Objectives - Topic 3

Students will be able to:

  • 3.1.6 Define the terms: protocol, data packet.
  • 3.1.7 Explain why protocols are necessary.
  • 3.1.8 Explain why the speed of data transmission across a network can vary.
  • 3.1.9 Explain why compression of data is often necessary when transmitting across a network.
  • 3.1.10 Outline the characteristics of different transmission media.
  • 3.1.11 Explain how data is transmitted by packet switching.

Protocols

A protocol is a set of defined rules or steps which must be followed in order to accomplish a specific task.

In computer networks, in order to transfer data from one device to another, certain protocols must be followed.

For example, the Transfer Control Protocol is respensible for breaking a file down into a sequence of packets.

The Internet Protocol is responsible for ensuring each packet contains the IP addresses of the sender and receiver, as well as other information.

The Reliable Internet

Scenario

2 gamers are online, communicating across the amazing public network we call the Internet.

As they send messages and other files to each other, look what happens. Click send.

Pack switching

The Internet is a packet switching network.

It employs thousands of routers to help data packets make their way from the source (eg a client) to the destination (eg a server).

Amazingly, each packet may take a different route.

And they may arrive at different times in the wrong sequence!

But used in data transmission eg TCP/IP ensure that packets are reordered at the destination and reassembled into the original file!

Watch this video and answer the accompanying questions.

Packet Switching Networks

So the Internet is a packet switching network.

Like any network, it uses a protocol (TCP) to decompose a file into a sequence of data packets.

Each packet includes:

  1. a sequence number
  2. the IP address of the sender
  3. the IP address of the receiver
  4. the data (payload) to be transmitted

Packets are routed across the Internet.

They will follow different routes.

But no problem! When they all arrive at the destination, they will be resequenced and reassembled into the original data file.

If any member of the sequence doesn't arrive, a request will be made to resend the missing packet.

This is another advantage of packet switching - individual packets can be resent rather than having to resend the entire file, which may be large.

Protocols ensure that every packet is ultimately delivered. This helps to ensure data integrity!

Redundancy

The above animation shows a packet-switching network with a few routers.

The Internet is a packet switching network which uses thousands of routers.

They all perform the same task. If one breaks, the Internet doesn't break - it can continue to operate normally.

The Internet utilises redundancy - it has many more than one component performing the same task so if one breaks, it is not a major problem.

Systems that utilise redundancy are fault-tolerant.

Compression

Large files require a lot of memory.

When TCP protocol transforms a file into a series of packets, it is not reducing the file size! A large file will require many packets to be produced, transmitted, and reassembled into the same large file.

If we can compress files to a smaller file size, fewer packets will be required, speeding up transmission times and using less bandwidth.

Standards

Imagine if 2 companies manufacture Ethernet cables for computer networks.

It is important that the cables match the devices that they will connect to.

And manufacturers of computer ports will also have to ensure that the ports support the cables that they are designed for.

Manufacturers have to follow international standards to ensure that their equipment is compatible with other equipment that it is designed for.

Glossary

packet

IP address

protocol

Transfer Control Protocol

router

reliability

fault tolerant

redundancy