Computer Networks

A Guide for Teachers

What is a computer network?Computer networks allow computers or other devices, like printers, to be connected together.

Many different devices can be connected to the network including:

o Files and documentso Internet connectiono Printerso Game systemso DVR's

What is the use of a computer network?

Imagine you had a computer on your desk.  This computer was not hooked up to anything, just the computer itself.

• How would you print?• Where would you save files?• How would you find information?

What use is there for this isolated computer? 

Types of computer networksThere are different types of computer networks.  The main networks you are likely to encounter are:

• Peer-to-Peer• Client/Server

Networks allow users to share computer resources.

• Print from your desk to a network printer or copier• Save files to a central drive.• Access information on the Internet.

Peer-to-Peer networkEach device on the network can share data.  Most common in homes.

Usually require the following equipment:

• Modem - Cable, DSL, Satellite• Router - routes data to the Internet • Devices - computer, laptop, printer

Client/Server networkEach computer (client) is connected together but a central server manages network activity.

Usually require the following equipment:

• Internet Connection - T1, Fiber, Cable, DSL • Firewall - control data in and out of network• Server - authenticates users, stores shared

files• Switches - connect devices together • Wireless Access Points - connect wireless

devices to network• Devices - computer, laptop, printer

What is a server?A server is a compter. Servers have server software installed to let them perform specialized tasks on the network.Some examples of servers are:

• Domain Controller - manages users on a network• File Servers - stores files in a central location• Web Server - hosts web sites• DNS Server - directs Internet traffic

 Generally servers are more powerful than desktop computers as they need to process requests from several computers.

What happens when you log into the school network?Your username and password are sent from your computer (client) to the server.The server checks your username and password.            

Your desktop is loaded and you can access the drives on the server from your computer.

How does data travel on the network?

The Ethernet cable from the back of your computer needs to be plugged into the wall in order to be connected to the network.

Ethernet cable has four pairs of wires that transfer data on the network.

The data travels from your computer to the server over Ethernet cable.

Connections on the NetworkFrom your computer the signal goes to the wiring closet  Inside the wiring closet there are a number of switches.        These switches connect all the network devices together.

Chunks of DataThe data flowing on the network is broken up into many small pieces.

These pieces are called network packets. Network packets are made up of the header, payload, and trailer.

The Packet

The header contains the address of the packet's destination.

The payload contains the data in the packet.  This could be part of a username, password, or a picture from a web site.

The trailer contains error checking to make sure the entire packet is delivered.

Network PacketsPackets travel from device to device over the Ethernet cable much like cars on a road.

These packets contain all the data for computer communication.

Packets are broken up and assembled by the computer.

Smaller to Larger NetworksThe computers in one school are generally on a LAN or Local Area Network.

Many LAN's can connect together to form a WAN or Wide Area Network.  This may be universities with buildings outside the main campus.

The connection of these networks form the largest network of all, the Internet.

Many WAN's joined together to make the Internet.

How data gets to the InternetWhen you request a web page, the request is broken up into several packets.  These packets travel from your computer and onto the LAN.   

If you want to go to Google's search page, you would type the URL into your browser and hit 'Enter'.

Your computer chops up this request into packets and sends them through the Ethernet cord onto the LAN.   

How data gets to the InternetThe packet header directs the router to accept the Packet.  Since it is destined for the Internet, the router sends the packets to the Internet Service Provider (ISP). The ISP directs the packets to the correct web server.   

The school ISP directs your request for the Google search page to Google's servers.   

How data gets to the InternetThe web server assembles the packets and sends the request back to your computer.  These are also sent in packets.

Google's servers send packets that contain the code needed to show their search page on your computer.

How data gets to the InternetYour computer assembles the packets and displays the web page on your monitor.

The same process is repeated if you were to use this page to perform a search.

Why is the network slow?The network can only handle so many packets at once.

If all the computers are trying to access the same web site there is packet congestion.  It is like too many cars on the road.

Network SlowdownA computer on the network may also be infected with a virus or malware.  This may cause the computer to generate a large number of packets on the LAN.

This flood of packets can cause the network to slow down.

It is important to keep the computers on the network updated and free from viruses.

Since all the computers are connected together even one infected computer can cause slowdowns for all devices on the network.

Resourceshttp://fcit.usf.edu/network/http://computer.howstuffworks.com/home-network.htm

http://www.tech-soup.com/learningcenter/networks/page4773.cfm Peer Network image - http://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/P2P-network.svg/500px-P2P-network.svg.png Client/Server image - http://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Server-based-network.svg/500px-Server-based-network.svg.png

Ethernet image - http://upload.wikimedia.org/wikipedia/commons/4/40/UTP_cable-blue.jpg

Twisted Pair image - http://upload.wikimedia.org/wikipedia/commons/a/a6/CAT6_twisted_pair.JPG Google server image - http://blog.itechtalk.com/2009/google%E2%80%99s-secret-works/

Other images assembled from GNU resources at Wikimedia Commons and screen captures.