Packet switching and circuit switching
Packet and circuit switching
When a message is being sent, it can be sent using either packet switching or circuit switching.
Packet switching
This is a method of sending data over networks. When an email message is sent across a network such as the Internet, the email is split up into packets of data, each of equal size. To each packet is attached some extra information. This includes:
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- The address of the sender of the packet.
- The destination of the packet.
- Email reassembling information.
- A check sum for error detection.
When the message is sent, the packets each try to find the best path to the destination. They invariably split up and go by different routes. The packets are passed on from one network to the next, ever forward to the final destination. This means that the network is being used efficiently because no single path is being tied up with just one message as it would do in a phone call using circuit switching, for example (no engaged tones). In addition, you are not relying heavily on one particular communication route. With phones, you are relying on the line working. You cannot phone your friend if their line is not working because they only have one circuit to their actual phone. With packet switching there may be a delay while each of the packets reaches their destination address. This can happen if, for example, a computer receives lots of packets at once. It will have to queue them all up and then deal with them in turn.
Circuit switching
This is a method of communication that sets up a communications channel just before a message is sent and then keeps that channel open exclusively for the entire duration of that transmission. The communication channel is released for other messages only when the transmission is totally finished. The classic example is the telephone system, when you make a call.
A phone call is immediate using circuit switching. There is no waiting for packets of information. You just talk and listen and communication is full duplex - you can speak and hear at the same time! However, this kind of transmission relies heavily on a particular channel being available (not engaged). Remember, with circuit switching, the communications channel is reserved for the entire transmission of a message and so nobody else can use it. In addition, security may be compromised because the whole of a message is being sent at once. While data security is still an issue with packet switching, a packet that is intercepted on its own may not compromise the whole message. It would be prudent to use data encryption with packet switching if the data was sensitive. Another issue with circuit switching, certainly in older parts of the telephone network (known as the 'PSTN') is that it involves setting up transmission channels by opening and closing electro-mechanical switches. This act causes electrical interference, which can corrupt data (causing interference) if adequate recautions are not taken.
