A test system for testing a communication system having a plurality of communication links is disclosed. The test system has a single tester for performing various measurement and diagnostic tasks on a single link. The test system also has a switching system for independently testing any link by coupling the tester into any one link. The tester is coupled into the link by coupling the tester input to the link's transmitter and the tester output to the link's receiver. The switching system couples the tester such that all remaining links of the communication system have a unique one of the plurality of transmitters coupled to a unique one of the plurality of receivers, whereby the operation of the communication system can be maintained while testing individual links.