Incomputer networks, asyncword,sync character,sync sequence orpreamble is used to synchronize adata transmission by indicating the end ofheader information and the start of data.[citation needed] The syncword is a knownsequence of data used to identify the start of a frame, and is also calledreference signal ormidamble inwireless communications.
Prefix codes allow unambiguous identification of synchronization sequences and may serve asself-synchronizing code.
In anaudio receiver receiving abit stream of data, an example of a syncword is 0x0B77 for anAC-3 encoded stream.
AnEthernet 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.
AllUSB 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.[1]
A receiver uses a physical layer preamble, also called a physical layer training sequence, to synchronize on the signal by estimating frequency and clock offsets.Some documentation uses "preamble" to refer to a signal used to announce a transmission, to wake-up receivers in a low-power mode.While some systems use exactly the same signal for both physical-layer training and wake-up functions, others use 2 different signals at 2 different times for these 2 functions, or have only one or the other of these signals.[2]
TheBisync protocol of the 1960s used a minimum of two ASCII "SYN" characters (0x16…0x16) to achieve character synchronization in an undifferentiated bit stream, then other special characters tosynchronize to the beginning of a frame of characters.[3][4]
The syncwords can be seen as a kind ofdelimiter. Various techniques are used to avoiddelimiter collision, or—in other words—to "disguise" bytes of data at thedata link layer that might otherwise be incorrectly recognized as the syncword. For example,HDLC usesbit stuffing or "octet stuffing", while other systems useASCII armor orConsistent Overhead Byte Stuffing (COBS).
In some communication systems, a receiver can achieve character synchronization from an undifferentiated bit stream, or start-of-header synchronization from a byte stream, without the overhead of an explicit syncword. For example, theFSK441 protocol achieves character synchronization by synchronizing on any "space" characters in the message—in effect, every "space" character in the message does double duty as a syncword. For example,CRC-based framing achieves character and start-of-header synchronization.
In aself-synchronizing code, every character is, in effect, a syncword, and can be used to achieve character synchronization in an undifferentiated bit stream.
Indigital communication, preamble is a sequence of known bits sent in each frame. It is used forframe synchronization such as inEthernet frames, as well as forchannel estimation.
In Ethernet and other protocols, the preamble can also be used forautomatic baud rate detection.