Download Computer Networks

Internet protocols and network
hardware
Protocols are sets of rules or standards that are used for
communication. They work together to help devices to
communicate. The rules apply to different layers used for
communication purposes.
A network protocol is a standard method for
transmitting data from one computer to another.
Protocols/Services
 Physical layer-data is transmitted bit by bit from one device
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to another e.g on modems
Data Link Layer: It acts upon requests for services from the
physical and network layer e.g wifi and ethernet
Network/Internet layer: It is responsible for forwarding
packets and routing e.g internet protocol
Transport layer: It divides data to be transmitted and adds
the address of the source and destination address e.g UDP
and TCP
Application: It consists of protocols that use the transport
layer to deliver services to the network or Internet layer.
Ethernet
It refers to a basic set of protocols used to operate a local
area network(LAN). It is made up of the following:
 Devices that send and receive data such as PCs,
printers and servers.
 Network devices such as hubs, switches and routers.
 Medium connecting devices such as twisted pair, fibre
optic and coaxial cables.
TCP/IP
 Transmission Control Protocol(TCP) ensures that data
is delivered correctly
 Internet Protocol(IP) ensures that the data is delivered
to the correct address. An IP address consists of 4
bytes which are given in decimal form
 Techniques such as subnetting and local addressing
are used to increase the number of addresses
TCP/IP
 TCP ensures that the correct number of packets reach
the destination and that there are no duplicates as a
result of retransmission.
 Any duplicates are deleted
 Each packet contains a destination address, source
address and a sequence number which will be used for
assembling the packets at the destination.
 It is used on the www, e-mails and for file transfer.
Handshaking
 It refers to the exchange of signals between devices to
establish a communication channel eg the rate of
transfer, parity checking and interrupts to be used.
 Parity can be odd or even. It refers to the total number
of bits within a transmitted message.
 The number of bits is checked to make sure that there
are no errors in transmission
UDP
The UDP is a basic protocol. It is considered to be a
connectionless service. No handshaking is required. It
has no congestion control mechanisms. It is unreliable
because packets of data may fail to reach the
destination. It is still better suited for real-time
transmissions e.g video on demand systems
FTP
FTP is used for the transfer of files when downloading
and uploading files. FTP makes use of client-server
systems. Clients have to log on in order to download
files.
SSL and SSH are used to enable encryption of files
during transmission.
HTTP
HTTP is the protocol used for transmitting data across
the internet/intranets. It uses a client-server system in
which a browser makes a request using a URL.
The URL is converted to an IP address. The server will
then provide the required resources
Telnet
 Telnet is a network protocol used on the internet or
within local area networks. It is used to enable a user
to gain access to another computer and use the
resources provided there.
 It works at the same layer as FTP. The remote compter
should have telnet client/server software.
 Telnet has major security problems which has
decreased its popularity.
 It is still used for network diagnostics and
troubleshooting problems.
SSH
 It allows data to be shared between two computers. It
allows users to log in remotely and also uses
encryption.
 It uses uses public-key authentication.
 Its main purpose is to log into a machine remotely and
execute commands. It is therefore used by
administrators.
Network Hardware
Devices used in a computer network
Network Hub
A hub is a devices that has a number of network devices
or computers connected to it. When a hub receives data
packets it broadcasts them to all computers on that
network including itself. It does not read the data
packets. Hubs that amplify the signal are called active
hubs and those that do not amplify the signal are
passive hubs.
Switches
A switch is more advanced than a hub. When hubs are
used congestion may occur when several computers are
transmitting data at the same time. The same packet of
data is send to all the ports on the hub.
In a switch the data from any computer is sent directly to
its destination port without congesting the other ports
unnecessarily.
A switch uses address tables to find the correct port to
send data packets. It stores the Media Access
Control(MAC) address. It is used for connecting local
computers and devices.
Routers
A router enables data to be sent transmitted between
different networks e.g from a LAN to a WAN. It provides
facilities for protocol conversion. It may also have a
firewall installed.
The router inspects packets of data and checks the IP
address so that it can forward it to the correct network.
The first part of an IP address is the network address
which is then used for routing.
Dedicated cabling
 Twisted pair: two conducting wires twisted around
each other which reduces magnetic interference and
data loss
 Coaxial: A conducting wire surrounded by a plastic
non-conducting layer, then conducting material and a
PVC
 Fibre optic cable: Optic fibres which are individually
coated by a plastic resin and sheathed in a plastic layer
Networks that require fast speeds use twisted pair and
fibre optic cables.
Servers
File Servers- Store user data. Users must log on to use
the servers.
Application Server-Distribute application software to
client machines e.g the isams servers
Other servers include print servers, e-mail servers, proxy
servers etc
One computer can be used as multiple servers. File
servers make use of groups and access rights in order to
manage security.
Network cards
A network interface card is a card that fits onto a
computer’s motherboard. A network interface card may
allow both wired and wireless communication.
