There are two main types of Public Domain Networks (PDN).
Packet Switched Networks
In packet switched data networks all data to be transmitted is first assembled into one or more message units, called packets, by the source DTK. These packets include both the source and the destination DTE network addresses. They are then passed by the source OTE to its local Packet Switching Exchange (PSE).
On receipt of each packet, the PSE inspects the destination address contained in the packet. Each PSE contains a routing directory specifying the outgoing links to be used for each network address. On receipt of each packet, the PSE forwards the packet on the appropriate link at the maximum available bit rate.
As each packet is received at each intermediate PSE along the route; it is forwarded on the appropriate link interspersed with other packets being forwarded on that link. At the destination PSE, determined by the destination address within the packet, the packet is finally passed to the destination OTE.
To prevent unpredictably long delays and ensure that the network has a reliably fast transit time, a maximum length is allowed for each packet. It is for this reason that a message submitted to the transport layer within the OTE might first have to be divided by the transport protocol entity into a number of smaller packet units before transmission. In turn, they will be reassembled into a single message at the destination DTE.
In packet switched data networks error and flow control procedures are applied on each link by the network PSEs. Consequently, the class of service provided by a packet switched network that is much higher than that provided by a circuit switched network.
Circuit Switched Network
In a circuit switched data network each connection-established result in a physical communication channel being set up through the network from the calling to the called subscriber equipment. The two subscribers use this connection for the duration of the call exclusively then. The main feature of such a connection is that it provides a fixed data rate channel, and both subscribers must operate at this rate.
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