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 DMX ring networks.

Derived DS1 with no synchronization auto-reconfiguration using externally timed ring

Part "a" of the figure below illustrates an externally timed access ring operating in its normal configuration. The Alcatel-Lucent 1665 DMX shelf at Site A is externally timed from a BITS clock referenced to a Primary Reference Source (PRS). The remaining Alcatel-Lucent 1665 DMX 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 DMX shelves. Therefore, a change in the sync message will not cause an automatic synchronization reconfiguration. In part "b", a fiber has been cut between Sites A and B. Immediately, Alcatel-Lucent 1665 DMX at site B changes the format of its derived DS1 to AIS. 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 6-21: External timing with derived DS1 output, part A and B

Synchronization auto-reconfiguration in an access ring

Part "a" of the figure below illustrates an access ring operating in its normal configuration. The Alcatel-Lucent 1665 DMX shelf at the CO is externally timed, and each of the other Alcatel-Lucent 1665 DMX 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 DUS 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.

In part "b", a fiber has been cut between sites A and B. Immediately, the Alcatel-Lucent 1665 DMX shelf at site B enters holdover and sends out the Stratum 3 message to site C. The Alcatel-Lucent 1665 DMX shelf at site B cannot switch to line time from site C because it is receiving the DUS message on that interface.

Figure 6-22: Automatic synchronization reconfiguration, part A and B

In part c of the figure below, the Alcatel-Lucent 1665 DMX shelf at site C detects the incoming Traceable Stratum 3 message and sends the message to site D. The Alcatel-Lucent 1665 DMX shelf at site C cannot switch to line timing from the other rotation because it is receiving the DUS message on that interface.

In part D of the figure below, the Alcatel-Lucent 1665 DMX 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 DMX shelf at site D switches to line timing from site A. After the switch occurs, the DUS message is sent back to site A, and the STU message is retransmitted to site C.

Figure 6-23: Automatic synchronization reconfiguration, part C and D

In part e of the figure below, the Alcatel-Lucent 1665 DMX 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 DMX shelf at site C switches to line time from site D. After the switch occurs, the DUS message is sent back to site D, and the STU message is retransmitted to site B.

In part f of the figure below, the Alcatel-Lucent 1665 DMX 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 DMX shelf at site B switches to line time from site C. After the switch occurs, the DUS 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.