data transmission Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point or ...

and

telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...

, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into

data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpret ...

. Stuffed bits should not be confused with overhead bits. Bit stuffing is used for various purposes, such as for bringing

bit stream A bitstream (or bit stream), also known as binary sequence, is a sequence of bits. A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet ma ...

s that do not necessarily have the same or rationally related

bit rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...

s up to a common rate, or to fill buffers or frames. The location of the stuffing bits is communicated to the receiving end of the

data link A data link is the means of connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a transmitter and a recei ...

, where these extra bits are removed to return the bit streams to their original bit rates or form. Bit stuffing may be used to synchronize several channels before

multiplexing In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - ...

or to rate-match two single channels to each other. Another use of bit stuffing is for

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''/ ...

coding: to limit the number of consecutive bits of the same value in the data to be transmitted. A bit of the opposite value is inserted after the maximum allowed number of consecutive bits. Since this is a general rule the receiver doesn't need extra information about the location of the stuffing bits in order to do the de-stuffing. This is done to create additional signal transitions to ensure reliable reception or to escape special reserved code words such as frame sync sequences when the data happens to contain them. Bitstuffing en

Bit stuffing does not ensure that the payload is intact (''i.e.'' not corrupted by transmission errors); it is merely a way of attempting to ensure that the transmission starts and ends at the correct places.

Error detection and correction In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable commu ...

techniques are used to check the frame for corruption after its delivery and, if necessary, the frame will be re-sent.

Zero-bit insertion

The NRZI coding scheme transmits a 0 bit as a signal transition, and a 1 bit as no change. In this case, bit stuffing is most easily described as the insertion of a 0 bit after a long run of 1 bits. It was popularized by IBM's SDLC (later renamed

HDLC High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides bot ...

), and is also used in Low- and full-speed USB. After a long sequence of 1 bits there would be no transitions in the transmitted data, and it would be possible for the transmitter and receiver clocks to lose synchronisation. By inserting a 0 after five (SDLC) or six (USB) consecutive 1 bits the transmitter guarantees a maximum of six (SDLC) or seven (USB) bit times between transitions. The receiver can synchronise its clock against the transitions to ensure proper data recovery. In SDLC the transmitted bit sequence "01111110" containing six adjacent 1 bits is the Flag byte. Bit stuffing ensures that this pattern can never occur in normal data, so it can be used as a marker for the beginning and end of the frame without any possibility of being confused with normal data.Kevin R. Fall and W. Richard Stevens, ''TCP/IP Illustrated Volume 1: The Protocols'', Second Edition, Addison-Wesley, 2012, Kindle Edition loc 3505 The main disadvantage of bit-stuffing is that the

code rate In telecommunication and information theory, the code rate (or information rateHuffman, W. Cary, and Pless, Vera, ''Fundamentals of Error-Correcting Codes'', Cambridge, 2003.) of a forward error correction code is the proportion of the data-st ...

is unpredictable; it depends on the data being transmitted. ''Source: from Federal Standard 1037C in support of MIL-STD-188''

Notes

{{reflist Line codes Synchronization

Zero-bit insertion

See also

Notes