A multiplex connection is an interface in a digital communication network, such as the ISDN network, which makes it possible to use a large number of connections over a single line. Such multiplex connections are often used by companies for the connection of large internal phone systems to the public telephone network. The proper name for the access connection for the ISDN network is Primary Rate Interface, often abbreviated to PRI or S2M.
In Germany a PRI uses the so-called E1 channel structure, with 30 individual user channels and a respective data rate of 64 kilobits per second. The traffic channels are often referred to as B-channels. In addition, a 64 kbit/s signalling channel and a 64 kbit/s synchronization channel are also in place. The gross bandwidth is therefore 2048 kbit/s, with a usable bandwidth of 1920 kbit/s. If a telephone system is connected to a PRI line with these characteristics, up to 30 parallel calls can be made through it.
In order to address a large number of phones or other devices, users of these kinds of connections are assigned entire blocks of numbers by the provider.
The respective phone numbers in the block are called direct inward dealing (DID) numbers. Within the telephone system these numbers are assigned to each terminal device and can be used for incoming or outgoing calls.