communication devices. These stations also referred as end devices are the communi-cating devices.
In the datagram approach, each packet is treated independently. In the Fig. 11.6. shown, the station A is assumed to send three packets of message namely P1, P2 and P3 (for explanation purpose named so). At first, A transmits these packets to node 1. Node 1 makes decision on routing of these packets. Node 1 finds node 4 as shortest compared to node 3. Thus it passes P1 and P2 to node 4. Accidently, if node 4 is not accessible, node 1 finds node 3 as shortest and sends packet P3 to node 3.
Node 3 and 4 sends its received messages to the destination C through node 6. It is shown that the order of the packet is changed due to the different routing of the packets. Thus in datagram, it is the responsibility of destination station to reorder the packets in proper sequence. Also if a packet crashes in a switching node, the destination C may not receive, all packets. In such a case also, it is the responsibility of station C to recover the lost packet.
Virtual circuit. In virtual circuit, a fixed route is selected before any data is transmitted in a call setup phase similar to circuit switched network. All packets belonging to the same data stream follow this fixed route called a vitrual circuit. Packet must now contain a virtual circuit identifier. This bit string is usually shorter than the source and destination address identifiers needed for datagram. Once the virtual circuit is established, the message is transmitted in packets. Fig. 11.7 shows the concept of virtual circuit.
In the Fig. 11.7 shown, suppose that end station B has two messages to send to the destination D. First B sends a control packet refered as call-request packet to node 2, requesting logical connection to D. Node 2 decides to route the request and the subsequent message packets through node 3 and 4 to destination D. If D prepared to accept the connection, it sends a call-accept packet to node 4. Node 4 sends the call-accept packet to B through node 3 and 2. Because the route is fixed for the duration of the logical connection, it is somewhat similar to a circuit switching network and is referred to as a virtual circuit.
Every data packet with virtual circuit identifier and data from B intended for D traverses node 2, 3 and 4. Similarly every data packet from D intended for B traverses nodes 4, 3 and 2. Any station can terminate the connection with a clear-request packet. At any time, each station can have more than one virtual circuit to any other station and can have virtual circuits to more than one station. As all packets follow the same route, they reach the destination in the same order. So there is no need of reording work for destination station. Error control is the additional advantage ofvirtual circuit. Error control is a service that assures error free reception. For example, if a packet in a sequence from node 3 to 4 fails to arrive at node 4, or arrives with an error, node 4 can request a retransmission of that packet from node 4. As there is no necessity of routing decision during transition of packets from source to destination, with virtual circuit, packets transit rapidly. Currently available packet switching networks make use of virtual circuits for their internal operations.
Packet size. If an organization has large amounts of data to send, then the data can be delivered to a packet assembler/disassembler (PAD). The PAD (software package) receives the data and breaks it down into manageable packets. In the data communication, a packet can be a variable length. Usually upto 128 bytes of data is in one packet. X-25 services have created packets upto 512 bytes, but the average is 128. The 128 byte capability is also referred to as fast select. There is a significant relationship between packet size and transmission time. The process of using more smaller packets (for example 30 byte information may be sent as a single packet with header of 3 byte or two packets with 15 byte each plus the header in each packet or 5 packets with 6 bytes plus header) increases the speed of transmission.
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