11.4. Comparison between different switching techniques
We can begin with two rough classifications. If a connection (path) between the origin and the end node is established at the beginning of a session we are talking about circuit or packet (virtual circuit) switching. In case it does not, we refer to message and packet (datagram) switching. On the other hand, when considering how a message is transmitted, if the whole message is divided into pieces we have packet switching (based either on virtual circuit or datagram) but if it does not, we have circuit and message switching.
In the following paragraphs we get into the details of different switching techniques
11.4.1. Circuit switching
In figure 11.11, the most import events in the life of a connection in a four-node circuit switching network (see figure 11.10) are shown. When a connection is established, the origin-node identifies the first intermediate node (node A) in the path to the end-node and sends it a communication request signal. After the first intermediate node receives this signal the process is repeated as many times as needed to reach the end-node. Afterwards, the end-node sends a communication acknowledge signal to the origin-node through all the intermediate nodes that have been used in the communication request. Then, a full duplex transmission line, that it is going to be kept for the whole communication, is set-up between the origin-node and the end-node. To release the communication the origin-node sends a communication end signal to the end-node.
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Figure-11.10:
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Figure-11.11:
11.4.2. Message switching
Figure 11.12 shows life connection events for a message switching network. When a connection is established, the origin-node identifies the first intermediate node in the path to the end-node and sends it the whole message. After receiving and storing this message, the first intermediate node (node A) identifies the second one (node B) and, when the transmission line is not busy, the former sends the whole message (store-and-forward philosophy). This process is repeated up to the end-node. As can be seen in figure 11.12 no communication release or establishment is needed.
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Figure-11.12:
11.4.3. Packet switching based on virtual circuit
Figure 11.13 shows the same events for a virtual circuit (packet) switching network. When a connection is established, the origin-node identifies the first intermediate node (node A) in the path to the end-node and sends it a communication request packet. This process is repeated as many times as needed to reach. Then, the end-node sends a communication acknowledge packet to the origin-node through the intermediate nodes (A, B, C and D) that have been traversed in the communication request. The virtual circuit established on this way will be kept for the whole communication. Once a virtual circuit has been established, the origin-node begins to send packets (each of them has a virtual circuit identifier) to the first intermediate node. Then, the first intermediate node (node A) begins to send packets to the following node in the virtual circuit without waiting to store all message packets received from the origin-node. This process is repeated until all message packets arrive to the end-node. In the communication release, when the origin-node sends to the end-node a communication end packet, the latter answers with an acknowledge packet. There are two possibilities to release a connection:
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Figure-11.13:
11.4.4. Packet switching based on datagram
The most important events in the life of a communication in a datagram switching network are shown in figure 11.14. The origin-node identifies the first intermediate node in the path and begins to send packets. Each packet carries an origin-node and end-node identifier. The first intermediate node (node A) begins to send packets, without storing the whole message, to the following intermediate node. This process is repeated up to the end-node. As there are neither connection establishment nor connection release, the path follow for each packet from the origin-node to the end-node can be different and therefore, as a consequence of different propagation delays, they can arrive disordered.
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Figure-11.14:
EJM 17/2/1999