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.