Transcript Session 6 Slideshow

Session 6: Networking
Teaching Computing to GCSE Level with Python
Topics today
 Networking
 Topologies
 The Internet & Packet switching
 IP addresses
 Protocols
Specification Links
OCR
(a) explain the advantages of networking standalone computers into a local area network
(b) describe the hardware needed to connect
stand-alone computers into a local area network,
including hub/switches, wireless access points
(c) explain the different roles of computers in a
client-server and a peer-to-peer network
(d) describe, using diagrams or otherwise, the ring,
bus and star network topologies
(e) describe the differences between a local area
network and a wide area network such as the
Internet
(f) explain the terms IP addressing, MAC
addressing, packet and protocols
(g) explain the need for security measures in
networks, such as user access levels, suitable
passwords and encryption techniques
(h) describe and justify network policies such as
acceptable use, disaster recovery, failover, back
up, archiving.
AQA
a) understand what a computer network
is
b) be able to discuss the advantages
and disadvantages of using a
computer network
c) be able to describe and explain the
bus, ring and star networking
topologies
d) be able to discuss the advantages
and disadvantages of each of these
topologies
e) understand the client-server model
f) be able to explain, in simple terms, the
handshake process used in most
modern networking protocols
g) be able to explain how coding for a
client-server model is different from
coding for a stand-alone application
Networked Computers
 Computers and peripherals connected together
 May be in one building or site (LAN)
 Or across large geographical area (WAN)
LAN
 A LAN is a Local Area Network.
 It is a connected set of computers and other
devices.
 Each device is called a node.
 A LAN is installed on one site.
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•
It does not cover a wide geographical area.
All the infrastructure is owned by the
organisation or individual who owns the LAN.
Stand-alone Computers
 Need own software installed
 Backups - own responsibility
 Need printer
 No access to centralised resources
Advantages of Networks
 Share resources
 Share information
 Centralised access to data
 Mail/messaging systems
 Access to own files from anywhere on network
Disadvantages of Networks
 Users’ dependence on network
 Busy network ---> delays
 Needs to be well-designed for optimum
performance
 Threat of viruses/hacking
Wide Area Network
 Spread over large geographical area
 May be public or private
 Communication may be by:
 telephone network
 satellite link
 dedicated cables
 microwave
Network Topologies
 Bus
 Star
 Ring
Network topologies: Bus
Bus Network Topology
One common or “omnibus” cable connects all nodes
Network topologies: Bus
 All workstations are connected by a single cable
(a bus)
 Max length of cable 500m
 Only suitable for LANs (Ethernet)
 One PC used as a file server/print server
Advantages of Bus
 Cheaper to install than ring
 Requires least amount of cable
 Easy to install and extend
 Failure of one workstation does not affect
network
Disadvantages of Bus
 Network performance degrades under heavy
load
 Dependent on reliable main cable
 Cable failure difficult to isolate
Network topologies: Star
 Suitable for LAN or WAN
 If one link or node (other than the central one) fails, the rest of the
network can still function
 All data passes through the central node, so if it fails the whole
network “goes down”
 Formerly commonly used for
multi-user systems
(terminals plus “mainframe”)
Network topologies: Star
 Suitable for central computer (hub) and dumb terminals
 Very popular for WAN where organisation relies on central
computer for bulk of processing tasks, limiting the nodes to their
local processing needs and the validation of data, prior to
transmission to central computer
Advantages of Star
 If one node or cable faulty the network can still function
 Centralised control of message switching allows high degree of
security control
 Methods of data transmission and speeds used can vary from one
spoke to another
e.g. public phone line or leased line
 No problems with collisions of data
Disadvantages of Star
 Needs a lot of cable (costly)
 Dependent on central node functioning
 Control of communication requires expensive
technology at the hub
Network topologies: Ring
Ring Network Topology
Each node connected to two neighbouring nodes
Network topologies: Ring
 No central server
 Not suitable for a WAN
 None of the nodes have overall control
 Messages flow in one direction only
Advantages of Ring
 No dependence on central computer
 High transmission rates are possible
 More predictable than bus
 Transmission of messages relatively simple
Disadvantages of Ring
 Difficult to extend as new cable must preserve
the ring topology
 If one node breaks down, transmission between
the other nodes is disrupted
Bus Network
The type of network topology in which all of the
nodes of the network are connected by a
backbone which has two endpoints. All data that
is transmitted between nodes in the network is
transmitted via the backbone.
Star Network
The 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,
which is usually a switch or a hub that then
retransmits the data to some or all of the other
nodes in the network.
 Network Topologies
Star Network with hub
Client
Workstati
on
Printer
Data
Packet
Hub
File
server
 Network Topologies
 Star Network with switch
Client
Workstati
on
Printer
Data
Packet
Switch
File
server
AQA Computing A Level © Nelson Thornes 2008
LAN hardware
 NIC – network interface card
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Each node needs a NIC to connect to the LAN.
Produces the electrical signals that carry data.
 Hub

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
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A device that connects nodes together.
Makes them act as a single segment.
Not smart – all data packets are transmitted.
Cheap devices – USB hubs useful in home
computer installations.
OCR Computing for GCSE
2011
© Hodder Education
LAN hardware
• Wireless access point

