A-2
Section 61246026L6-5, Issue 2
61246026L6-5B
Loopback Process
In general, the loopback process for the HDSL system
elements is modeled on the corresponding DS1 system
process. Specifically, the HTU-C loopback is similar
to an intelligent office repeater loopback and the
HTU-R loopbacks are similar to an inline T1 repeater
loopback.
Each HDSL system element is independently
described by the state diagram shown in Figure A-2.
The four states are disarmed, loop-up, armed, and
loop-up/timeout-disable.
State transitions result from in-band, ESF data link
sequences, and timeout operations. The sequences
and timeouts are as follows:
• Arming (in-band and ESF)
• Activation
• Deactivation
• Disarming (in-band and ESF)
• Loop-up Timeout
• Arming Timeout
A summary of timeout and control sequences is given
in Table A-1.
In-band control code sequences are transmitted over
the DS1 link by either the unframed or overwrite
method. The HDSL elements respond to either
method.
The unframed method produces periodic control
sequences, and the normal DS1 framing bit is omitted.
The overwrite method produces periodic control
sequences. However, once per frame, the framing bit
overwrites one of the bits in the control sequence.
The unit can detect the loopback activation or
deactivation code sequence
only
if an error rate of
1E
-03
or better is present.
In all control code sequences presented, the
in-band codes are shown leftmost bit
transmitted first, and the ESF data link codes
with rightmost bit transmitted first.
Disarmed State
The disarmed state is the normal mode of operation.
Each HDSL element is transparent to the data flow.
However, the in-band data flow and the ESF data link
are monitored for the arming sequence.
The in-band control code sequence used to
simultaneously arm the loopback capability of all
HDSL elements is the standard 5-bit in-band sequence
used for NIU Smartjack loop-up. Each HDSL
element arms after receiving the following code for
five seconds:
Arm Sequence
11000 for 5 seconds
The arming process ensures unambiguous race-free
operation of HDSL element arming and Smartjack
loop-up. The HDSL unit can detect the sequence
without interfering with the detection by the
Smartjack. Presently, the Smartjack loop-up response
requires a duration of at least five seconds. The
objective of the HDSL detection scheme is to arm the
HDSL elements without interfering with the
Smartjack loop-up.
The requirement imposed on the arm sequence is that
the Smartjack should loop-up and all HDSL elements
make a transition from the disarmed state into the
armed state. All other control code sequences are
ignored in the disarmed state.
Figure A-2. HDSL Element State Diagram
ARMED
STATE
LOOP-UP / NO TIMEOUT
STATE
DISARMED
STATE
ACTIVATION
LOOP-UP
TIMEOUT
ARMING
TIMEOUT
DEACTIVATION
DISARM
LOOP-UP
TIMEOUT
DISABLE *
* The Loop-up Timeout Disable function is currently not supported.
LOOP-UP / TIMEOUT
STATE
