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OSN 500 550 580 V100R008C50 Alarms and Performance Events Reference 02

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Pointer Justification

Pointer Justification

With the pointer justification, rate asynchronization and phase difference of payload signals are allowed. The pointer justification adjusts the rate of the information payloads so that they are synchronized with the STM-N frame.

SDH payload pointers can be classified into administrative unit pointer (AU_PTR) and tributary board pointer (TU_PTR). Pointer justification involves administrative unit pointer justification and tributary board pointer justification.

Generation Mechanism of AU Pointer Justification

In the AU-4 frame shown in Figure 1-10, several bytes in specific locations (the first nine bytes in the fourth row) are used to record the starting point of data information (to represent the data information phase). These bytes are called pointers. H1 and H2 are pointers, and three H3s are negative pointer justification opportunities.

Figure 1-10  Location and content of AU_PTR

When the network is synchronous, the pointer is used to perform phase alignment among the synchronous signals. If the NEs work under the same clock, the signals that are transmitted from various NEs to a certain NE have the same clock frequency. Therefore, rate adaptation is unnecessary. Transiently, the rate may be either a little higher or lower. In this case, phase alignment is required.

When the network is not synchronous, the NEs work at different frequencies, and the pointer is used for frequency justification. Pointer justification is also required to tolerate the frequency jitter and wander in the network.

If the frame rate of the virtual container (VC) is different from that of the administrative unit group (AUG), information is stuffed in the H3 bytes of the AU pointer area. The idle bytes are stuffed with pseudo-random information and are inserted before the VC to decrease or increase the frame rate of the VC. At the same time, the pointer value is raised or dropped to increase or decrease the frame rate of the VC. Therefore, positive and negative pointer justifications are performed. See Table 1-4.

Table 1-4  Pointer justification states
State Name Byte Numbering and Content of the Fourth Row in the STM-1 Frame Rate Relationship
7 8 9 10 11 12
Pointer zero justification H3 H3 H3 Information Information Information Information = container
Positive pointer justification H3 H3 H3 Stuffing Stuffing Stuffing Information < container
Negative pointer justification Information Information Information Information Information Information Information > container

The information rate is the VC frame rate. The container rate is the AU encapsulation rate.

All the NEs in the SDH network are generally well synchronized, and pointer justification seldom occurs. Actual performance monitoring for pointer justification of the network proves that AU pointer justification and TU pointer justification seldom occurs.

It is difficult to guarantee that all the NEs are well synchronized all the time during long-term network operation. If one or several NEs are not synchronized, even for a very short time, a great amount of pointer justifications could occur. Consecutive positive or negative pointer justification adjusts the phase forward or backward to realize the frequency justification.

Generation Mechanism of TU Pointer Justification

Causes of TU pointer justification are as follows:

  • TU pointer justification is transformed from AU pointer justification.

    TU pointer justification does not occur when the E1 signal is adapted into VC-12, and multiplexed into STM-1. If there is frequency offset between the E1 signal of the switch and the SDH clock, the signal is adapted to realize synchronization. Therefore, TU pointer justification that is detected on the tributary board is generally transformed from AU pointer justification.

  • TU pointer justification occurs during demultiplexing.

    If the system clock is inconsistent with the received clock, TU pointer justification occurs during demultiplexing.

  • When the upstream NE that the service passes through has pointer justification, TU pointer justification occurs at the local NE during demultiplexing.

Detection and Reporting of Pointer Justifications

There are two modes of detection and reporting of AU pointer justification: remote detection and local detection.

  • Remote detection

    The information about AU pointer justification that is generated at the local NE is transferred to the remote NE through the H1 and H2 bytes. The remote NE reports the AU pointer justification by interpreting the H1 and H2 bytes. Therefore, if the remote NE reports an AU pointer justification event, the local NE has pointer justification. The remote NE is the downstream NE in the service direction.

  • Local detection

    AU pointer justification that is generated at the local NE is detected and reported at the local NE. Therefore, if the local NE reports an AU pointer justification event, the local NE has pointer justification.

In the SDH system, the AU pointer justification events on a majority of optical interface boards are detected and reported through the detection of the H1 and H2 bytes. This is also called remote detection.

As the transformation from AU pointer justification into TU pointer justification could occur at the upstream NE instead of the local NE, the local NE does not necessarily have pointer justification if the tributary board reports pointer justification events.

Generally, AU pointer justification is generated at the upstream NE, but it is detected and reported at the downstream NE. TU pointer justification is generated at the NE where AU pointer justification is transformed into TU pointer justification. It is detected and reported at the tributary board of the NE where the service is terminated.

Updated: 2019-01-21

Document ID: EDOC1100020975

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