Question

Describe clearly, step by step, and layer by layer, the operation that takes place when passing a datagram through an intermediate node in a switching network.

Answer 1

Datagram packet-switching is a packet switching technology by which each packet, now called a datagram, is treated as a separate entity. Each packet is routed independently through the network. Therefore packets contain a header with the full information about the destination. The intermediate nodes examine the header of a packet and select an appropriate link to another node which is nearer to the destination. In this system, the packets do not follow a pre-established route, and the intermediate nodes do not require prior knowledge of the routes that will be used.

The individual packets which form a data stream may follow different paths between the source and the destination. As a result, the packets may arrive at the destination out of order. When this occurs, the packets will have to be reassembled to form the original message.

Because each packet is switched independently, there is no need for connection setup and no need to dedicate bandwidth in the form of a circuit.

Datagram packet switches use a variety of techniques to forward traffic; they are differentiated by how long it takes the packet to pass through the switch and their ability to filter out corrupted packets.

There are three primary types of datagram packet switches:

• Store and forward: buffers data until the entire packet is received and checked for errors. This prevents corrupted packets from propagating throughout the network but increases switching delay.
• Fragment free: filters out most error packets but doesn't necessarily prevent the propagation of errors throughout the network. It offers faster switching speeds and lower delay than store-and-forward mode.
• Cut through:does not filter errors; it switches packets at the highest throughput, offering the least forwarding delay.

A datagram network is a best effort network. Delivery is not guaranteed. Reliable delivery must be provided by the end systems (i.e. user's computers) using additional protocols.

The most common datagram network is the Internet, which uses the IP network protocol. Applications which do not require more than a best effort service can be supported by direct use of packets in a datagram network, using the User Datagram Protocol (UDP) transport protocol. Applications like voice and video communications and notifying messages to alert a user that she/he has received new email are using UDP. Applications like e-mail, web browsing and file upload and download need reliable communications, such as guaranteed delivery, error control and sequence control. This reliability ensures that all the data is received in the correct order without errors. It is provided by a protocol such as the Transmission Control Protocol (TCP) or the File Transfer Protocol (FTP).