What are Jitter and Wander? - Technopediasite


Sunday, October 28, 2018

What are Jitter and Wander?

What are Jitter and Wander?: Today I would like to discuss about this topic.In real life, various interfering factors prevent perfect synchronization. All systems are subject to ┬¬jitter┬║ and/or "wander", which can cause bit errors, slips, data loss and/or frequency interference, thereby impairing transmission quality. The jitter test & measurement facilities are one important component. Jitter/wander measurements at all major bit rates (electrical and optical): E1, E3, E4, STM-1/4/16/64 or DS1, DS2, DS3, STS-1/3/12, OC-1/3/12/48/192.

Using the Zoom function of Jitter, the graphical results presentation format lets you detect errors in the details even with long term measurements (also useful for acceptance reports).Practically any quantity of results and setups can be stored to hard disk. Floppy disk drive for exchanging data. Offline analysis of stored results is possible on any PC. PCMCIA slots simplify installation of modems and/or LAN cards.
Jitter/wander measurements
Telecom Tester

What are jitter and wander?
Jitter: "Jitter" is the term used to designate periodic or stochastic deviations of the significant instants of a digital signal from the ideal, equidistant values (Below Figure). Otherwise stated, the transitions of a digital signal invariably occur either too early or too late when compared to a perfect squarewave (reference clock).
Jitter is the deviation of clock transitions from an ideal squarewave.
Very slow jitter is known as "Wander".Limit between jitter and wander at 10 Hz.

Sources of jitter and wander
Impulsive noise and crosstalk can produce phase fluctuations composed mainly of higher frequency components, thereby causing non-systematic (stochastic) jitter.

Pattern jitter
Digital signal distortion leads to "intersymbol" interference, which is a sort of crosstalk interference between neighboring pulses. Pattern-dependent systematic jitter is the result.

Phase noise
Although clock generators are usually synchronized to a reference clock in SDH/SONET systems, there are still phase fluctuations due to thermal noise or drift in the oscillators, for example. The faster phase variations caused by the noise lead to jitter, whereas the drift caused by temperature variations and aging produces slower phase changes (wander).

Delay fluctuations
Changes in the signal delay on a communications path result in corresponding phase fluctuations, which are generally relatively slow. For example, delay variations of this sort occur on an optical fiber due to daily temperature fluctuations. This generally results in wander.

Stuffing and delay jitter
During multiplexing, asynchronous digital signals must be adapted to the transmission speed of the higher speed system by inserting stuffing bits. The stuffing bits are removed during the demultiplexing process. The gaps which then occur are evened out by a smoothed clock. This compensation is never perfect, and the result is stuffing and delay jitter.

Mapping jitter
Plesiochronous and asynchronous signals are mapped into synchronous containers using stuffing techniques. At the next terminating multiplexer, the plesiochronous tributaries are then unpacked. Due to the stuffing that occurred, there are gaps in the recovered signal, which are compensated using PLL circuitry. There is still some leftover phase modulation, which is known as mapping or stuffing jitter

Pointer jitter
Clock differences between two networks or between SDH network elements are compensated by pointer movements. These pointer jumps correspond to 8 or 24 bits, depending on the multiplex hierarchy. When the tributary signal is unpacked at the end point, the phase variations are still present but are smoothed out using PLL circuitry. The residual phase modulation is known as pointer jitter. Besides pointer jitter, the unpacked signal also exhibits mapping jitter, so the sum total of both, known as "combined" jitter is always measured.

Disruptions caused by jitter

It is the job of clock recovery circuitry used in network elements to correctly sample the digital signal, i.e. as close as possible to the center of the bit, using the recovered bit clock. If the digital signal and the clock both have identical jitter, then the position of the sampling instant does not change despite significant jitter error. Sampling still occurs properly, and no bit errors arise. Strictly speaking, however, this is the case only with lowfrequency jitter for which the clock recovery circuitry can keep up with digital signal phase variations with no problems. At higher jitter frequencies, however, the clock recovery circuitry cannot keep up with the fast phase variations of the digital signal. Phase shifts result, and for values 40.5 clock periods (UI = Unit Interval), the result is incorrect sampling of the bit element and thus bit errors.

Due to additional digital signal distortion, the decision range is much smaller in real life. At very large jitter amplitudes, bit errors become so common that a loss of frame (LOF) will occur.

Disruptions caused by wander

Unlike jitter, the phase variations due to wander do not lead to bit errors since the recovered clock can easily follow these slow changes in phase. However, wander amplitudes can accumulate to produce very large values over longer time intervals. Digital signals arriving at network and exchange nodes from different directions can have very high wander amplitudes relative to one another. Since digital signals are processed internally with a common clock, buffers are required to compensate for the wander.

At SDH/SONET nodes, these buffers can be relatively small since adaptation is possible using pointer actions. However, pointer actions can lead to a high jitter amplitude in the transported payload signal at the tributary output.

At exchange nodes, however, if the buffer overflows the only way to compensate involves an intentional frame slip. Parts of the transmitted signal are lost, producing error bursts. However, these error bursts do not trigger alarms due to a loss of frame (LOF) or errors in the frame alignment signal (FAS).

How do you measure jitter and wander?
Jitter effects
To measure jitter effects, the incoming signal is regenerated to produce a virtually jitter-free signal, which is used for comparison purposes. No external reference clock source is required for jitter measurement. The maximum measurable jitter frequency is a function of the bit rate and ranges at 10 Gbit/s (STM-64/OC-192) up to 80 MHz. The unit of jitter amplitude is the unit interval (UI), where 1 UI corresponds to an error of the width of one bit. Test times on the order of minutes are necessary to accurately measure jitter.

Wander effects
Wander test equipment requires an external, extremely precise reference clock source. The most practical unit of wander amplitude is the absolute magnitude in ns (10±9 seconds), and not the UI unit preferred for jitter measurements. The extremely low frequency components (mHz range) require rather long test times ranging up to 106 s.

The differences between jitter and wander are also reflected in the various test applications, even though in both cases we are dealing with phase fluctuations that must be measured and evaluated details in below table.
Comparison between jitter and wander
jitter and wander

1 comment:

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