When you or your computer places a call (Telephone), the switching equipment within the telephone system search for a physical copper path all the way from your telephone to receiver’s telephone, this technique is called as circuit switching.
Once the call has been setup, a dedicated path between both ends exists & will continue to exist until the call is finished.
An important feature of circuit switching is the need to set-up an end-to-end path before any data can be sent. The time between the end of dialing and start of ringing can easily be 10 seconds and may increase on long-distance or international calls. During this time interval, the telephone system is hunting for a copper path.
As a consequence of copper path between the calling parties, once the set up has been completed, the only delay for data is he propagation time for electromagnetic signal.
Once the call has been put through, you never get busy signals; you might get busy signals before the connection has been established due to lack of switching.
A Message is a logical unit of information and can be of any length. In this method, if a station or a switching office wishes to send a message to another, it first attaches the destination address to message. When the sender has a block of data to be sent, it is stored in the first switching office and then forwarded later. This method is known as store-and-forward. Each block is received entirely, checked for errors and then retransmitted.
In message switching, no physical connection is required between the source and destination.
But one disadvantage is that the message length is unlimited i.e. each switching node must have sufficient storage to buffer message and another one is that a message is delay at each node because of time required to receive the message plus a queuing delay waiting for a chance to retransmit message to next node.
This method works similar to message switching, however it overcomes the disadvantage of message switching because in this method routing is done on packet basis and not on message basis.
Here, a message is split up into packets of fixed size. Besides the block of data to be sent, a packet has a header that contains source and destination addresses, control information, acknowledgement and error checking bits. With message switching, there was no limit on block size, which means that routers must have disks to buffer long blocks. It also means that a single block may tie up a line for minutes restricting switches for the other traffic. To get around these problems, packet switching was invented.
Packet switching makes sure that no user reserves the transmission line for very long time.
Thus computer networks usually use packet switching, occasionally circuit switching, but never message switching.