Network Types
 Local Area Networks- These are house within a
building or the same geographical area e.g schools,
offices etc
 The networks normally use switches/hubs
 The switch is in turn connected to a broadband
modem in order to connect to the internet.
Network Topologies
There are 3 main types of network
topologies:
The Ring
Network
The Bus
Network
The Star
Network
Network Topologies
The Ring Network:
Computers are
connected
together to form a
ring shape so that
none of them is
more important
than any of the
others.
Network Topologies
The Ring Network:
Advantages:
Disadvantages:
•They are cheap to expand.
•If there are a lot of users on the
network, it could slow down as all
the data is sent along a single
line.
•The data flows around the
network in one direction so it is
fast. It performs better than a
star network when traffic is
heavy.
•There is no reliance on a central
computer.
•If one computer in the ring
stops working, the whole network
stops.
•Adding a new computer may
cause disruptions to the network.
Network Topologies
The Bus (or line) Network:
Computers are
connected like bus
stops on a main
road. None of them
is more important
than any of the
others. The spine
has cables which
connect to each
network.
Network Topologies
The Bus (Line) Network:
Advantages:
Disadvantages:
•It is cheap as it uses the least
amount of cable.
•With a lot of users, the network
will be slow as data has to travel
through the same central cable.
•More computers can be added
without disruption.
•If one of the devices fails the
network will continue to function
as normal
•No hubs/switches are required
•Failure of the central cable will
stop the network from working.
•It is difficult to troubleahoot
when the whole network breaks
down.
Network Topologies
The Star Network:
Computers are
connected like a
star to a central
hub/switch/serve
r/router.
Network Topologies
The Star Network:
Advantages:
Disadvantages:
•If a cable fails, it will only affect
one workstation.
•It is more expensive as it uses
the most cabling.
•More computers can be added
without disruption.
•Failure of the central
computer/network device will
stop the network from working.
Hybrid Networks
It is made up of star networks where each hub/switch is
connected to one backbone. The backbone cable forms a
bus-type network. The switches may also connect to
each other forming multiple interconnected star
networks.
Network Topologies
The Hybrid Network:
Advantages:
Disadvantages:
•It employs the advantages of a
star network and bus network
•Sections can stop working if a
hub/switch breaks down.
•More computers can be added
without disruption.
•The whole network will not work
if there is a fault in the spine.
Intranets
Local Area Networks are used for creating intranets. An
intranet can be defined as a private company network
that makes use of internet technology. It is for the use of
members of that organisation.
Users are given usernames and passwords
Dedicated telecommunication links may also be used for
linking different locations such as fibre optic cable and
telephone lines.
It uses internet protocols.
Uses of Intranets
 Providing local e-mail
 Video conferences
 Sharing information
 Fast access to documents and statistics
 Sharing diaries
 Organising meetings
 Discussion forums
 Policy information
Local e-mail and Business
Networks
 Intranets are very secure and businesses use them put
up information that employees need to access.
 Intranets reduce paper costs
 Lan-based e-mail reduces the cost of printing
 An ISP can be used for providing the e-mail service
WLANs
A WLAN uses either spread spectrum( a signal with an
initially low bandwidth is spread to have a wider
bandwidth) radio waves or infrared signals for
communication.
Computers or devices must have WNICs
Devices can be added easily to the network
problems with the WLANs include security issues,
interference and poor speeds
Wi-fi
Wifi is short for wireless fidelity. Data is transmitted into
radio signals and then transmitted. The wireless router
decodes the signal.
The following hardware devices are used:
Wireless Network Cards
Wireless Routers
Wireless Access Points
Wireless Network Cards
 It transmits data as radio waves. Most laptops a fitted
with WNICs by manufacturers
 Desktop computers can also have these fitted
 Most mobile devices have wireless NICs
Wireless Router
It converts data signals into radio signals as well as radio
signals back to data signals.
Features include:
 Wifi protected access(WPA), a complexi encryption
system
 MAC address filtering and IP address filtering
 Invisible mode
Wireless Access Points
They are used for connecting wireless devices to a wired
network. They cover a limited area and number of
computers.
List Devices that have wireless capabilities:
Methods of Data transmission
 Spread spectrum radio waves are used for transmitting
data by increasing its bandwidth.
Methods of Data transmission
contd.
The same random number generators are used by the
receiver and sender to generate random frequencies and
stay synchronised.
It allows computers and other devices to communicate
through walls.
Another method is to use infrared signals. As it pulses on
and off it willl be transmitting data in bits. It is secure
because it cannot be used across walls. There is also no
interference. The main disadvantage of the system is the
lack of range.
WPAN
 Pg 141-143 Don’t have to do network security.
Network Security
Network security is there to ensure that unauthorised
users do not view data or understand it if they manage to
intercept/access the data.
