The Up0 interface is an interface for devices within a private ISDN installation of telephone systems. It is functionally equivalent to the UK0 interface on the public network. Compared to this, it has a much shorter range, which is due to the fact that telephone equipment, only bridge the distances that occur within a building, while the public telephone network to eight kilometers Length of connecting cable are the rule. Differences between the two interfaces are available in the way how bits are encoded and transmitted. The advantage of Up0 interface compared to the S0 bus used also for private installations is the greater range. This is, depending on line quality about two to four kilometers. This operation of digital terminals ( telephone system, but not regular ISDN telephone ) is possible even with larger internal building cabling.

In contrast to the S0 bus of the connection of digital devices via a single twisted pair. Thus, an existing two-wire cabling buildings continue to be used. The connection doses used are usually RJ45 sockets, the wires are placed on this pin 4 and 5. The transmission on this connection is in half-duplex mode: Both sides send alternating at twice the speed. This method is also called a ping-pong method. In contrast, the UK0 interface uses the same frequency location method.

While several terminals can be connected to an S0 bus, only one device can be operated at a Up0 interface without additional technical measures. Still standing at this interface as the S0 bus, the two B- channels at 64 kbit / s and a D channel with 16 kbit / s.