Nortel Networks 311

Architecture 899

Switchover may occur in the case of referencedegradation or reference

failure. When performance of the reference degrades to a point where

the system clock is no longer allowed to followthe timing signal, then the

reference is said to be out of speciﬁcation. If the reference being used is

out of speciﬁcation and the other reference is still within speciﬁcation, an

automatic switchover is initiated without software intervention. If both

references are out of speciﬁcation, the clock controller provides holdover.

See "Reference switchover"(page 914).

Autorecovery and chatter

If the software command "track to primary" is given, the clock controller

tracks to the primary reference and continuously monitors the quality of

both primary and secondary references. If the primary becomes out of

speciﬁcation, the clock controller automatically tracks to secondary 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 secondary recovers ﬁrst, then the clock controller tracks

to the secondary, but switchesover to the primar y wheneverthe 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 switchesover 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.