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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 fe ...
, a non-return-to-zero (NRZ)
line code 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 ...
is a
binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...
code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negative voltage, with no other neutral or rest condition. For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the baseband bandwidth required by the Manchester code (the passband bandwidth is the same). The pulses in NRZ have more energy than a
return-to-zero Return-to-zero (RZ or RTZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is s ...
(RZ) code, which also has an additional rest state beside the conditions for ones and zeros. When used to represent data in an
asynchronous communication In telecommunications, asynchronous communication is transmission of data, generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream. Any timing required to recover data f ...
scheme, the absence of a neutral state requires other mechanisms for bit synchronization when a separate clock signal is not available. Since NRZ is not inherently 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 withi ...
, some additional synchronization technique must be used for avoiding bit slips; examples of such techniques are a run-length-limited constraint and a parallel synchronization signal.


Variants

NRZ can refer to any of the following serializer line codes: The NRZ code also can be classified as a polar or non-polar, where polar refers to a mapping to voltages of +V and −V, and non-polar refers to a voltage mapping of +V and 0, for the corresponding binary values of 0 and 1.


Unipolar non-return-to-zero level

"One" is represented by a
DC bias In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias. A waveform with no DC ...
on the transmission line (conventionally positive), while "zero" is represented by the absence of bias – the line at 0 volts or grounded. For this reason it is also known as "on-off keying". In clock language, a "one" transitions to or remains at a biased level on the trailing clock edge of the previous bit, while "zero" transitions to or remains at no bias on the trailing clock edge of the previous bit. Among the disadvantages of unipolar NRZ is that it allows for long series without change, which makes synchronization difficult, although this is not unique to the unipolar case. One solution is to not send bytes without transitions. More critically, and unique to unipolar NRZ, are issues related to the presence of a transmitted DC level – the power spectrum of the transmitted signal does not approach zero at zero frequency. This leads to two significant problems: first, the transmitted DC power leads to higher power losses than other encodings, and second, the presence of a DC signal component requires that the transmission line be DC-coupled.


Bipolar non-return-to-zero level

"One" is represented by one physical level (usually a positive voltage), while "zero" is represented by another level (usually a negative voltage). In clock language, in bipolar NRZ-level the voltage "swings" from positive to negative on the trailing edge of the previous bit clock cycle. An example of this is
RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such ...
, where "one" is −12 V to −5 V and "zero" is +5 V to +12 V.


Non-return-to-zero space

"One" is represented by no change in physical level, while "zero" is represented by a change in physical level. In clock language, the level transitions on the trailing clock edge of the previous bit to represent a "zero". This "change-on-zero" is used by
High-Level Data Link Control 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 both c ...
and
USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
. They both avoid long periods of no transitions (even when the data contains long sequences of 1 bits) by using
zero-bit insertion In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits. Bit stuffing is used fo ...
. HDLC transmitters insert a 0 bit after 5 contiguous 1 bits (except when transmitting the frame delimiter "01111110"). USB transmitters insert a 0 bit after 6 consecutive 1 bits. The receiver at the far end uses every transition — both from 0 bits in the data and these extra non-data 0 bits — to maintain clock synchronization. The receiver otherwise ignores these non-data 0 bits.


Non-return-to-zero inverted

Non-return-to-zero, inverted (NRZI, also known as ''non-return to zero IBM'', ''inhibit code'', or ''IBM code'') was devised by Bryon E. Phelps ( IBM) in 1956. It is a method of mapping a
binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
to a physical signal for transmission over some transmission medium. The two-level NRZI signal distinguishes data
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 ...
s by the presence or absence of a transition at a clock boundary. ''Which'' bit value corresponds to a transition varies in practice, and the name NRZI is used for both.
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'' ...
(RLL) codes are generally described using the convention that a logical 1 is transmitted as a transition, and a logical 0 is transmitted as no transition. The HDLC and
Universal Serial Bus Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
protocols use the opposite convention: a logical 0 is transmitted as a transition, and a logical 1 is transmitted as no transition. A long series of no-transition bits can be difficult for a receiver to count accurately, so some means for forcing a transition at reasonable intervals is generally used in addition to NRZI. Magnetic disk and tape storage devices generally use fixed-rate RLL codes, while HDLC and USB use
bit stuffing In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits. Bit stuffing is used for ...
: they insert an additional 0 bit (forcing a transition) after 5 or 6 (respectively) consecutive 1 bits. While bit stuffing is efficient, it results in a variable data rate because it takes slightly longer to send a long string of 1 bits than it does to send a long string of 0 bits.


Synchronized non-return-to-zero

Synchronized NRZI (NRZI-S, SNRZI) and '' group-coded recording'' (''GCR'') are modified forms of NRZI. In NRZI-S, each 8-bit group is extended to 9 bits by a 1 in order to establish a transition for synchronisation.


Comparison with return-to-zero

Return-to-zero Return-to-zero (RZ or RTZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is s ...
describes a
line code 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 ...
used in
telecommunications 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 fe ...
in which the signal drops (returns) to zero between each
pulse In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the n ...
. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format. The ''zero'' between each bit is a neutral or rest condition, such as a zero amplitude in
pulse-amplitude modulation Pulse-amplitude modulation (PAM) is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulse ...
(PAM), zero
phase shift In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it ...
in phase-shift keying (PSK), or mid-
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
in frequency-shift keying (FSK). That ''zero'' condition is typically halfway between the significant condition representing a 1 bit and the other significant condition representing a 0 bit. Although return-to-zero contains a provision for synchronization, it still may have a DC component resulting in ''baseline wander'' during long strings of 0 or 1 bits, just like the line code non-return-to-zero.


See also

*
Universal asynchronous receiver-transmitter A universal asynchronous receiver-transmitter (UART ) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significa ...


References


Further reading

* * *


External links


CodSim 2.0: Open source simulator for Digital Data Communications Model at the University of Malaga written in HTML
{{Bit-encoding Line codes