A network contains a large number of computers and various intermediate devices. All these devices and computers must be joined so as to transfer data from one computer of network to any other computer. This can be possible by using two methods:-
The first method is to create an individual wire link from every computer to every other computer (like fully connected topology) this method requires a lot of wiring and will increase the system cost and efficiency. The other method is to use a technique of switching. In a switched network, special devices called switches, are used to join two or more computers and intermediate devices with each other. Switches can create temporary connection between two devices attached to the switch. These devices are hardware devices that may have a software component in them. When a particular virtual path is to be set, the switch is set accordingly, to create a path between two nodes.
There are three different methods, establishing communication links between data sources and receivers between communication networks. These are:-
- Circuit Switching
- Message Switching
- Packet Switching
Circuit Switching:- It is a simplest method of data communication in which a dedicated physical path is established between the sending and the receiving station through the nodes of the network. This method is used to connect two subscriber in a telephone network. Computers and terminals connected to a telephone network also use this method of establishing communication path among them.
There is a switching node of a communication network. When a message is to be communicated a physical path is established between the sending and receiving stations by physically connecting the incoming and outgoing line of each of the intermediate switching node that fall on the path. Once a circuit is established between the two stations it is exclusively used by the two parties and the dedicated physical link between both ends continue to exist until the call is terminated either by the sender or the receiver. As soon as the connection is terminated by one of the two stations, the dedicated resources are deallocated and can now be used by other stations also.
Thus, circuit switching involves three phases i.e. circuit establishment, circuit transfer and circuit disconnection.
Advantages of Circuit Switching
- The main advantages of circuit switching is that once the circuit is established data is transmitted in no delay other than the propagation delay which is negligible.
- It is suitable for low speed communication between a terminal and a compute because this simple and requires more speed facility.
- since, a dedicated, continuous path is established for long continuous transmission.
- Prior to actual data transfer, the time required to establish a physical link between the two stations is of the order of 10sec or more depending on the distances.
- Since, the physical link is a dedicated to one source for the entire duration of a connection. Hence, the network resources are not properly utilized.
- This method proves to be very uneconomical when used for communication between computers that are connected using expensive high speed transmission line.
Message Switching:- In this form of switching, no physical copper path is established in advance between sender and receiver. Instead, when a sender has block of data to be sent, it is stored in the first switching office (i.e. router) and then forwarded later. A network using this technique is called a store-and-forward network. With message switching there is no limit on block size this means that routers must have disks (secondary memory) to buffer (store) long blocks (data). It also means that a single block may tie up a router-to-router lining for long time.
Packet Switching:- Circuit Switching was designed for voice communication in a telephone conversation. For e.g. once a circuit is established it remains connected for the duration of session. Circuit Switching create a temporary (telephone) or permanent dedication links (lease lines), Ethernet link, that are well suited to this type of communication and is not feasible for data and other non-voice transmission. A better solution for data transmission is packet switching. In packet switching network, data are transmitted in discrete units of potential variable length blocks called packets. The maximum length of the packet is established by the network. Each packet contains not only data but also a header with control information. The packets are send over the network from node-to-node. At each node the packet is stored briefly and routed according to the information in its header.
Packet Switching network plays an important role, it tights a upper limit on block size allowing packets to be buffered in router main memory instead of a disk. It makes sure that no user can monopolize (only he can use) any transmission line very long. One more advantage of Packet Switching over message switching is that the first packet of a multi-packet can be forwarded before the second has fully arrived, reducing delays and improving the throughput. For these reasons computer networks are usually Packet Switching but never Message Switched.