Last Updated: August 16, 2025
7 min read
Some of the FAQs for Optical System Timing are:
Q. How do I time SONET NEs shelves in a central office environment?
A. Each SONET NEsshelf should be externally referenced to the BITS clock in the office. If a BITS clock is not available in the office, a traffic-carrying DS1 from the local switch may be bridged (for example, using a bridging repeater) as the reference to the SONET NEsshelf. Line timing may also be used, but at least one SONET NE shelf in the network must be externally timed.
Q. Where do I use the DS1 timing output feature?
A. The primary application is for supplying a timing reference to the office BITS clock. This allows the BITS clock to be slaved to a BITS clock in another office that is, in turn, traceable to the primary reference source (PRS). Typically, the SONET NE supplying the DS1 timing output will, in turn, be externally timed by the BITS clock. If there is no BITS clock, the DS1 timing output can be used to time a switch or switch remote (if the switch remote is equipped for that option) directly or even another SONET NE Multiplexer.
Q. How do I prevent my BITS clock from using a DS1 timing output when a failure in the network results in this DS1 being timed from a SONET NE in holdover?
A. SONET sync messaging informs the local SONET NE of this condition, and AIS is inserted on the DS1 timing output.
What is the advantage of using the DS1 timing output instead of a multiplexed DS1 as the timing reference?
The DS1 timing output is derived from the optical line rate and is superior because:
The DS1 is virtually jitter-free
Sync messages guarantee the traceability of the timing
Administration of traffic DS1s for timing is eliminated.
Q. Can I ever use the SONET NE in the free-running timing mode?
A. If a PRS traceable external reference is available, it is the recommended timing mode for any/all CO applications. The free running timing mode can be used but a slight increase in jitter will result. If one SONET NE is provisioned for free running, all other SONET NEs in the network must be line timed and SONET interfaces to other equipment are not allowed. The DS1 timing output should not be enabled with a free running network.
Q. How do I provide timing to a central office host switch that does not have the option for an external reference?
A. DS1 carried over SONET may contain significant jitter/wander and be unacceptable to the switch as a timing reference. If the central office has a BITS clock, the recommendation is to use the output from the BITS clock into an unused DS1 traffic port on the switch. If the central office does not have a BITS clock, the recommendation is to use the DS1 timing output from the SONET NE as the line timing reference into an unused DS1 traffic port on the switch.
Q. Can a DS1 carried over SONET ever be used as a timing reference?
A. YES! In many applications there is no other choice. Most switch remotes, for instance, obtain their timing from a specific DS1 signal generated by their host switch, so these remotes must line time from the DS1 signal. In addition, DLC equipment, channel banks, and PBXs will not likely have external references and may be allowed to line time from a DS1 carried over SONET.
Q. Are there any specific concerns when using a DS1 carried over SONET to time equipment such as a switch remote or DLC?
A. Yes. The major concern is to make sure all the equipment is synchronous. The SONET NEs should be synchronous to each other to prevent pointer adjustments. This can be accomplished by having one source SONET that is externally timed. The other SONET NEs in the network should be line timed, or they should be externally timed to a clock to which they provide a DS1 timing output. The SONET NEs should also be synchronous to the switch to prevent excessive mapping jitter. This can be done by synchronizing the host switch to the BITS clock used to reference the SONET .
Q. Will I have any problems providing timing to a customer that has a high quality PBX or switch?
A. If the network is completely synchronous, as described in the previous answer, there should be no problems. If the PBX is sensitive to the jitter produced, even under the synchronous conditions, the DS1 timing output of SONET may be required to be used as a timing reference to this equipment.
Q. Why does Bellcore say that DS1s carried over SONET should not be used for timing?
A. Bellcore has provided this recommendation because there are several limitations. Bellcore says that DS1s carried over SONET must be used in applications such as switch remotes and will be acceptable, provided pointer adjustments are not created.
Q. Can pointer adjustments be prevented?
A. Neither random nor periodic pointer adjustments will occur if the SONET shelf is provisioned for line timing.
Q. How do I time SONET at a remote site?
A. Line time.
Q. How many SONET NEs can I chain together in an add/drop configuration before the timing becomes degraded?
A. The Stratum level traceability of the nth node in an add/drop chain is the same as that in the first node. Also, while timing jitter will theoretically increase as the number of nodes is increased, the high quality timing recovery and filtering on the SONET allows add/drop chains to be extended to any practical network limit without detectable increases in jitter levels. In practice, the only effects on timing at the nth node will occur whenever high-speed protection switches occur in any of the previous n-1 nodes. These effects should be rare.
Q. How do I time a SONET ring network?
A. An interoffice ring should have each node externally timed if BITS clocks are available. All other rings should have one node externally timed (two in some dual homing architectures) and the rest of the nodes line timed. Synchronization reconfiguration is automatic.
Q. Why are there more issues related to timing with SONET equipment than there are with asynchronous equipment?
A. SONET equipment was designed to work ideally in a synchronous network. When the network is not synchronous, mechanisms such as pointer processing and bit-stuffing must be used and jitter/wander increases.
Q. Can DS3 signals be used to carry DS1 timing signals without the worry of having the network synchronous?
A. Yes, although this option is more expensive.
Q. What are the limitations on automatic synchronization reconfiguration?
A. Automatic synchronization reconfiguration is only available when the SONET is provisioned for line timing mode. This allows the timing direction of an OC-n (OC-12, OC-48, or OC-192) ring network to change automatically in response to a failure. When the SONET is provisioned for external timing, automatic synchronization reconfiguration is not available. When an OC-n fault is detected in the timing direction, AIS is inserted on the derived DS1s which forces the BITS to switch to another good timing source or into holdover preventing timing loops.
Q. How do I synchronize a BITS clock and maintain automatic synchronization reconfiguration on a SONET ring?
A. Provision all but the host node (node with co-located PRS) for line timing. Provide each non-host BITS clock with a pair of derived DS1s. The SONET will detect faults and provide the BITS clocks with good inputs if available. Timing loops will be prevented. The host node should be set for external timing and get its timing from an externally timed BITS clock. To prevent a timing loop, the host BITS clock should get its timing from a PRS traceable source. The non-host nodes should not be timed from the co-located BITS clock since this would disable the automatic synchronization reconfiguration feature.
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