Connection-oriented transport, with connection-oriented network

A connection-oriented transport protocol can use all of the connection-oriented network service primitives (see Network Layer). All the transport protocol data units (TPDUs) are sent within the N.DATA and N.EXPEDITED_DATA network primitives.

To see how this works we examine the mapping during the three phases of a transport connection:

Connection establishment:

A T.CONNECT.request is issued by the TS-user. The transport connection must then be initiated by sending out a CR (connect request) transport unit. To do this the transport layer must map the transport connection to a network connection. The transport entity can use an already established network connection or create a new one. To use an existing network connection it must have the same quality of service and expedited data option as the transport connection to be set up. That is, a network connection must be compatible with the transport connection, otherwise a new network connection is set up.

In the diagram below a transport connection is established. In this case a new network connection has to be set up using the N.CONNECT primitive. Once the network connection is set up the transport CR-TPDU is sent as data within the network N.DATA service.

Thus there is quite an overhead involved in setting up a transport connection, when a network connection must also be set up.

Data Transfer:

Once the transport connection has been established, TPDUs are sent across the network using the N.DATA service. As mentioned in the expedited data section, a class 1 transport protocol will use the N.EXPEDITED_DATA primitives to send ED and EA TPDUs.

Connection termination:

The transport connection is closed by sending a DR (disconnect request) TPDU in an N.DATA primitive. Once all transport connections on a network connection have closed, the network connection can be closed itself or kept open for later use.

However class 0 transport protocol does not do this, but instead terminates the network connection by using N.DISCONNECT network primitive, which of course ends the class 0 transport connection.

Back to Mapping Transport Services to Network Services

Michael Peirce mepeirce@tcd.ie