Networks

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Networks
VCE Software Development ¾
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What is a network?

• A network is two or more computers connected
  together for the purpose of
• sharing resources (printers, files, software,
  hardware)
• communication

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How are they classified?

• Their size LAN, WAN
• Their configuration client-server, peer-to-peer
  (no servers)
• The rules they use to exchange data protocols
• How they are linked together cable, wireless
• Their logical shape (topology) bus, tree, star,
  ring, mesh

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Types of networks

• Local Area Networks (LAN)
• One location
• Wide Area Networks (WAN)
• Centralised WAN Servers in one location
• Distributed WAN Servers distributed throughout
  the network (the Internet is an example of this)

LAN
WAN
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Network configurations

• Client-server
• File server
• Files stored and accessed from central location
• Application server
• Applications installed and executed from central
  location.
• Often used in combination with file servers
• Peer-to-peer
• No servers
• Applications installed on each PC, files shared

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Protocols

• A protocol is a convention or a standard that
  controls or enables the connection, communication
  and data transfer between computers on a network
• Contains rules governing the syntax, semantics
  and synchronization of communication
• May be implemented by hardware, software or a
  combination
• Like two people agreeing to speak the same
  language

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Communication protocols

• Communication protocols are specifically designed
  to control data transfer over a communications
  channel
• Some common examples
• TCP/IP Two protocols that control communication
  across the Internet and some networks
• POP3, IMAP, SMTP Protocols that can be used to
  send and receive email
• HTTP Method of publishing and retrieving web
  pages
• FTP Protocol for sending and receiving files

These examples are often referred to as the
Internet protocols
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Network Protocols

• Rules and conventions for communication between
  network devices
• The commonly used are Ethernet, Wi-Fi
• Others exist but are not as widely used

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Ethernet

• Uses an access method called CSMA/CD (Carrier
  Sense Multiple Access/Collision Detection)
• CS Computers (or nodes) listen to the network
  to see if there are any signals being transmitted
• MA All computers (or nodes) can hear what is
  being transmitted on the network at any one time
• CD Collision detection two computers (or
  nodes) could transmit at the same time, causing a
  collision

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Ethernet

• What happens when there is a collision?
• Each computer (or node) waits a random amount of
  time and then retransmits
• The random time delay is usually enough to ensure
  the collision does not reoccur
• The Ethernet specified speed is 100Mb/s
• The Fast Ethernet standard increases this to
  1000Mb/s

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Wi-Fi

• Based on the 802.11x standard
• Cannot do collision detection and instead uses
  RTS/CTS
• RTS Request to send packet sent by the sender
• CTS Clear to send packet sent by the intended
  recipient that alerts all other nodes to be
  quiet

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TCP/IP

• TCP/IP consists of two protocols TCP and IP
  protocol
• TCP (Transport Control Protocol)
• Used to divide data that needs to be transmitted
  into a number of packets, each with a sequence
  number
• The TCP at the destination address assembles the
  data and notifies the sender if any packets are
  not received within a certain time (timeout)
• IP (Internet Protocol)
• Defines how information gets sent between network
  nodes

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TCP/IP IP Address

• In order for packets to be able to be sent across
  the Internet or a network, each node must have a
  unique IP address (just like a street address)
• Domain Name Servers (DNS) translate URLs to IP
  addresses
• An IP address consists of 4 bytes (32 bits)
• For example 255.255.255.255

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TCP/IP Packets

• TCP divides data to be sent into packets
• Each packet contains
• A header consisting of
• a destination address
• a source address
• the packet number
• the total number of packets
• a checksum
• and some other things
• The data itself (known as the payload)
• A trailer

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TCP/IP From source to destination

• If a packet arrives at a destination with an
  incorrect checksum, it is discarded
• The path from the source to destination is in
  flux, therefore packets get lost, are corrupted
  or are duplicated
• If a timeout occurs or a packet is corrupted, a
  request for a resend of that packet is sent
• After several unsuccessful attempts at receiving
  a packet, the connection is broken

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Network Hardware

• Network hardware includes
• Network Interface Controllers (NICs)
• Switches / Hubs
• Repeaters
• Bridges
• Routers
• Transmission media (cabling)

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Network interface controller

• NICs provide a physical connection from a
  computer (or node) to the network
• Can be integrated on the motherboard
• Each has a unique 48-bit MAC address
• Stored in ROM
• Media Access Control
• Acts like a name for the card
• No two the same

http//en.wikipedia.org/wiki/Network_card
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Hubs

• Connects a number of NICs together
• Consists of a number of ports
• Signals arriving at one port and sent to all
  other ports
• Large number of collisions a dumb device

http//en.wikipedia.org/wiki/Ethernet_hub
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Switches

• Sends data only to the port that requires it a
  clever hub
• Reduces collisions and in many cases can
  eliminate them altogether
• Much better than hubs traditionally were much
  more expensive, but this is no longer the case

http//en.wikipedia.org/wiki/Network_switch
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Repeaters

• As a network signal degrades as it travels over
  large distances, a repeater can be used to boost
  the signal
• Both switches and hubs act as repeaters

http//www.unitedsale.net/products/image-02/D-Link
-DWL-G710-Range-Extender-2.jpg
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Bridges

