Synchronization messaging
Overview
Alcatel-Lucent 1665 DMX provides a synchronization messaging feature to ensure the integrity of network synchronization during both normal and abnormal conditions. Through the use of synchronization messaging, the current quality of the timing source can be conveyed from one Alcatel-Lucent 1665 DMX to the next. This capability allows the Alcatel-Lucent 1665 DMX to automatically change its timing reference in order to always maintain the highest quality timing available. The capability also allows Alcatel-Lucent 1665 DMX to inform a local BITS clock when the DS1/E1 timing output has been degraded and should no longer be used as a reference. This synchronization messaging feature is compliant with the scheme developed in the ANSI T1X1 standards committee.
Also, the same parameter used to choose between DS1 and E1 timing outputs is employed to choose between SONET sync messaging (ANSI T1X1 mentioned above) and SDH sync messaging.
Applications
The applications supported with the synchronization messaging feature can be divided into three categories:
DS1 timing output integrity
The derived DS1 timing outputs are typically used as a synchronization reference to a BITS clock which provides the timing reference to an externally-timed Alcatel-Lucent 1665 DMX. The synchronization reference is derived from the SONET transmission facility which is synchronized from an upstream timing reference. In this way, the timing from the BITS clock in one office (master) is distributed to the next office (slave) using the SONET transmission facilities between them as the synchronization vehicle.
The BITS are typically capable of synchronizing to a Stratum 3 or better accuracy. Alcatel-Lucent 1665 DMX equipped with the embedded Stratum 3 timing generator is capable of synchronizing to a ± 4.6 ppm clock or better. The Stratum timing hierarchy requires that clocks of equal or better Stratum level be used to synchronize other clocks. In this way, the Stratum timing hierarchy is preserved under all failure conditions. Under non-failure conditions, Alcatel-Lucent 1665 DMX does not introduce its own internal timing source onto the SONET facility, but merely transfers the quality of its timing reference. A failure of all derived DS1 timing references to the BITS at the master office will cause the BITS to enter holdover mode, whose minimum accuracy is dependent on its internal clock. If the BITS internal clock is of equal or better Stratum level than Alcatel-Lucent 1665 DMX, the externally-timed Alcatel-Lucent 1665 DMX will use this reference to synchronize all outgoing SONET transmission facilities. This preserves the required hierarchical structure of the timing network and should be maintained at all times.
If the externally-timed Alcatel-Lucent 1665 DMX at the master office enters holdover due to a disconnected reference cable or a local BITS failure, the quality of the derived DS1 timing output at the slave office will now be traceable directly to the Alcatel-Lucent 1665 DMX. The Stratum timing hierarchy is violated if the slave office BITS requires timing accuracy better than the Stratum level provided by the master Alcatel-Lucent 1665 DMX's internal clock. Because the Alcatel-Lucent 1665 DMX contains the embedded Stratum 3 Timing Generator, Stratum 3 accuracy is maintained indefinitely. This provides acceptable timing for Stratum 3 NEs at slave offices.
Stratum timing rules are also violated when at least one line-timed Alcatel-Lucent 1665 DMX exists at a site between the master and slave offices. In this scenario, a fiber cut between the master office and the line-timed site will cause the line-timed Alcatel-Lucent 1665 DMX to enter holdover (with accuracy dependent upon its internal clock). If the BITS at the slave office requires a higher accuracy clock than that generated by the Alcatel-Lucent 1665 DMX in holdover, the Stratum timing hierarchy is violated.
Automatic synchronization reconfiguration
SONET was designed to operate optimally in a synchronous environment. Although plesiochronous and asynchronous operation can be supported through the use of pointer adjustments, transmission quality is affected by the generation of additional jitter and wander due to pointer adjustments. Because of this, it is desirable to maintain synchronous operation whenever possible. Through the use of synchronization messages, the quality of the different timing references can be made available at each Alcatel-Lucent 1665 DMX NE. Alcatel-Lucent 1665 DMX can be optioned to determine the best timing reference available and switch to that reference. Through this mechanism, the synchronous operation of the subnetwork can be maintained. The switching of timing references is hitless, and the synchronization messages also allow it to be done without creating timing loops in the process.
Access ring network
Under normal operation, the access ring network, shown below, has one Alcatel-Lucent 1665 DMX shelf externally timed and the others line timed. If a fiber failure occurs between the first two Alcatel-Lucent 1665 DMX shelves, the synchronization auto-reconfiguration feature causes Alcatel-Lucent 1665 DMX shelves to change their line timing directions to the opposite direction. The result is that the ring is again operating synchronously. The ring already provides self-healing of the traffic, so it is especially important to maintain synchronous operation during this type of failure to prevent service degradation due to increased jitter and wander.
Figure 6-20: Synchronization reconfiguration - access ring
Synchronization provisioning integrity
Synchronization messaging also helps prevent provisioning errors. Provisioned timing loops on Alcatel-Lucent 1665 DMX are quickly detected through the synchronization messaging algorithm and prevented by forcing a shelf into holdover. The system can then be reprovisioned correctly.