The OSI protocols of the transport layer support five different transport classes. These correspond to the different network options provided by layers 1 to 3. The transport classes are defined in ISO 8072 and ISO 8073 or in X.214 and X.224 of the International Telecommunication Union (ITU). The selected transport class is usually fixed, but can also be changed once the connection is established.
The transport classes are distinguished by the quality of service (QoS) and are defined as follows:
TransportClass 0 (TP0) is the basic class for connectionlesstransport protocols used in circuit networks with good error handling and low transmission interruptions. In order to use TP0, the ConnectionModeNetwork Service (CONS) must be implemented. The best known application of TP0 is Telex.
Transport Class 1 (TP1) adds an error handling routine(Basic Error RecoveryClass) for line networks that have many interruptions Class 1 was developed by the ITU for X.25. If TP1 is used, error handling of these interrupts by the application is avoided.
Transport class 2 (TP2) has all the capabilities of class 0, plus transport class 2 can multiplex multiple transport connections onto one network connection
Transport class3 (TP3) combines the capabilities of classes 1 and 2.
Transport class 4 (TP 4) for connection-oriented transport protocols can detect errors and data outside the data sequence that were not reported by the lower layers. Transport class 4 does not require a reliable underlying network and is suitable for networks using datagram technology. TP4 is the only transport class that is based on the Connectionless Mode Network Service (CLNM) and provides all services that guarantee a secure end-to-end connection over a data network.
In general, transport classes 0 or 2 are suitable for networks that have a largely error-free environment for service functions. Networks with a high interruption rate should be operated with class 1 or 3. The class 4 transport protocol is particularly suitable for connectionless networks.