Meridian Nortel 1 Option 11C Technical Reference Manual page 491

NTAK79 2.0 Mb PRI card Page 491 of 544
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.
Option 11C and 11C Mini Technical Reference Guide