HDSL4 for General Distribution Installation and Maintenance Practice
HDSL NEW ENHANCED FEATURE OVERVIEW
61223HDSL4L2-5C
B-11
Fast Retrain Feature
Fast Retrain is an ADTRAN proprietary feature whose intent is to minimize downtime when an
intermittent non power-related impairment (bad splice, noise burst, etc.) affects the HDSL
loop and cannot be bridged.
HDSL2 and HDSL4 transceivers normally train in approximately 25 to 30 seconds. For an
initial circuit turn-up, this is not a big issue. However, once service has been established on
the circuit, any large down-time will interrupt communications on the circuit. A loss of
synchronization on the HDSL loop can cause excessive down times due not only to the 30-
second HDSL retrain time, but also further delays due to the higher level protocols in the
network going through re-synchronization. On the older generation HDSL2 and HDSL4 units,
a 1-second loss of HDSL frame synchronization would cause the units to retrain. This retrain
would take approximately 25 seconds during which AIS would be sent to the terminating
equipment. The reception of AIS by the terminating equipment then might trigger higher level
protocol re-synchronizations.
In an effort to minimize this down time, the Fast Retrain feature has been implemented. If an
impairment (bad splice, for example) causes the HDSL unit to lose frame synchronization for
500 msec or longer, a fast retrain will be attempted instead of a retrain. This abbreviated train
can achieve data mode in 5 to 7 seconds. A successful fast retrain should be evident by
watching the Span Status screen and by reduced unavailable seconds (UAS) in the PM data
for each LOS alarm recorded.
NOTE
Fast-Retrain capable units must be installed on both ends of the
HDSL4 circuit for this feature to function properly. Also, if there is
a failure of a fast retrain attempt, for any reason, then the tradi-
tional (25-30 second) retrain will be initiated.
