Sync messaging examples

Overview

In this section, some detailed examples are given to show specifically how the sync messages propagate through network and assist in the recovery from a fiber failure. Through these examples, one can extend the same concept to any other network that may include different topologies, number of sites, failure locations, and number of BITS clocks. Some examples are depicted in a linear configuration because the particular sync messaging concept is more easily conveyed in linear terms. Those concepts can be applied to Alcatel-Lucent 1665 DMXtend ring networks.

Synchronization reconfiguration using an externally timed access ring

The left side (Part A) of the figure below illustrates an externally timed access ring operating in its normal configuration. The Alcatel-Lucent 1665 DMXtend shelf at Site A is externally timed from a BITS clock referenced to a Primary Reference Source (PRS). The remaining Alcatel-Lucent 1665 DMXtend shelves (Sites B, C, and D) are externally timed from a BITS referenced to a derived DS1 timing reference output. The STU message is sent to indicate where timing is traceable to an external BITS and where it is valid to be used. Automatic synchronization reconfiguration is not an option for externally timed Alcatel-Lucent 1665 DMXtend shelves. Therefore, a change in the sync message will not cause an automatic synchronization reconfiguration. On the right side (Part B), a fiber has been cut between Sites A and B. Immediately, Alcatel-Lucent 1665 DMXtend at site B changes the format of its derived DS1 to AIS. This forces the BITS clock at Site B to enter holdover or switch input source (if a valid one is available). Because automatic synchronization reconfiguration is not available, the synchronization status messages are not used. All other non-host nodes will track the holdover clock at Site B. Although no timing loops have been formed, the timing of all non-host nodes will differ from the host node by the accuracy of the holdover clock at Site B.

Figure 6-20: External timing with automatic synchronization reconfiguration, part A and B

Synchronization reconfiguration in an access ring

The left side (Part A) of following figure illustrates an access ring operating in its normal configuration. The Alcatel-Lucent 1665 DMXtend shelf at the CO is externally timed, and each of the other Alcatel-Lucent 1665 DMXtend shelves are line timed in a counterclockwise direction. The STU message is sent to indicate where timing is traceable to an external BITS and where it is valid to be used. The DON'T USE message is sent on the interface that is being used as the line timing reference and, thus, where using that timing would create a timing loop. Sync messaging and automatic synchronization have both been enabled for this network.

On the right side (Part B), a fiber has been cut between sites A and B. Immediately, the Alcatel-Lucent 1665 DMXtend shelf at site B enters holdover and sends out the Stratum 3 message to site C. The Alcatel-Lucent 1665 DMXtend shelf at site B cannot switch to line time from site C because it is receiving the DON'T USE message on that interface.

Figure 6-21: Automatic synchronization reconfiguration, part A and B

In the figure below, the Alcatel-Lucent 1665 DMXtend shelf at site C detects the incoming Traceable Stratum 3 message and sends the message to site D. The Alcatel-Lucent 1665 DMXtend shelf at site C cannot switch to line timing from the other rotation because it is receiving the DON'T USE message on that interface.

The Alcatel-Lucent 1665 DMXtend shelf at site D detects the incoming Traceable Stratum 3 message. The STU message is a better quality message than the Traceable Stratum 3 message, so the Alcatel-Lucent 1665 DMXtend shelf at site D switches to line timing from site A. After the switch occurs, the DON'T USE message is sent back to site A, and the STU message is retransmitted to site C.

Figure 6-22: Automatic synchronization reconfiguration, part C and D

On the left side of the figure below (Part E), the Alcatel-Lucent 1665 DMXtend shelf at site C detects the incoming STU message from site D. The STU message is a better quality message than the Traceable Stratum 3 message being received from site B, so the Alcatel-Lucent 1665 DMXtend shelf at site C switches to line time from site D. After the switch occurs, the DON'T USE message is sent back to site D, and the STU message is retransmitted to site B.

On the right side of the figure below (Part F), the Alcatel-Lucent 1665 DMXtend shelf at site B detects the incoming STU message from site C. The STU message is a better quality message than the internal holdover capability, so the Alcatel-Lucent 1665 DMXtend shelf at site B switches to line time from site C. After the switch occurs, the DON'T USE message is sent back to site C, and the STU message is forwarded to site A. When the failure clears, the synchronization remains in the new configuration unless it is manually switched back.