The transport layer provides the application layers with a uniform transport service( TS), the transport classes, which satisfies the requirements in terms of cost, data throughput and data security, i.e. quality of service( QoS).
The transport service recognizes the following quality-of-service parameters: The delay of the connection setup; the maximum time span between T-Connect-Request and associated T-Connect-Confirm. The connection setup failure probability, which is the number of setup failures for which the transport system is responsible divided by the number of all setup attempts. The throughput per transmission direction and the number of successful transmissions per time. The transit delay with the maximum time span between T-Data request and corresponding T-Data indication. In addition, the residual error rate, the interference probability of the data transfer, the delay of the connection termination, the protection and the priority of a transport connection.
It is the task of the transport layer to select the appropriate network for the required quality of service and, if necessary, to provide the quality of service through its own protocol. If this is not possible, the connection setup is rejected.