This approach uses a different, more dynamic scheme, to determine the route through the network links. Each packet is treated as an independent entity, and its header contains full information about the destination of the packet. The intermediate nodes examine the header of the packet, and decide to which node to send the packet so that it will reach its destination. In the decision two factors are taken into account:

·         The shortest ways to pass the packet to its destination – protocols such as RIP/OSPF are used to determine the shortest path to the destination.

·         Finding a free node to pass the packet to – in this way, bottlenecks are eliminated, since packets can reach the destination in alternate routes.

Thus, in this method, the packets don’t follow a pre-established route, and the intermediate nodes (the routers) don’t have pre-defined knowledge of the routes that the packets should be passed through.

                                              Datagram Packet switching

Packets can follow different routes to the destination, and delivery is not guaranteed (although packets usually do follow the same route, and are reliably sent). Due to the nature of this method, the packets can reach the destination in a different order than they were sent, thus they must be sorted at the destination to form the original message. This approach is time consuming since every router has to decide where to send each packet. The main implementation of Datagram Switching network is the Internet, which uses the IP network protocol.

Advantages:

·         Call setup phase is avoided (for transmission of a few packets, datagram will be faster).

·         Because it is more primitive, it is more flexible.

·         Congestion/failed link can be avoided (more reliable). Problems:

·         Packets may be delivered out of order.

·         If a node crashes momentarily, all of its queued packets are lost.

Packet Size

In spite of increase in overhead, the transmission time may decreases in packet switching technique because of parallelism in transmission as shown in Fig.

 Reduction of transmission time because of parallelism in transmission in packet switching technique

However, question arises about the optimal size of size of a packet. As packet size is decreased, the transmission time reduces until it is comparable to the size of control information. There is a close relationship between packet size and transmission time as shown in Fig. In this case it is assumed that there is a virtual circuit from station X to Y through nodes a and b. Times required for transmission decreases as each message is divided into 2 and 5 packets. However, the transmission time increases if each message is divided into 10 packets.

                Variation of transmission time with packet size