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Outline Notes
8/22/99

Government’s Advance Research Products Agency funded a small program operated by Bolt, Beranek, and Newman (BBN), a computer and technology firm in Cambridge, MA to figure out how to allow computer to interact with each other. BBN figure out a way to tie several computers together using phone line and modem (converts digital 1s and 0s into sound wave) to transmit data that has been packaged into small fixed-length pieces called packets or datagrams. The technology of converting data into packets was called packet-switching, and it is one of the most important development in the history of computer networking.

What is Packet-Switched Data?

• Packet-switching is how all computer networks move data around.
• Allows more than one stream of data to travel over a wire at a time.
• Inherently ensures error-correction, that data transmitted over a wire is free of error.
• Allows data to be sent from one computer to another over multiple routes, depending on which routes are currently open.
• Example of Packet Switching (In normal everyday scenario, assume you have a large document that need to be send to your customer in New York. You are currently in San Jose. You document is 5000 pages. The post office only allows 500 pages per package to be sent at a time. Packet switching would work similar to this example.):

1. Your document is the data needed to be sent over the network. Because the network (post office) only allows 500 pages of document to be send at a time. The whole document of 5000 pages then need to be break into 10 individual packages. This process of breaking up the document (data) into equal size is the first part of packetizing.

2. Once the document is break into its own individual packages. The receiver must be able to put the document back in its original order. The sender would have to label each package (packetizing) using a method called checksum. Checksum help receiver put the individual packages into its original order.

3. After the label has been written on the package, the user has to put the document into the package in sequential order from 1 to 500 for package 1 and 501 to 1000 for package 2 and so on.

4. Once step 1 – 3 is completed, you would then send the document using the post office service. The route the document travel is unimportant. Data packet can follow several paths across the Internet.

5. When the receiver receive the packages, it will then try to put the package together using the label (checksum).

For example, it only packages 1, 2, 3, 4, 6, 8, 9, and 10 arrived. The receiver would wait for the missing packages 5 and 7. If those packages do not show up, the receiver either send back signal that packages 5 and 7 are missing and wait for them or terminate the process or assembling the packages.

Another example would be that all the packages got to the receiver. The receiver would then reorganize the package in order of sequence from 1 to 10 and then assembling the document in its original order of package. The checksum would be used to check if the sending total of 500 pages (data) sent is correct when it open up the package. Example, if the package#7 only has 497 pages, then the package#7 (data) is corrupted and new package#7 is need to be send again by the sender.

• Any data send over a computer network is packetized. (Packetizing – the process by which a computer breaks a single large chunk of data into smaller pieces.)
• You can have lots of packets of data from multiple machines without confusion because each data packet must have the following elements:

1. Source address – return address or where the packet come from.
2. Destination address – where the packet is going.
3. Sequence number – this explained where the packet fits in with the remainder of packets.
4. Checksum – this ensures that the data is free of errors.

Benefits of networking:

• Allow the sharing of all network resources: printer, file, hard disk space, and applications (know as GroupWare application – application that allows users to use networked computer to work together).