Editing Synchronous optical networking (section)

==Synchronization==
Clock sources used for synchronization in telecommunications networks are rated by quality, commonly called a ''[[Synchronization in telecommunications|stratum]]''.<ref>{{cite book |chapter-url=http://oreilly.com/catalog/t1survival/chapter/ch05.html |archive-url=https://web.archive.org/web/20010818163152/http://www.oreilly.com/catalog/t1survival/chapter/ch05.html |url-status=dead |archive-date=18 August 2001 |title=T1: A Survival Guide |author=Matthew Gast |date=August 2001 |isbn=0-596-00127-4 |chapter=Chapter 5: Timing, Clocking, and Synchronization in the T-carrier System |publisher="O'Reilly Media, Inc." |access-date=28 September 2012 }}</ref> Typically, a network element uses the highest quality stratum available to it, which can be determined by monitoring the synchronization status messages (SSM) of selected clock sources.

Synchronization sources available to a network element are:{{citation needed|date=September 2012}}
;Local external timing
:This is generated by an [[Cesium clock|atomic cesium clock]] or a satellite-derived clock by a device in the same central office as the network element. The interface is often a DS1, with sync-status messages supplied by the clock and placed into the DS1 overhead.
;Line-derived timing
:A network element can choose (or be configured) to derive its timing from the line-level, by monitoring the S1 sync-status bytes to ensure quality.
;Holdover
:As a last resort, in the absence of higher quality timing, a network element can go into a [[Holdover in Synchronization Applications|''holdover'']] mode until higher-quality external timing becomes available again. In this mode, the network element uses its own timing circuits as a reference.

===Timing loops===
A timing loop occurs when network elements in a network are each deriving their timing from other network elements, without any of them being a "master" timing source. This network loop will eventually see its own timing "float away" from any external networks, causing mysterious bit errors—and ultimately, in the worst cases, massive loss of traffic. The source of these kinds of errors can be hard to diagnose.<ref>{{cite book |chapter-url=http://www.cisco.com/en/US/tech/tk482/tk607/technologies_q_and_a_item09186a008016c442.shtml#qa10 |chapter=Why is a timing loop so bad, and why is it so difficult to fix? |title=Optical Timing: Frequently Asked Questions |publisher=[[Cisco Systems]]|date=2 December 2005 |access-date=28 September 2012}}</ref> In general, a network that has been properly configured should never find itself in a timing loop, but some classes of silent failures could nevertheless cause this issue.