A EuroISDN Primary Rate Interface contains 30 B Channels and one D channel. In North America and some other countries a PRI contains 23 B channels and one D channel.

A B channel operates at 64 000 bits per second. (56 000 bits per second in some parts of North America and other countries.)

A D channel operates at 64 000 bits per second in a Primary Rate Interface. Note that this is different to the BRI where the D channel runs at 16 000 bits per second.

The animation below shows the multiplexing in a European Primary Rate Interface.

The Primary Rate Interface spends equal amounts of time transmitting data for each of the B channels and for the D channel because they all operate at the same speed.

Notice that there is one timeslot which has no channel assigned to it. This is reserved for your network supplier to use for diagnostic purposes. The D channel appears between B channel 15 and B channel 16.