Transcript 6.2 Networks (AQA)

Section 6.2
Section 6.2: Networks
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Objectives
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Section 6.2
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Section 6.2
Local Area Networks
Emerged in 1970s to replace mainframe computers
More economical
Covers a small geographic area
High speed communications
Low error rates
Links
Twisted pair
Baseband coaxial
Broadband coaxial cable
Optical fibre
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Section 6.2
Network Topologies
Network topology is the study of the elements of a network:
the nodes (e.g. the computers and other devices) and
the interconnections between them.
The most common network topologies are star and bus.
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Wide Area Network
WANs are a set of links that connect geographically
remote computers and LANs
Mobile phone network is a WAN
WAN connections may use:
Public switched telephone network
Dedicated leased lines
Fibre optic cable
Microwave links
Satellite
Radio waves
Internet
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Section 6.2
Internetworking
Two LANs connected by a WAN so that computers
on one network can communicate with computers
on the other network.
The Internet grew from LANs on University
campuses that were interconnected by a WAN
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Network Adapter (card)
Used by a computer to communicate on a network
Plugs into the computer’s motherboard and a
network cable
Coverts parallel data from the computer to serial
data on to the network (vice versa)
Uses an area of memory called a buffer
Checksum is calculated for the block of data in the
buffer
Address information added to the block of data
Data transmitted one bit at a time on to the cable
Has a unique 48 bit MAC address
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Bus Network
All computers are attached through a NIC to a linear
transmission medium e.g. wired bus network using coaxial
cable
Each computer attached can be identified by the NICs MAC
address
Two computers transmitting on to bus at same time causes a
problem called ‘collision’
To reduce collision, the transmission duration is limited to one
frame of pulses
Computers follow a transmission protocol called ‘carrier sense
multiple access with collision detection’ or CSMA/CD for short
CSMA/CD reduces collision problems
Ethernet is a bus network which uses CSMA/CD
Break in cable stops whole network working
Not secure against eavesdropping as frames can be read by all
connected nodes
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Bus Network
Data Packet
Client
Workstation
Printer
Backbone
File server
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Section 6.2
Switched Ethernet
LAN is wired in star topology with computers connected to a
central switch
LAN still behaves as a bus
Central switch queues frames
Switch ensures that collisions do not occur
A separate cable is run from switch to each workstation
If a node goes down, traffic between other nodes are
unaffected
Unplugging a network cable will not affect the rest of the
network
Out performs Ethernet LANs based on coaxial cable
Performance can be affected when traffic volumes high,
because the buffers in the switches suffer overflow.
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Section 6.2
Segmentation
Performance in a non-switched Ethernet network drops as
more computers are added
Non-switched Ethernet bus networks are often split into smaller
parts called network segments. This improves performance
Segmentation is a process of splitting a non-switched Ethernet
into segments linked by bridges or routers
Segmentation reduces collisions
Bridges and routers enable communication between two
computers on different segments
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Star Network
The star network is a type of network topology in which
each of the nodes of the network is connected to a
central node.
All data that is transmitted between nodes in the network
is transmitted to the central node.
If the central node is a hub the data will be
retransmitted to all of the other nodes in the network.
If the central node is a switch the data will be
retransmitted to one node in the network.
If link to a node goes down, the other links and nodes are
unaffected
If central node goes down, the whole network will fail
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Star Network with hub
Data Packet
Printer
Client
Workstation
Hub
File server
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Star Network with switch
Data Packet
Printer
Client
Workstation
Switch
File server
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Peer to Peer Networks
There are no dedicated servers
All computers are equal and are known as peers
Each computer functions as a client and a server
The user at each computer determine what data,
disk space and peripherals on their computer are
shared
Password protection to resources e.g. a directory
can be given
Peer networks are organised into workgroups of up
to 10 computers.
