32 Discuss the various Switching Methods available.

For systems to communicate effectively, a communication path has to be set up between the two. It is the job of a switch to provide a path between two points for communication and route the data on the correct path. The three types of switching that are used in networks are:

Packet Switching: In this form of switching, messages are broken down into smaller pieces. These pieces are called packets. After this breaking, the packet is assigned a source, an intermediate address and a destination. This information is required, as the packet may not always the same path to reach its destination. This is termed as independent routing and is an advantage of packet switching.

This feature allows the data packet to take a path that is not crowded. The data packet can also take an alternative path for reaching the destination. The best route available is chosen by the device that is to send the data. The data may not always be received in a sequential order. The receiving device waits for all the data packets to come in, and then the same are arranged. The figure given below illustrates an example of packet switching.

- Virtual-Circuit Packet Switching: When this switching method is used, it forms a logical connection between the source and the device. The connection is initiated when a sending device starts a conversation with a receiving device. The line of communication remains active for the entire period the devices are available. There are two types of this particular method:

- Permanent Virtual Circuit (PVC): As the name suggests it is a permanent dedicated virtual link. It is set up between the sending device and the receiving device. It can be used to replace a hardwired dedicated end-to-end line.

- Switched Virtual Circuit (SVC): It is a temporary virtual circuit. It is set up only for the period for which data transfer has to take place. The virtual circuit is cleared after the data transfer is completed.

- Datagram Packet Switching: In this system a logical connection between a sending and a receiving device is not formed. The packets are allowed to take independent paths to reach their designated destinations. For this, every packet carries the address. The packets take the easiest route to the destination. The packets do not arrive in a sequential order. The reconstruction of the data is done based on the header on every packet. It is a connectionless network. It does not require dedicated and active connections between the sending and the receiving device.

Circuit Switching: This method asks for a dedicated and a physical connection between the devices meant for sending and receiving data. When one side of the connection disconnects the connection between the two devices breaks. The communication channel is closed down when the transmission of the data is complete. The advantage of using this application is that the connection is consistent and reliable. The technology in this system assures a guaranteed rate of transfer. The biggest disadvantage of this technique is that a dedicated line spells inefficiency. Once the connection is made, the line becomes unavailable for the other processes and sessions. The figure given below illustrates the working of a circuit switching.

Figure 52: Circuit Switching

Message Switching: This technology is similar to packet switching. This technology divides the data to be transmitted into messages. Every message contains the destination address. The message is temporarily stored by the intermediate device before forwarding it to the next device in the network. It is a store and forward method. The advantage that this method has is that it does not require a dedicated connection. It can be used to keep traffic congestion on a low. The drawback of this method is that it is not suitable for real time applications. The temporary storing of the message on the intermediate device is disruptive.