A Technical Assurance Center, or TAC, is a department within a company that's primary purpose is networking, or an ISP. The TAC also works closely with a company's Network Operations Center, or NOC

Network Operations Center

A network operations center is one or more locations from which control is exercised over a computer, television broadcast, or telecommunications network....

. A TAC can only monitor items with a return signal, such as a modem

Modem

A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

 or a MTA. Forward-only systems, such as cable boxes, are currently unable to be monitored.