BitTorrent and Skype are examples of P2P Internet
protocols
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Server Based Networks
A network in which resources, security,
administration are provided by dedicated servers
A dedicated server functions solely as a server
Servers ensure the security of files and directories
Clients use servers to authenticate users logging on,
file storage and printing
Multiple servers used in large networks e.g. file
server, web server
ECC’s network is server based.
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Section 6.2
Web 2.0
Software becomes a service that is accessed over
the Internet
Examples of Web 2.0 include the Google search
engine and Social Networking sites
Software never needs to be distributed to users,
only performed
Google’s service is delivered by a massive collection
of Internet servers
iTunes is another example which seamlessly
reaches from the hand-held device to a Web
backend
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Web Services
These are self-contained, modular applications that
can be described, published, located and invoked
over the Web
An API is published for use by other services on the
network that encapsulates implementation details
Software as a service (SaaS) is a model of software
deployment where an application is hosted as a
service provided to customers across the Internet
SaaS eliminates the need to install and run the
application on the Customer’s own computer
SaaS software vendor may host the application on
its own web server
See examples of web services used in business
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Wireless Networking
No need to run cables through walls and ceilings
A Wireless Access Point (WAP) allows devices to
connect to the network
WAP allows data to flow between wireless devices
and wired devices
WAPs use IEEE 802.11 standards e.g. Radio
frequencies
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Wi-Fi
A standard for connecting computers wirelessly in a
LAN
Slower than bus and star networks which use cable
Vulnerable as radio waves carrying the data can be
intercepted
Wired Equivalent Privacy (WEP) gives a level of
security to wireless LANs
Wi-Fi Protected Access (WPA) provides a better
level of security than WEP
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Bluetooth
A wireless protocol for exchanging data over short
distances
Used by fixed and mobile devices e.g. mobile
phones, laptops, GPS
Uses Frequency Hopping Spread Spectrum (FHSS)
Data rate of 1Mbps
Data chopped into chunks
Data chunks are transmitted on up to 79 frequencies
Uses a secure, unlicensed short range radio band at
2.4 GHz (normally reserved for industrial, scientific
and medical use)
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Section 6.2
Routers
A device that receives packets from one computer,
uses the destination IP address contained in the
packet to pass them to another computer correctly
formatted
Route chosen is determined by the destination IP
address
Usually connected to a network of other routers
Maintains a table of other routers it is connected to
A packet of data may pass through several routers
before reaching its destination
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IP Addresses
This defines where a host computer is on the
Internet
Public (routable) or private (non-routable) addresses
Internet Assigned Numbers Authority (IANA)
responsible for IP addressing systems
Two versions of IP addresses
IPv4 most commonly used 32 bit e.g. 192.168.0.0
IPv6 128 bit numbers
ISPs assign IP addresses to users
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Connecting two LANs by routers
See page 195-196 in A2 Computing by Bond and
Langfield
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Gateways
A device used to connect networks using different
protocols so that information can be passed from
one system to another
LAN which does not use TCP/IP protocol is
connected to a gateway
Gateway translates the LAN packets to WAN
packets and sends on to the Internet (and vice
versa)
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Setting up a Computer on a LAN
Each computer with a public presence on the
Internet has a unique IP address
Each computer on the Internet has to have TCP/IP
settings
IP address
Subnet mask
Gateway or router IP address
DNS servers
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Setting up a Computer on a LAN
Subnet mask e.g. 255.255.255.0
Defines the size of the network
Tells computer which LAN its connected to
Tells computer which addresses it cannot reach
directly
DNS servers
Domain name system servers
Keep track of association between domain
names and its IP address
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Section 6.2
Class work
1.
2.
3.
June 2002 CPT5 Q4 b) ii) and iii)
January 2005 CPT5 Q10
2010 Specimen question paper COMP3 Q5
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Section 6.2
Homework
1.
2.
COMP3 2010 Q9
COMP3 2011 Q8
Hand-in Wednesday 9th May 2012
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