Circuit Switching, Methods of Circuit switching, Cut through, Store and forward, Fragment free switching

Circuit switching is the most familiar technique used to build a communications network. It is used for ordinary telephone calls. It allows communications equipment and circuits, to be shared among users. Each user has sole access to a circuit (functionally equivalent to a pair of copper wires) during network use. Consider communication between two points A and D in a network. The connection between A and D is provided using (shared) links between two other pieces of equipment, B and C.

Circuit-switching systems are ideal for communications that require data to be transmitted in real-time. Packet-switching networks are more efficient if some amount of delay is acceptable.

Circuit-switching networks are sometimes called connection-oriented networks. Note, however, that although packet switching is essentially connectionless, a packet switching network can be made connection-oriented by using a higher-level protocol. TCP, for example, makes IP networks connection-oriented.

The three most common switch methods are:

1. Cut-through - Streams data so that the first part of a packet exits the switch before the rest of the packet has finished entering the switch, typically within the first 12 bytes of an Ethernet frame.

2. Store-and-Forward - The entire frame is copied into the switch's memory buffer and it stays there while the switch processes the Cyclical Redundancy Check (CRC) to look for errors in the frame. If the frame contains no errors, it will be forwarded. If a frame contains an error, it will be dropped. Obviously, this method has higher latency than cut-through but there will be no fragments or bad frames taking up bandwidth.

3. Fragment-free Switching - Think of this as a hybrid of cut-through and store-and-forward. The switch reads only the first 64 bytes of the frame into buffer before forwarding it (think of a truck...