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 ...

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 represente ...

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 ...

rates of the transmitting and receiving devices. One cause of bit slippage is overflow of a receive

buffer Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Buffering agent, the weak acid or base in a buffer solution * Lysis buffer, in cell biology * Metal ion 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 pr ...

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 (such as a signal modulated using

OQPSK Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs a ...

) or using a line coding such as

Manchester encoding In telecommunication and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no D ...

. 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 In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...

, , 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 In cryptography, a block cipher mode of operation is an algorithm that uses a block cipher to provide information security such as confidentiality or authenticity. A block cipher by itself is only suitable for the secure cryptographic transform ...

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