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