6.2.2 Circuit and packet switching

Today's switched networks in the area of voice telephony (in the fixed and the mobile network areas) operate according to the principle of "circuit switching". The characteristic features of circuit switching are the following: establishing a connection, the data transmission phase, and discontinuing a connection. When establishing a connection, the destination of the connection (and the type of connection) is indicated.

The "resources" reserved in a network in the course of establishing a connection (for example, the transmission capacities in all relevant transmission paths, as well as in the switching exchanges) will then remain as an exclusive right of access for the terminal systems of that connection until the end of the connection - even if no useful data is transmitted at a specific moment. The transmission path has been physically "connected through" between the two terminal systems, the useful data therefore needs no addressing information in order to reach its destination, but simply follows the established physical path.

A typical example of this would be the previously mentioned 8-bit speech data transmitted every 125 micro-seconds in the framework of a speech connection. However, this only applies to a user channel connection, in modern voice telephony networks the signalling data is transmitted separately in a separate signalling network (see section 6.2.4.1). The transmission method used here corresponds to the principle of a packet-switched datagram service.

Packet switching is characterised in that - for a connection - there is no exclusive reservation of transmission capacities along the different sub-sections between the terminal points.

The useful data is split up into packets, each of which is passed on from the terminal system to the network, together with their destination information. In case of packet switching, there are also connection-oriented and connectionless transmission variants within a network. In the case of the connection-oriented option, the path is determined only once in all switching exchanges when establishing the connection, on the basis of the information on their destination (target address). A logical connection number is allocated to the respective connection and transmitted to the terminal systems. During the transmission phase, the terminal systems will only forward the logical connection number in the individual data packets, which greatly simplifies the search for the path in the switching exchanges. In addition, the utilisation of the individual network nodes can be taken into consideration when establishing the connection, which is of benefit for the "quality of service" of a service (example of an application: ATM networks).

Contrary to that, in the case of connectionless packet switching, the terminal system will forward the complete target address with every data packet, which the switching exchanges will evaluate.

As no connection-specific data is stored in the different switching exchanges with this system (this is why one speaks of "connectionless"), it may also happen that subsequent data packets will take different paths in a network and will arrive in a different sequence at the receiver (example of an application: the Internet, which is based on the IP protocol).