Transcript Class notes
Day 2 – OSI Model What’s in the phone network? – Phones – Wire – Switches • If you include wireless phones – Cell towers – Wireless phones Wireless Telephone System Phone connections were physical 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 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 The Big Picture of Networks (continued) LAN Network 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 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 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. 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. 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 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. 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 ISO model Application IE Presentation SSL Session Sync Points Transportation End-to-End error/flow control Network Routing Data Link Frames Physical Wires 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 TCP/IP • Only 4 layers – No Session. • Application – Application & Presentation • Transport – Transport • Network – Network • Network Access – Datalink & Physical