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).
If a circuit mean a cable I guess it is transparent, only 0 and 1 and never altered. But if a circuit include network element then transparency should mean data carried over it is not affected and can be reconstructed exactly the same form. Is not filtered or packetized such that data could not be reconstructed properly. Normally all network element has a role to play when data is sent to them and before sending out to transmission media, otherwise many vendors will be out of customers. And Cisco will sure be a small company.