View source for Bit slip

{{Short description|Loss or gain of bits in digital transmission}}
{{Refimprove|date=October 2014}}
{{About|the digital transmission concept||Slippage (disambiguation){{!}}Slippage}}

In [[Digital data|digital]] transmission, '''bit slip''' is the loss or gain of a [[bit]] or bits, caused by [[clock drift]]{{snd}}variations in the respective [[Clock signal|clock]] rates of the transmitting and receiving devices.

One cause of bit slip is [[Buffer overflow|overflow]] of a receive [[Buffer (telecommunication)|buffer]] that occurs when the transmitter's [[clock rate]] exceeds that of the receiver. This causes one or more bits to be dropped for lack of [[Computer data storage|storage]] capacity.

One way to maintain timing between transmitting and receiving devices is to employ an [[Asynchronous communication|asynchronous protocol]] such as [[Asynchronous start-stop|start-stop]].  Alternatively, bit slip can be prevented by using a [[self-clocking signal]] (such as a signal modulated using [[OQPSK]]) or using a [[line coding]] such as [[Manchester encoding]].

Another cause is "losing count", as on a hard drive: if a hard drive encounters a long string of 0s, without any 1s (or a string of 1s without 0s), it may lose track of the frame between fields, and suffer bit slip. When a pulse of N consecutive zero bits are sent, clock drift may cause the hardware to apparently detect N-1 zero bits or N+1 zero bits{{snd}}both kinds of errors are called bit slip.<ref name="everett">{{cite book |editor=John Everett |url=https://books.google.com/books?id=MWuXmf4V4NwC |title=VSATs: Very Small Aperture Terminals |section=6.22 Demodulator failure: data bit slips |page=117 |date=1992}}</ref><ref>{{cite book |author1=Yongquan Fan |author2=Zeljko Zilic |url=https://books.google.com/books?id=-hTTWWSVBbEC |title=Accelerating Test, Validation and Debug of High Speed Serial Interfaces |page=127 |date=2010}}</ref> Thus one prevents long strings without change via such devices as [[run length limited]] codes.

Many communication systems use [[linear-feedback shift register#Scrambling|linear-feedback shift register scrambling]] to prevent long strings of 0s (or other symbol),
including VSAT,<ref name="everett" /> [[1000BASE-T]], {{IETF RFC|2615}}, etc.
While a [[scrambler]] makes the "losing count" type of bit slip error occur far less often,
when bit slip errors do occur (perhaps for other reasons), 
scramblers have the property of expanding small errors that add or lose a single bit into a much longer burst of errors.

The optimized cipher feedback mode (OCFB), the statistical self-synchronization mode, and the "one-bit CFB mode" also expand small bit-slip errors into a longer burst of errors, but eventually recover and produce the correct decrypted plaintext.
A bit-slip error when using any other [[block cipher mode of operation]] generally results in complete corruption of the rest of the message.<ref>
Oliver Jung, Christoph Ruland.
"Analysis of the Statistical Self-Synchronization Mode of Operation".
published in:
[https://books.google.com/books?id=rwvY5oPE6i4C "Fifth International ITG Conference on Source and Channel Coding (SCC)"].
p. 121.
2004.
</ref><ref>
William Millan and Ed Dawson.
"On the Security of Self-Synchronous Ciphers".
published in:
[https://books.google.com/books?id=5tln5w-Hb6UC "Information Security and Privacy: Second Australasian Conference, ACISP '97, Sydney, NSW, Australia, July 7-9, 1997 Proceedings"].
p. 159-160.
</ref>

== See also ==
* [[Clock drift]]
* [[Deletion channel]]
* [[Erasure code]]
* [[Self-synchronizing code]]

==References==
{{Reflist}}

[[Category:Data synchronization]]