It refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.
Most modern Wide Area Network (WAN) protocols, includingTCP/IP, X.25, and Frame Relay, are based on packet-switching technologies. In contrast, normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it's sent. This is the case with most real-time data, such as live audio and video. Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages andWeb pages.
A new technology, ATM, attempts to combine the best of both worlds -- the guaranteed delivery of circuit-switched networks and the robustness and efficiency of packet-switching networks.
What is the idea behind it?
Telephones have been around for over one hundred years. When you want to make a call to someone else then a dedicated connection is set up between you. Whilst that call is taking place you both have sole use of the telephone line - no one else can use it. Once the call is finished the connection is broken and the line becomes available for somebody else to use.
This method has worked fine for many years. However, as said above, the line is tied up for the whole length of the call. This is called 'circuit switching'
Now imagine if this method were used in networks. For every person using the network a dedicated line would be needed. Large companies can employ thousands of staff around the world, all logged into the company network at the same time. Imagine how much that would cost to set up? And what about all that cable?
What about the Internet? Nobody owns the Internet, so who would be responsible for setting up and paying for the lines? And worse still could you imagine just how many lines would be needed for everyone to use the Internet at the same time? Impossible.
So this is where 'packet switching' becomes important.
How does packet switching work?
Imagine that you have a data file, perhaps an email or a document which is 2 Megabytes in size. You want to send this file to someone in another country.
When you send the file, it isn’t sent as one document (remember the telephone call), instead it is broken up into lots of small 'data packets'. Our 2MB file would be broken up into chunks of 512 bytes in size.