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In coding theory, especially in telecommunications, a self-synchronizing code is a uniquely decodable code in which the symbol stream formed by a portion of one code word, or by the overlapped portion of any two adjacent code words, is not a valid code word.^[1] Put another way, a set of strings (called "code words") over an alphabet is called a self-synchronizing code if for each string obtained by concatenating two code words, the substring starting at the second symbol and ending at the second-last symbol does not contain any code word as substring. Every self-synchronizing code is a prefix code, but not all prefix codes are self-synchronizing.

Other terms for self-synchronizing code are synchronized code^[2] or, ambiguously, comma-free code.^[3] A self-synchronizing code permits the proper framing of transmitted code words provided that no uncorrected errors occur in the symbol stream; external synchronization is not required. Self-synchronizing codes also allow recovery from uncorrected errors in the stream; with most prefix codes, an uncorrected error in a single bit may propagate errors further in the stream and make the subsequent data corrupted.

Importance of self-synchronizing codes is not limited to data transmission. Self-synchronization also facilitates some cases of data recovery, for example of a digitally encoded text.

Examples

UTF-8 is self-synchronizing because the leading byte (11xxxxxx) and subsequent bytes (10xxxxxx) of a multi-byte code point have different bit patterns.
High Level Data Link Control (HDLC)
Advanced Data Communication Control Procedures (ADCCP)
Fibonacci coding

Counterexamples:

The prefix code {00, 11} is not self-synchronizing; while 0, 1, 01 and 10 are not codes, 00 and 11 are.
The prefix code {ab,ba} is not self-synchronizing because abab contains ba.
The prefix code b^∗a (using the Kleene star) is not self-synchronizing (even though any new code word simply starts after a) because code word ba contains code word a.

References

^ "Self-synchronizing code – Glossary".
^ Berstel, Jean; Perrin, Dominique; Reutenauer, Christophe (2010). Codes and automata. Encyclopedia of Mathematics and its Applications. Vol. 129. Cambridge, UK: Cambridge University Press. p. 137. ISBN 978-0-521-88831-8. Zbl 1187.94001.
^ Berstel, Jean; Perrin, Dominique (1985). Theory of Codes. Pure and Applied Mathematics. Vol. 117. Academic Press. p. 377. Zbl 0587.68066.