Wireless technology has many
advantages over wired
networks.
•
No building work
needed
•
Quick to add new
workstations
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Flexible deployment of
nodes
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Visitors can access
network / internet
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But there are drawbacks
 Slower performance
 Security issues
Peer-to-peer networks
All computers have the same status.
May be very slow because of
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data collisions
shared processor power
Only suitable for small low traffic
networks.
Client–server networks
 One or more computer is
designated as a server.
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•
•
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A high specification machine.
It offers services to client machines,
such as centrally held software, data,
file storage and peripheral access.
Servers control network traffic.
Log usage of resources.
Provide security.
Clients have to log in – this identifies
users and assigns rights.
Big networks may have hundreds of
servers.
OCR Computing for GCSE
2011
© Hodder Education
Protocols
Protocol
Meaning
Application
 Protocols are the rules for
communication between devices.
DNS
Domain Name
System
Translates domain names
such as ocr.org.uk into IP
addresses.
 Standard protocols encourage
network development.
TLS/SSL
Transport Layer
Security / Secure
Sockets Layer
Cryptographic protocols
designed for secure
communications.
FTP
File Transfer
Protocol
For copying files from one
host to another.
 Network protocols are constructed
in layers.
Gopher
 This allows independent
development of small parts of a
protocol.
 Internet Protocol is a group of
protocols that underlies the
internet.
An early means of
searching for files on the
Internet.
HTTP
Hypertext
Transfer
Protocol
For distributing
hypermedia files –
essentially web pages.
IMAP
Internet
Message Access
Protocol
One method for
accessing emails.
POP3
Post Office
Protocol (version
3)
Another method for
accessing emails, used
by most webmail
services.
 TCP/IP now the most important set
of protocols.
Telnet
 Includes many special protocols.
OCR Computing for GCSE
2011
Allows bidirectional text
communications on a
network.
© Hodder Education
IP Addresses
 Every Web site has a ‘Home page’ with a unique address
known as its IP address
 4 numbers, each in range 0 to 255, separated by full-stops.
e.g. 177.234.243.186
 IP addresses are difficult to remember, so can use Domain
Name instead
 Domain name system maps the domain names onto the IP
addresses
Ethernet Addressing
 Each Ethernet card that is manufactured is assigned a
unique Ethernet hardware address.
 Ethernet header contains two addresses:
- 48-bit source address that uniquely identifies the
source of the frame
- 48-bit destination address that uniquely
identifies the destination of the frame
 The Ethernet address enables the frames to be routed
between two computers that are in communication.
The frame just looks for a matching hardware address
in each network card it encounters over the Ethernet
cable.
 IP addresses are different from Ethernet addresses.
They may be moved from computer to computer.
Sending a file over a
network
 User sends file using FTP
 File is passed to another layer of software – the
TCP/IP protocol stack
(A protocol stack has multiple layers)
 TCP/IP software breaks file up into packets
What is a packet?
 “A packet is a discrete collection of information
that includes user data plus protocol information,
such as destination and source addresses, and
controls for error detection/correction,
sequencing, flow control etc.”
Packet Switching
 Messages are broken down into smaller blocks
(packets)
 Packets routed individually through the network
 Packets from a particular message may travel
along different routes through network
 At the receiver, packets are reassembled to
construct complete message.
Datagram Packet Switching
Network
Original message
Network
EDCBA
Message packets
D C
E D C B A
A
1
D
A
3
5
C
Computer X
E
D
B
Computer Y
A
E
2
Packet switch node
2
E
4
C
B
E D C B A
6
EDCBA
Re-assembled message
Packet switching demo
 Packet switching demo here
Client/Server Model of the
Internet
 A server provides a service requested by a client
 A client that requests and uses services provided
by a server
 In the client-server model of the internet the
client and server are software processes that fulfil
these roles
 HTTP is a client/server protocol
Protocols
A protocol is a set of rules that is used to ensure
successful communication between different
computers
FTP
 File Transfer Protocol
 An application-layer protocol
 Enables on one computer to be copied to
another computer
 Uses port 20 for data and port 21 for commands
 A port is an address for a software process
HTTP
 Hypertext Transfer Protocol
 An application-layer protocol
 Used to enable web pages to be viewed
 A client machine sends a request message to the
server and the server responds with a message,
usually in the form of a web page.
 Uses port 80 and 8080
POP3
 Post Office protocol 3
 Application layer protocol
 Used to retrieve stored mail
 Uses port 110
SMTP
 Simple Mail Transfer Protocol
 Application layer protocol
 Used by email clients to send email.
 Recipients of the message are specified and the
message text is transferred to a mail server
listening on port 25
HTTPS
 HyperText Transfer Protocol over Secure Sockets
layer
 Application-layer web protocol
 Encrypts and decrypts user web page requests
 Uses port 443
Security
 Transmitted data is vulnerable.
 Users on a network do not need to see all the data.
 Access levels
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Users granted privileges.
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Associated with login names.
 Passwords
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Must be changed regularly.
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Passwords should be strong.
 Encryption
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Especially important on wireless networks.
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WEP or WPA should be used.
OCR Computing for GCSE
2011
© Hodder Education
Policies
 Network owners should have policies to protect their
assets.
 Users have responsibilities.
 Acceptable use policies are common.
 Policy ensures that users understand what is expected of
them.
 Sanctions used if users misuse resources.
OCR Computing for GCSE
2011
© Hodder Education
Disasters
 Disasters do happen.
 Need to plan for worst case scenario.
 Example strategies:
• mirrors of systems
• use of disk protection technology such as RAID
• surge protectors — to minimise the effect of power surges on
delicate electronic equipment
• uninterruptible power supply (UPS)
• back-up generator in case of a power failure
• fire preventions — alarms, fire extinguishers
• anti-virus software and other security measures
• failover systems
OCR Computing for GCSE
2011
© Hodder Education