There are different technical specs on this depending on what technology you are talking about. When speaking of network connections, such as dsl and adsl, synchronous dsl would be when the upload and download speeds are the same and asynchronous dsl would be when the download speed is much higher than the upload speed, (this is the standard dsl you would get from yahoo, etc.)
another way at looking at the two terms is how two systems communicate over a physical link. This is more in depth. please look at the following article:
Last Wiki Answer Submitted: April 18, 2013 5:03 pm by Buddyfarr6,850 pts.