Physical Security
This prevents users from gaining physical access to
computers. This can be achieved by:
-Securing Computer Rooms- All doors and windows
should be locked when computers are not in use. Burglar
alarms may also be used. Some Servers are lockable.
CCTVs can also be used for deterring thieves.
Pass cards can be used for giving users different times
and levels of access.
-Security Guards for patrolling and responding to
alarms.
Software Security
Most computer threats come from hackers who access
computer systems remotely. The following methods may
be used for preventing access:
Firewall
Encryption
Digital Certification
Authentication Techniques
Anti-virus Software
Firewalls
A firewall can be in the form of hardware or software or a
combination of both. Firewalls prevent unwanted
network traffic from accessing a computer network.
Packets of data are inspected and if their ip
address/network address is not wanted then they are
prevented from accessing the computer network.
Domains, words, phrases can be filtered out. Packets
with these phrases are prevented from entering the
system. At school we have a firewall that does this.
Encryption
Encryption refers to encoding data by scrambling or
encoding it. This is for preveting hackers from
understanding data sfter intercepting it.
The sending computer uses an encryption key for
encoding data.
The receiving computer has a decryption key for
decoding it.
Public key/Private key encryption is used. The sender
computer uses the public key for encrypting data and
the receiver will use a private key for decoding it.
Digital Certification
Digital certificates are issued by a certification authority.
The authority certifies that both receiving and sending
computers are who they say they are.
Once certified each one may get the public key of the
other.
Authentication Techniques
This can be achieved by using a for example a username
and password, card and pin code, biometrics such as
facial recognition and finger print scanners.
Anti-virus software
It is used to find viruses and remove them.
The anti-virus software has a database to remove known
viruses. One way is to scan the hard disk or monitor ports for
suspicious behaviour.
Actions taken by viruses include the following:
-Repair the files
-Quarantine the file by not allowing any program to access it.
-Delete the file
Anti-virus software must be updated regularly. Antivirus
definitions are becoming longer
Other Types Of Viruses
 Trojan horses
 Worms
User ID
 These are used for identifying users and must match a
password in the database. Users can also be grouped
and be given privileges.
 Users are given a limited number of attempts when
logging in usually three attempts
 Good password policies must be maintained such as ?
Chip and PIN cards
 Chip and pin card readers are slotted into keyboards
Anti-spam
 This is software for rejecting unsolicited e-mails
Anti-pop-up software
Pop up blockers are usually incorporated into the
browser. It identifies part of html code that will open
another window and prevents it from being executed.
Anti-spyware software
Spyware software allows people to access and control
your computer. It allows pop ups, changes browser
settings. In its worst form it searches for user names and
passwords.
Anti-spyware software searches the hard disk for known
spyware software and removes it. It needs constant
updating.
Key logging software may also be used to detect the keys
presssed.
Wired Equivalent Privacy of
information(WEP) and Wi-fi
Protected Access(WPA)
 WEP is used for encrypting data on wireless networks
but uses a weak key
 WPA uses stronger encryption keys and also includes
user authentication
Electronic conferencing
Hardware Requirements
Electronic conferencing requires additional hardware
such as microphones, earphones/speakers, webcams
including in some cases phone handsets.
Software Requirements
 Conference Software
 VOIP software
 Instant messaging
Coference Software
 Compression Decompression software is used to
reduce the size of the packets of data transmitted. The
packets of data must be transmitted in real time.
 Echo cancellation software is used for keeping
communication synchronised
 VOIP software may also be used for conducting
telephone conferences.
VOIP Software
 It is cheaper than using conventional telephone lines.
The software coverts the speech into digital data and
compresses the audio data so that it can be
transmitted in real time.
Instant Messaging
It is the exchange of text messages across the internet in
real time. Businesses and Institutions can also make use
of IM on Local Area Networks.
Video Conferencing
Advantages
Meetings can be called at short notice
No time spend on travelling
No money spend on travelling
No money spend on booking facilities
It is environmentally friendly
Meetings are kept shorter
It is safer
No payments for travelling time
Video Conferencing
Disadvantages
Confidential documents may need to be seen in their
original form
There is no direct eye contact
Delegates may be put off by the camera
Quality of the video depends on the bandwidth
Transmission breakdowns may occur
Environmental factors such as power cuts may also affect
the video conference
Phone Conference
Advantages
Cheaper than a video conference because there are no
webcams ot large screens
Less bandwidth
No need to worry about appearance
Phone Conference
Disadvantages
Documents cannot be seen
Facial expressions cannot be seen
Number of participants is limited
Power cuts may interrup services
Instant messaging
Advantages
It is instant
Saves time and money when resolving problems
Easy to use
Instant messaging
Disadvantages
Users may be exposed to dangerous people
It may lead to obescity
It is disruprive in workplaces when used for socialising
It may also pose a security risk when files are sent.
Uses Of Electronic Conferencing
Businesses
Schools
Research Meetings