Nortel Networks 553-3001-211 manual Reference clock selection through software

Page 720 of 894 NTAK10 2.0 Mb DTI card

secondary, but switches over to the primary whenever the primary recovers. If the primary recovers first, then the clock controller tracks to the primary.

If the software command “track to secondary” is given, the clock controller tracks to the secondary reference and continuously monitors the quality of both primary and secondary references. If the secondary becomes out of specification, the clock controller automatically tracks to primary provided that it is within specifications. On failure (both out of specification), the clock controller enters the HOLDOVER mode and continuously monitors both references. An automatic switchover is initiated to the reference that recovers first. If the primary recovers first, then the clock controller tracks to the primary, but switches over to the secondary whenever the secondary recovers. If the secondary recovers first, then the clock controller tracks to the secondary.

A time-out mechanism prevents chatter due to repeated automatic switching between primary and secondary reference sources.

Reference clock selection through software

The 2MB DTI card has the necessary hardware for routing its reference to the appropriate line on the backplane.

Software is responsible for the distribution of the secondary references and ensures that no contention is present on the REFCLK1 backplane line. Software designates the 2MB DTI card as a primary reference source to the clock controller. The secondary reference is obtained from another 2 Mbps DTI card, which is designated by a craft person. No other clocks originating from other 2MB DTI packs are used.

The clock controller provides an external timing interface and is capable of accepting two signals as timing references. In this case, an external reference refers to an auxiliary timing source which is bridged from a traffic carrying signal. This is not intended to be a dedicated non-traffic bearing timing signal. The clock controller uses either the two external/auxiliary references or the 2MB DTI references.

553-3001-211 Standard 2.00 September 2004