In the case where S and T (see Reference Points) are coincident, NT1 is connected to a TE1 or TA by means of a passive bus called the S-Bus. This is a four wire connection to allow transmission of physical layer 1 protocol data from the TE1 to the NT1 and onwards to the network and receive data from the network via the NT1 on the receive pair. Up to eight TE1s may be connected to the S-Bus. The data between the NT1 and TE1 is formatted in frames which contain B and D channel data. All three channels may be active simultaneously transferring and receiving information from a variety of TE1s.

The information is transmitted and received between the NT1 and the ET in the form of electrical signals.

Note: not all networks support information transfer over the D channel.