• Network devices that can be used to connect two
  different physical networks together
• For example, a bridge could be used to join an
  ethernet network and a fast ethernet network or a
  star and a bus topology

http//www.stream-shop.ru/Modems/P6212.jpg
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Routers

• Acts as a junction between two or more networks
• Directs traffic in the same way as a switch, but
  can also translate and filter (firewall)
• Used to connect LANs to the Internet
• For example, a router may connect to the Internet
  and be assigned an IP address by an ISP
• IP addresses may exist within the LAN that
  duplicate those on the Internet, but the router
  ensures these are private to the LAN

http//shopproducts.howstuffworks.com/LinksysWRT5
4GSWirelessRouter/SF-1/PID-20664068
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Bridges vs Routers

• Bridges and routers work in different ways (on
  different OSI layers look up the definition of
  this if you want to know more)
• Bridges are not able to convert from one protocol
  to another where routers can

http//images.amazon.com/images/P/B000H1L5W4.01._A
A280_SCLZZZZZZZ_V61775592_.jpg
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Network Cabling

• Cables transmit data from one network node to
  another
• Several types of cable exist, each having a
  particular bandwidth
• Coaxial cable
• UTP cable
• Fibre optic cable
• Different types can be used within the same
  network, but this can affect bandwidth

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Coaxial cable

• Consists of a round conducting wire, surrounded
  by an insulating spacer, surrounded by a
  cylindrical conducting sheath
• Highly resistant to signal interference
• Not used much today
• Bayonet style connectors
• Often associated with bus networks
• Bandwidth 10Mb/s

http//searchnetworking.techtarget.com/sDefinition
/0,,sid7_gci211806,00.html
http//www.fotosearch.com/results.asp?keywordcoax
ialcablecategorysearchtypesss
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UTP Cable

• Unshielded Twisted Pair (UTP) is the most popular
  cabling used currently
• Also comes in a Shielded Twisted Pair version,
  that prevent interference
• Cables are twisted together to decrease
  interference
• Length limit is 100 metres
• Cheap

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UTP Cable

• Connector on the end is called an RJ45
• CAT5 is most common version and provides a
  bandwidth of 100Mb/s
• CAT5e can provide 1000Mb/s (or a gigabit/s)
• CAT6a is a future specification designed for
  10Gb/s!

http//en.wikipedia.org/wiki/ImageUTP_ethernet_ca
ble_8pin_modular_jack_endings.jpg
http//en.wikipedia.org/wiki/Category_5_cable
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Fiber Optic Cable

• Transmits light using the concept of critical
  angle
• As a result, cannot bend through tight angles
• Not affected by interference
• Can carry multiple signals at the same time
• Very hand bandwidth possible
• Expensive and difficult to install

http//www.fotosearch.com/PDS084/aa031778/
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Wireless LANs

• Wireless network hardware is available
• Wireless router
• Wireless NIC
• Wireless Access Point (equivalent to a switch)
• Relatively cheap and easy to install
• Encryption a must!
• WAP (Wireless Application Protocol)
• WPA, WPA2 are better

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Wireless LANs

• 802.11x is a set of standards defining the way
  signals are formatted, the frequencies they use
  and ultimately, transfer speed
• 802.11a (1999), max 54Mb/s
• 802.11b (1999), max 11Mb/s
• 802.11g (2003), max 54Mb/s
• 802.11n (draft) (2007), max 540Mb/s
• Typical speeds are often half the max speeds due
  to other network traffic
• Most devices are backwards compatible, for
  example a device might be 802.11bg

http//en.wikipedia.org/wiki/ImageLinksys_Router.
png
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Topology

• The arrangement of nodes within a network
• Main types
• Bus
• Star
• Ring
• Tree
• Mesh
• Not examinable

http//en.wikipedia.org/wiki/ImageNetworkTopologi
es.png
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Bus

• Contains a backbone which usually runs the length
  of the network
• Ends are normally terminated using special
  connectors
• Often associated with coaxial cable

http//compnetworking.about.com/od/networkdesign/i
g/Computer-Network-Topologies/Bus-Network-Topology
-Diagram.htm
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Star

• In a star network, a central point is used to
  connect all other network nodes
• The central point is usually a hub or a switch
• Common network topology
• Easy and cheap to implement

http//compnetworking.about.com/od/networkdesign/i
g/Computer-Network-Topologies/Ring-Network-Topolog
y-Diagram.htm
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Ring (not examinable)

• All network nodes are connected in a ring
• A token is passed around the network from one
  computer to another
• Only the node that holds the token can transmit
  data
• No collisions
• Used to be implemented using coaxial cable and is
  sometimes used with fibre optic cable today
• Two rings transmitting data in different
  directions can be used

http//compnetworking.about.com/od/networkdesign/i
g/Computer-Network-Topologies/Ring-Network-Topolog
y-Diagram.htm
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Tree

• A tree topology combines star and bus topologies
• Usually this takes the form of star networks
  linked by a network backbone (or a bus network)

http//compnetworking.about.com/od/networkdesign/i
g/Computer-Network-Topologies/Tree-Network-Topolog
y-Diagram.htm
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Mesh (not examinable)

• A mesh topolgy is one in which paths between
  network nodes are often changing as the status of
  nodes changes
• The Internet is a perfect example of a mesh
  topology

http//compnetworking.about.com/od/networkdesign/i
g/Computer-Network-Topologies/Mesh-Network-Topolog
y-Diagram.htm