Nortel Networks 553-3001-211 manual NTAK79 2.0 Mb PRI card Page 749

NTAK79 2.0 Mb PRI card Page 749 of 894

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

Holdover and free-run

In the temporary absence of a synchronization reference signal, or when sudden changes occur on the incoming reference due to error bursts, the clock controller provides a stable holdover. The free-run mode is initiated when the clock controller has no record of the quality of the incoming reference clock.

If the software command “free run” is given, the clock controller enters the free-run mode and remains there until a new command is received. Note that the free-run mode of operation is automatically initiated after the clock controller is enabled.

Reference clock selection through software

The NTAK79 has the necessary hardware for routing its reference to the appropriate line on the backplane.

The software is responsible for the distribution of the secondary references and ensures that no contention is present on the REFCLK1 backplane line. The software designates the NTAK79 as the primary reference source to the clock controller. The secondary reference is obtained from another NTAK79 card, which is designated by a technician. No other clocks originating from other NTAK79 circuit cards 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 NTAK79 references.

Circuit Card Description and Installation