An ISDN channel has two and only two ends. B channels terminate at a user. A B channel can therefore connect two and only two users. A B channel cannot be Y-shaped. B channels are therefore described as end-to-end.

In the case of the D channel one end is with the user. The other end is in the network.

A D channel is not end-to-end. You cannot normally,

Notice how the D channels (the red lines) do not pass through the network. Notice also how each user has only one D channel and it is not connected in any way with the D channel of the other user.

The B channel (the blue line) passes directly across the network.