LEGACY OPTICAL SYSTEM TIMING FAQ

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 .