QUESTION: I work as a system engineer for the carrier-grade high density media gateway. Our media/signaling gateway interconnects PSTN, IP and ATM networks. We support E1/T1, SDH/SONET STM-1 PSTN trunks, Gigabit Ethernet IP STM-1 links and ATM STM-1 links.
Is it logical to request Stratum 2 performance on this GW in order to support any carrier-grade applications/services?
In the case that we provide E1/T1 PSTN interfaces and IP Ethernet
interfaces only, do we need to provide performance better than Stratum
3?
Can we declare stratum compatible performance with the limitation
during the clock rearrangement?
ANSWER: There is no cut-and-dried answer, but here are some considerations and guidelines. First, you must establish the environment into which the Gateway is being introduced. If the equipment has an external timing option and is deployed in a Central Office that has a Timing Signal Generator (TSG) or Sync Supply Unit (SSU) (called Building Integrated Timing Supply, or BITS, in North America and Stand Alone Sync Equipment, or SASE, in other parts of the world) from which the Gateway is receiving a timing reference, then an internal Stratum 3 clock is adequate. However, I do recommend the use of 3E or better. If the holdover capability of the TSG is 3E, then an internal clock of better than 3E may not be required.
There may be deployment scenarios where the "Conceptual BITS" approach is being followed. In this situation one piece of equipment, usually the most expensive, or largest, or newest, is considered the "master" and all equipment in the location connected to this device extracts timing from the interconnection (E1/DS1 or STM-x/OC-n). In this situation the external timing may come from a PRS (Primary Reference Source) (or a derived reference from a SONET/SDH terminal) and the equipment has to perform the holdover function. In this situation I recommend Stratum 2/2E. This improved holdover would make the equipment all the more valuable to the Service Provider since maintenance/repair does not have to be "immediate" in the case of a sync reference outage.
Depending on the sync architecture and design of the equipment, clock re-arrangements can introduce phase jumps. If the equipment has a "phase-following clock" (typical of a Stratum 3) then phase jumps are not "played-out"; a "filtering clock" (typical of 3E and better) is superior in suppressing such phase transients. My recommendation is that if the clocking is not of the filtering variety, then any outgoing SSM should be "Do Not Use for Sync".
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