In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data.
[citation needed] The syncword is a known sequence of data used to identify the start of a frame, and is also called reference signal or midamble in wireless communications.
An Ethernet packet with the Ethernet preamble, 56 bits of alternating 1 and 0 bits, allowing the receiver to synchronize its clock to the transmitter, followed by a one-octet start frame delimiter byte and then the header.
All USB packets begin with a sync field (8 bits long at low speed, 32 bits long at high speed) used to synchronize the receiver's clock to the transmitter's clock.
Some documentation uses "preamble" to refer to a signal used to announce a transmission, to wake-up receivers in a low-power mode.