
Client/Server and Peer-to-Peer
Server-based LAN
Peer-to-Peer LAN
Novell, Banyan amd Windows
NT/2000 server
Windows for workgroups, NetWare
Lite and Lantastic
Workstations either server or client
machines
Workstations are peers
Network OS loaded onto server and
client OS loaded onto clients
Network OS loaded onto all
machines
Server provides all required services Workstations can request or provide
services
Server is dedicated with superior
resources and hardware
No server
One point of admin
Several points of administration
Client/Server and Peer-to-Peer
Server-based LAN
Peer-to-Peer LAN
Suitable where high performance is
required
Good where performance is not an
issue
Needs technical expertise to install,
configure and maintain
Installation and maintenance simple
High Security
Limited Security
Expansion simple
Expansion can be complex
Setup cost is high
Fairly inexpensive
Suitable for large networks
Suitable for smaller networks
Protocols
A protocol is a set of rules that govern how and
when data is transmitted between nodes in a
network
 CSMA/CD Ethernet Protocol
 Token-passing Protocol
 Polling Protocol
 TCP/IP Protocol
 IPX/SPX Protocol
 NetBEUI Protocol
CSMA/CD






Carrier Sense Multiple Access with Collision Detection
When a node wants to transmit it listens to the
medium to check if any other nodes are transmitting
data
If the medium is quiet the node transmits else the
node waits till the medium is quiet
Nodes listen while transmitting data, therefore
If two nodes try to transmit simultaneously a collision
occurs and both nodes release the wire and wait a
random amount of time before trying again
Random time is important because if the two nodes
transmit at the first available opportunity after the
collision they will collide again.
Token Passing






Used in ring and bus topologies
A “token” is passed around amongst the nodes
If a node needs to transmit data it must wait till it
receives the token
Once the node that want to transmit has the token, it
sets the token’s state to busy and puts its data on the
token
The token is then passed to the destination node
which sends confirmation via the token to the node
that sent the data
When the sending node receives the token it marks
the token as free and passes it to the next node
The Rest

Polling



TCP/IP






Transmission Control Protocol / Internet Protocol
Used for internet access
IPX/SPX


A master device asks each of the nodes in turn whether it has data to
transmit
Rather slow as every node gets the chance to transmit even if it does
not need to
Internet Packet Exchange / Sequenced Packet Exchange
NetBEUI
SAN
Apple Talk
SLIP and PPP
Network Security

User Security





Passwords should be changed periodically and
never duplicated
Users (or trustees) given limited rights and access
to certain files and folders
Users can be restricted to certain times of the day
Accounts can be locked if incorrect password is
entered too many times
Users no longer needing the network must me
deleted
Network Security (cont.)

Physical Security



Equipment should be kept separate from the rest of
the network
Access to expensive equipment should be
controlled
Data Security


UPS (Uninterrupted Power Supply) provides
enough power for Server to be shut down in case of
power failure
Backups should be done regularly
WAN Connectivity Devices

Routers
Connect two logically separate network
segments
 Forms logical boundary and reduces traffic
 Routers operate independent of medium,
method, topology or architecture
 NOT a bridge!


Gateways
Examples of WANs


Public Telephone Service
Internet
 Compuserve
 Sabre
 Amadeus and Galileo
 SWIFT
 Bitnet
 Diginet
 Beltel
 Saponet
 Telkom400
Virtual Private Networks





Employees of large companies that travel
need to connect to company’s network
Data is sent via the internet in packet-form
Each packet has data, address of sender and
address of the recipient
Each packet also contains a unique tag so that
the data can be reassembled on arrival at it’s
destination
Data is encrypted before sending and
decrypted when it reaches it’s destination
The End