Circuit Switching and Packet Switching Networks

            Networks are devices that are connected together using special hardware and software that allows them to exchange information. The most important word in that sentence is the final one: information. One fundamental way to differentiate between networking technologies is on the basis of the method used to determine the path between devices over which information will flow. In highly specified terms, there are two approaches: a path can be set up between the devices in advance, or the data can be sent as individual data elements over a variable path.

Circuit Switching

            In the circuit switching networking method, a connection called a circuit, which is used for the whole communication, is set up between two devices. Information about the nature of the circuit is maintained by the network. The circuit may be either a fixed one that is always present or one that is created on an as-needed basis. The classic example of a circuit switched network is the telephone system. When you call someone and s/he answers, you establish a circuit connection and can pass data in a steady stream. That circuit functions the same way, regardless of how many intermediate devices are used to carry your voice. You used it as long as you need it and then terminate the circuit. The next time you call, you get a new circuit, which may use different hardware than the first did, depending on what’s available at that time in the network.

Packet Switching

            In the packet switching network type, no specific path is used for data transfer. Instead data is chopped up into a small pieces call packets and sent over the network. You can route, combine, or fragment the packets as required to get them to their eventual destination. On the receiving end, the process is reversed- the data is read from the packets and reassembled to form the original data. A packet switched network is more analogous to the postal system than it is to the telephone system.

Which Switching Method to choose

            A common temptation when considering alternatives such as these is to ask which is better; the answer is neither. There are places for which one is more suited than the other. One important issue in selecting a switching method is whether the network medium us shared or dedicated. Your phone line can be used for establishing a circuit because you are the only one which use it. However, this doesn’t work well with LANs, which typically use a single shared medium and baseband signaling. If two devices were to establish a connection, they would lock out all the other devices for a long period of time. It makes more sense to chop the data into small pieces and send them one at a time. Then, if two other devices want to communicate, their packets can be intersperse, and everyone can share the network.

            The ability to have many devices communicate simultaneously without dedicated data paths is one reason why packet switching is becoming more predominant today. However, there are some disadvantages of packet switching compared to circuit switching. One is that since all data does not take the same predictable path between devices, it is possible that some pieces of data may get lost in transit or show up in the incorrect order. In some situation this does not matter, but in others it is very important indeed.

            Although the theoretical difference between circuit and packet switching is pretty clear-cut, understanding how to use them is a bit more complicated. One of the major issues is that in modern networks, they are often combined. For example, suppose you connect to the internet using a dial-up modem. You will be using UP datagrams (packets) to carry higher-layer data, but it will be over the circuit switched telephone network. Yet the data may be sent over the telephone system in digital packetized form. So in some ways, both circuit switching and packet switching are being used concurrently. Another issue is the relationship between circuit and packet switching, and whether a technology is connection-oriented or connectionless.

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