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Day 2 – OSI Model

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  1. Day 2 – OSI Model

  2. What’s in the phone network? • Phones • Wire • Switches • If you include wireless phones • Cell towers • Wireless phones

  3. Wireless Telephone System

  4. Phone connections were physical

  5. Terms • Workstation • Your computer • Server • Computer which answers requests • Mainframe • Big server typically accessed remotely • Hub/Switch/Router/Bridge • Device to connect networks • Wire • Used to connect from one location to another

  6. Computer Networks - makeup • Computers • Servers • Network Interface Cards (NICs) • Hubs/Switches/Access Points • Routers/Hubs • Wire (Cat 5) • Phone networks • Fiber Optics • Wireless transmission • Software

  7. The Big Picture of Networks (continued)

  8. LAN Network

  9. Logical Connection • Unlike Physical connection • No wiring actually changed • No moving parts • Connection only exists in software • Computers create virtual connection • Dismantle connection when users are finished. • Phone network is now all logical • All data connections are logical

  10. Where do web pages come from? • Open Internet Explorer (or Firefox, Safari…) • Type in the address of the web site • E.g http://kahuna.clayton.edu/~enda • Wait a moment • Page appears

  11. How does it work? • Web browser: • Lookup DNS name of server • Connects to IP address of server on port, asks for page • Reads resulting page and decides if more pages are now required • E.g. Images, stylesheets… • Presents the information to you.

  12. How does IE know where cnn.com is? • It doesn’t. • IE asks the operating system to do a “DNS lookup” on the name (e.g. www.cnn.com) • This results in an IP address 64.236.24.28 • Now IE asks the operating system to connect to that machine.

  13. So Windows is the smart one? • Well not exactly… • Windows has no idea where the IP is either, it hands the request to its closest networking device • Router • Access Point • Modem

  14. Long sequence of events • Each network device can change the request to suite itself • Pass the request on to the next device • E.g. • Modem -> modem -> router -> router -> router -> router -> web server • The same happens in reverse.

  15. This is complex… • The smart people at the International Organization for Standardization (ISO) came along to describe what happens • They came up with the ISO model • Which helps a little

  16. ISO model

  17. IP • The OSI model is just that a MODEL • There is no real implementation of OSI • Most people today use IP (Internet Protocol) for all communications • IP is a suite of protocols, many of which you use daily • The most common are TCP, UDP and ICMP

  18. TCP/IP • Only 4 layers • No Session. • Application • Application & Presentation • Transport • Transport • Network • Network • Network Access • Datalink & Physical

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