A circuit is always transparent, isn’t it ? It just transports data from one end to another end..
(until quantum theory kicks in that is…)
An equipment, such as a router for example, is just transferring data from one cable to another, therefore it is transparent to the information contained in the IP Packet.
in telecommunications, the BSC is tranparently transferring some data from BTS to MSC (voice packets, for example, are not analyzed by the BSC), while analyzing some others (measurements results from the BTS are gathered by BSC, and are not sent to the MSC).