Aug 24 2009 6:35PM GMT
||Telegraph companies quickly learned that it was important to be able to track messages and minimize relaying errors. They developed the idea of a preamble that contained the information describing the message and providing the means to trace it back to its origins. The preamble concept proved so useful that it has been carried forward into the terminology of the Internet and computer networks.
The ARRL Ham Radio License Manual
Preamble is still used in the very same way – the information at the head of a transmission contains information about the message that follows. If you examine a transmission of data on a garden-variety Ethernet network, you will find that each packet of data has a preamble with a unique number and address, among other things.