John,
There is no such thing as an S, T or U INTERFACE. There are S, T, U, R etc., "REFERENCE POINTS". There may or may not be a physical interface at anyone of these reference points. The reference point is a point in space, not a physical appearance of one sort or another. If there is an isolation device at one of the reference points, that isolation device provides isolation, not the interface, not the reference point. Ciao, Vic
