In computing a syncword is used to synchronize data. For example an audio receiver is receiving a bit stream of data. It needs to know where the header information starts. In computing a predefined syncword is used to indicate the start of data. An example of a syncword is 0x0B77 for an AC-3 encoded stream. The bisync protocol used a minimum of two ASCII "SYN" characters (0x16…0x16) to achieve character synchronization in an undifferentiated bit stream[1][2], then other special characters to synchronize to the beginning of a frame of characters.
Various techniques are used to "disguise" bytes of data at the data link layer that might otherwise be (incorrectly) recognized as the sync word. For example, HDLC uses bit stuffing or "octet stuffing", while other systems use ASCII armor.
Alternatives to syncwords
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, the FSK441 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 a self-synchronizing code, every character is, in effect, a syncword, and can be used to achieve character synchronization in an undifferentiated bit stream.
See also
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