G.821

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The recommendation G.821 Error performance of an international digital connection operating at a bit rate below the primary rate and forming part of an Integrated Services Digital Network of the ITU-T is an international agreement on the transmission quality of lines of the telephone network , which also applies to the Transmission of digital data can be used. Telephone lines are not always suitable for data transmission , regardless of whether they are laid as local subscriber lines or whether they are used for international telephone traffic within the network. So technical criteria had to be found and agreed with which one could assess the suitability of a telephone line for data transmission, and also measurement methods with which one could check these criteria. The most important criterion was agreed on the bit error rate . An agreement was reached that with a bit error rate of 10 −4 a line is still suitable for analog voice transmission, but at 10 −3 a line would be completely unusable. The bit error rate of 10 −3 was equated to a complete signal failure, in which the alarm indication signal (AIS) is inserted into the line instead of the faulty useful signal . For digital voice transmission in ISDN and for data transmission, the transmission quality of a line should be so good that bit errors occur with a frequency of less than 10 −6 . These criteria and suitable quality parameters were worked out in detail in Recommendation G.821. Multiplexers and other devices used in digital transmission technology continuously measure these parameters and record them in a (performance) database (MIB).

History

The original recommendation dates back to 1980 and dealt with a 64 kbit / s channel of the ISDN. Since it was the first quality description of a digital line, it was also used for speeds for which it was not designed. Since 1993 the recommendation G.826 describes all speeds from 2 Mbit / s upwards, since 2002 the G.821 describes all speeds below that.

application

The G.821 is actually a theoretical treatise, but is used in practice worldwide. In the first version it describes a measurement with a test text on the 27,500 km long International Hypothetical Reference Line (Hypothetical Reference Digital Path) . The measurement duration is not defined (Original: A period of the order of any one month is suggested ... ), the suggested time is not applicable.

In practice, the cable length is usually neglected and a separate measurement period is used.

Test texts

Test texts in synchronous data transmission are bit patterns that repeat themselves after a certain number of bits. Since they appear to be an arbitrary combination, they are also known as quasi-random sequence (QZF) or pseudo random text . These test samples are not random, they are based on mathematical formulas and can be traced at any time.

There are at least ten different test texts used. Although there is a standardization, the manufacturers of bit error measuring devices do not agree on the bit sequence, for example 2047r (r = reverse) or 2047i (i = inverted).

The generation of these bit patterns is relatively simple and is also offered as an IC in hardware .

The G.821 does not prescribe any test samples.

  • In ITU-T recommendation O.152 , a test text 2 11 −1 = 2047 bits is recommended for speeds up to 2 Mbit / s .
  • O.151 and O.153 are further international recommendations for test texts at different speeds.
  • For test texts for asynchronous data transmission, see R.52 .

Error evaluation

Since bit errors can always occur with a sufficiently long measurement period (months, see above), these are tolerated.

Evaluation criteria:

  • Faulty second (ERRORED SECOND, ES): One second measurement time in which one or more bit errors are present.
    • Limit value: Less than 8% of the measurement time
  • Severely errored second (SES): one second with a bit error rate ≥ 1 · 10 −3 . Example: 63 bit errors / s are faulty on a 64 kbit / s line; 64 are considered unacceptable.
    • Limit value: Less than 0.2% of the measurement time
  • Degraded minute (DM): This error analysis was given in the first edition of G.821; it was hardly usable in practice and was deleted without replacement. However, it is still available in older bit error measuring devices.
    • Limit value: Less than 10% of the measurement time

In practice, ratios are formed from these values, for example error-free to disturbed seconds; the result is a quality profile of the measured line at the time of the measurement.

See also

literature

Karl-Heinz Schmidt (Ed.): Message transmission measurement technology, Volume 1 Line measurement technology , R. v. Decker's Verlag, G. Schenk GmbH, Heidelberg 1990, ISBN 3-7685-4989-5

Web links