What exactly is transmission time?
Time taken by a message to reach receiver : Propagation delay or time
Time taken to push message on the link by the sender : Transmission time   


Transmission time is the time from the first bit until the last bit of a message has left the transmitting node. Or, in simple word it's the amount of time required for the router to push out the packet.

Packet transmission time = Packet size / Bit rate   


Reference :  


in2p3.fr/actions/formation/InfoTempsReel/IN2P3-Ch1-datatran-tr.pdf