System Functions. Structure of Functional Units. Functional Units. Synchronous Timing Unit and Clock Interface Unit, страница 6

The fiber-shared virtual path protection is to logically divide one optical channel such as STM-64/STM-16/STM-4 optical path into several paths. These logical paths then are collocated with other links to form logic rings at path level. These rings will be provided with appropriate protection schemes, such as MSP and SNCP.

In Figure 3-2 and Figure 3-3, the OptiX OSN 9500 products build up a STM-64 MSP ring and form a STM-16 fiber-shared virtual path protection ring with other equipments respectively.

Figure 3-2 Fiber-shared virtual path protection

Figure 3-3 Fiber-shared virtual path protection

6.  Mesh Protection

Mesh network achieves a higher reliability because there are multiple routes available between two nodes. This effectively protects the traffic against node congestion and node failure. Compared with the ring network, the mesh features the following advantages:

n  Excellent bandwidth availability, scalability and survivability.

n  Suitable for the area with large traffic volume but evenly distributed.

3.3.3  Clock Synchronization Protection

In the SDH network, it is of vital importance to keep all NE clocks synchronous. Usually, there are many timing reference sources available for one NE. These reference sources may be from either the same master timing source or clock reference sources of different qualities. Therefore, it is necessary to learn the quality of each reference source to execute automatic protection switching of the synchronous timing source.

The S1 byte defined in ITU-T Recommendation is responsible for transmitting the quality information of the timing source. The four bits b5–b8 in S1 of the SOH contains quality information of 16 timing sources. The following table shows the SSM encoding mode specified in ITU-T Recommendation. With this quality information and together with certain switching protocols, the automatic protection switching of the synchronization clock can be realized in the synchronous network.

Table 3-3 SSM encoding mode

In the SDH optical synchronous transmission system, the automatic clock protection switching follows the protocols below:

n  Above all, the NE will select a timing source with the highest level in the S1 byte from all current available timing sources, and then transmit its quality information (that is, S1 byte) to downstream NEs.

n  When the information contained in S1 bytes of multiple timing sources are the same, the system will select the one with higher priority in the priority list as its timing source. Then, the quality information of this timing source will be transmitted to downstream NEs.

n  If NE B is tracing the synchronous timing source of NE A currently, the clock of NE B then is an unavailable synchronization source to NE A.

3.4  On-Line Upgrade of Unit Functions

The board software and main control software can be on-line upgraded through the NM.

Two sets of main control software can be held in the JSCC board simultaneously, supporting new software upgrading without affecting the current software running. The old software will be replaced once the new software is confirmed as correct. The two sets of software also support manual switching, without damaging the already set configuration information or causing NE service interruption.