digital Digital usually refers to something using discrete digits, often binary digits. Technology and computing Hardware *Digital electronics, electronic circuits which operate using digital signals **Digital camera, which captures and stores digital i ...

transmission, bit slip is the loss or gain of a

bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented a ...

or bits, caused by

clock drift Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to ...

– variations in the respective

clock A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and t ...

rates of the transmitting and receiving devices. One cause of bit slippage is overflow of a receive buffer that occurs when the transmitter's

clock rate In computing, the clock rate or clock speed typically refers to the frequency at which the clock generator of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the ...

exceeds that of the receiver. This causes one or more bits to be dropped for lack of storage capacity. One way to maintain timing between transmitting and receiving devices is to employ an asynchronous protocol such as start-stop. Alternatively, bit slip can be prevented by using a

self-clocking signal In telecommunications and electronics, a self-clocking signal is one that can be decoded without the need for a separate clock signal or other source of synchronization. This is usually done by including embedded synchronization information with ...

(such as a signal modulated using OQPSK) or using a

line coding In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...

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—both kinds of errors are called bit slip. John Everett
"VSATs: Very Small Aperture Terminals"
Section "6.22 Demodulator failure: data bit slips". p. 117. 1992. Thus one prevents long strings without change via such devices as

run length limited Run-length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. RLL codes are defined by four main parameters: ''m'', ''n'', ''d'', ''k''. The first two, ''m'' ...

codes. Many communication systems use linear-feedback shift register scrambling to prevent long strings of 0s (or other symbol), including VSAT, 1000BASE-T, , etc. While a

scrambler In telecommunications, a scrambler is a device that transposes or inverts signals or otherwise encodes a message at the sender's side to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Wher ...

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. William Millan and Ed Dawson. "On the Security of Self-Synchronous Ciphers". published in
"Information Security and Privacy: Second Australasian Conference, ACISP '97, Sydney, NSW, Australia, July 7-9, 1997 Proceedings"
p. 159-160.

References

{{Reflist Data synchronization

See also

References