 Its decision was signalled back to the MCU which then instructed the interrogator-markers to mark the chosen pair of paths, starting from the link out through the C, B and A stages to the subscriber, and then from the other side of the same link, through C, B and A-stages to the register.The register then checked the connection to the subscriber and sent dial tone.Normally, the whole process took about one-fifth of a second, less than the time required for the subscriber to lift the handset to his ear.