Abstract: A spare receiver in a CMTS is used to non-invasively test the upstream signal quality of a network without disrupting a subscriber's operations. A modem registered with a receiver on the network is selected as a testing modem. The spare receiver is RF connected to the receiver and the testing modem is tuned to the spare receiver. The testing modem is used to test the signal quality of the network, such as by using a SNR test. The testing modem remains registered with the network during the testing operation. Other modems are prevented from registering with the spare receiver. If other modems attempt to register on the spare receiver, the system overrides their attempts and moves them back to another receiver. The testing modem is returned to its original receiver when testing is completed.