An upstream PCM link accommodated in a new switch is bridged to a downstream PCM link accommodated in an old switch in the normal operating condition. A test unit sends the number of a subscriber to be tested to an old and new switches. The old switch sends call control information corresponding to the...http://www.google.de/patents/US5187732?utm_source=gb-gplus-sharePatent US5187732 - Board-to-board test unit for a digital subscriber line
