Abstract: A communication network signalling system couples a signal transfer point to a plurality of channel banks through links which include link monitor and test units. The link monitor and test units, which monitor the links in a passive, non-obtrusive manner and which at the same time are capable of being serially coupled into the link so as to insert signals in series with the link and to otherwise accomplish in-depth link testing and analysis, each include a pair of channel units coupled to opposite portions of an associated link and which are coupled together through a metallic through connection in the form of a normally unenergized relay. The metallic through connection provided by the relay maintains the signal path through the link during a variety of fault conditions, while at the same time permitting electronic portions of the channel units to be cross-connected to accomplish monitoring and testing functions.