Download - Day 2 – OSI Model
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