DATAGRAM NETWORKS

In data communications, we need to send messages from one end system to another. If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. The size of the packet is determined by the network and the governing protocol.

Topics discussed in this section:

• Routing Table
• Efficiency
• Delay
• Datagram Networks in the Internet

Packet Switching

1. In packet switching, there is no resource allocation for a packet.Resources are allocated on demand.
2. The allocation is done on a first-come, first-served basis.
3. When a switch receives a packet, no matter what is the source or destination, the packet must wait if there are other packets being processed.
4. In a datagram network, each packet is treated independently of all others.
5. Datagram switching is normally done at the network layer.
6. The datagram networks are sometimes referred to as connection-less networks.

Note

In a packet-switched network, there

is no resource reservation;

resources are allocated on demand.

A datagram network with four switches (routers)

Routing Table

1. In Packet Switching network, each switch (or packet switch) has a routing table which is based on the destination address.
2. The routing tables are dynamic and are updated periodically.
3. The destination addresses and the corresponding forwarding output ports are recorded in the tables.
4. This is different from the table of a circuit-switched network in which each entry is created when the setup phase is completed and deleted when the teardown phase is over.

Routing table in a datagram network

Note

A switch in a datagram network uses a

routing table that is based on the

destination address.

Note

The destination address in the header of

a packet in a datagram network

remains the same during the entire

journey of the packet.

Efficiency

• The efficiency of a datagram network is better than that of a circuit-switched network; resources are allocated only when there are packets to be transferred.
• If a source sends a packet and there is a delay of a few minutes before another packet can be sent, the resources can be reallocated during these minutes for other packets from other sources.

Delay

There may be greater delay in a datagram network than in a virtualcircuit network.

Although there are no setup and teardown phases, each packet may experience a wait at a switch before it is forwarded.

Delay in a datagram network

The packet travels through two switches. There are three transmission times (3T), three propagation delays (slopes 3P of the lines), and two waiting times (W1 + w2) We ignore the processing time in each switch. The total delay is = 3T+3P+W1+W2

Note

Switching in the Internet is done by

using the datagram approach

to packet switching at

the network layer.