A-1
Section 61246026L6-5, Issue 2
61246026L6-5B
Appendix A
HDSL LOOPBACKS
This Appendix describes the use and operation of
loopback control code sequences used in ADTRAN’s
HDSL system. Loopback control codes are governed
by the HTU-C (and HRE(s) if deployed). Two types
of HTU-Cs exist which enable two different sets of
loopback codes -- Standard or Enhanced loopbacks.
The Standard loopbacks are those that have been
contained in ADTRAN’s HDSL product family
beginning with 2
nd
Generation products. The
Enhanced loopbacks are contained in selected
ADTRAN HTU-C units. The following table denotes
whether the HTU-C (part number) contains Standard
or Enhanced loopback capabilities.
Standard Loopback
Part Number ......... Description
1242002LX .......... 220/E220 HTU-C
1242016L1 ........... 3192 HTU-C
1242023L1 ........... DDM+ HTU-C
1244001L1 ........... E220/220 Low Voltage T1 HTU-C
1244002L1 - L3 ... Litespan AHDSL
1244002L4 - L6 ... Litespan AHT1U
1245001L1 ........... E220/220 Low Voltage 5
th
GEN
HTU-C
1245001L2 & L4 .. E220/220 HTU-C M
1245003L1 & L2 .. DDM+ 5
th
GEN HTU-C
1245004L1 & L2 .. 3192 5
th
GEN HTU-C
Enhanced Loopback
Part Number ......... Description
1181101L1 ........... Total Access HTU-C
1245001L6 - L8 ... E220/220 HTU-C M R
1245002L6 ........... Litespan HTU-C
1245003L6 - L8 ... DDM+ HTU-C M R
1245004L6 - L8 ... 3192 HTU-C M R
1246001L4 - L8 ... E220/220 HTU-C M
1246003L4 - L8 ... DDM+ HTU-C M
1246004L4 - L8 ... 3192 HTU-C M
The loopback capabilities of both the HRE and the
HTU-R loopback capabilities are controlled from the
central office unit (HTU-C).
NOTE: If the HTU-C on a circuit contains
Standard loopbacks, then refer to
subsection 1 of this Appendix to determine
its loopback capabilities. If the HTU-C on
a circuit contains Enhanced loopbacks,
then refer to subsection 2 of this Appendix
to determine its loopback capabilities.
1. Standard Loopbacks
This subsection describes operation of the HDSL
system in detection of in-band and ESF facility data
link loopback codes. The operation of the loopback
commands in the ADTRAN HDSL system is
compliant with the recommendation to ANSI recorded
in T1E1.4/92. The HDSL network loopback points
described below are illustrated in Figures A-1 and A-2.
The HTU-C loopback is a regenerative loopback of
the DSX-1 signal toward the network.
The HTU-R loopback is a regenerative loopback of
the DS1 signal toward the network. This loopback is
in addition to a separate Smartjack loopback.
Separate activation sequences are provided for the
HTU-R and the Smartjack loopback initiation. The
HDSL loopbacks are implemented such that the
downstream HDSL elements (toward the customer)
remain synchronized.
Upon deactivation of a loopback, the HDSL system
will synchronize automatically. It should be noted
that the synchronization process of the HDSL system
upon deactivation of the HRE loopback, could take up
to 15 seconds to ensure all system elements are
synchronized.
Figure A-1. HDSL Loopback Points
HTU-C
HTU-R
DSX-1
Interface
HDSL
Loops
HDSL
Loops
DS1
Interface
HDSL
Repeater
NIU
