ADTRAN T200 H2TU-R Installation And Maintenance Manual page 20

Loopback Process Description
In general, the loopback process for the HDSL2
system elements is modeled on the corresponding
DS1 system process. Specifically, the H2TU-C
loopback is similar to an Intelligent Office Repeater
loopback and the H2TU-R loopbacks are similar to an
inline T1 Repeater loopback.
Each HDSL2 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/time out disable.
State transitions result from in-band and ESF Data
Link sequences as well as time out operations. The
sequences and time out values are as follows:
1. Arming Sequence (In-band and ESF)
2. Activation Sequence
3. Deactivation Sequence
4. Disarming Sequence (In-band and ESF)
5. Loop up Time out
6. Arming Time out
In-band control code sequences are transmitted over
the DS1 link by either the unframed or overwrite
method. The HDSL2 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
-03
1E or better is present.
States and State Transitions
A summary of time out and control sequences is given
in Table A-1.
NOTE
In all control code sequences presented, the
inband codes are shown left-most bit transmitted
first, and the ESF data link codes with right-
most bit transmitted first.
A-2
The Disarmed State is the normal mode of operation.
Each HDSL2 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 of
the HDSL2 elements is the following 5-bit pattern:
Arm Sequence ............................ 11000
Note that this sequence is the standard NIU loop up
code. If the NIU loopback feature for the HDSL2
circuit is enabled (see H2TU-C Switch Options), the
arming sequence will activate the NIU loopback in the
H2TU-R. If the NIU loopback feature is disabled and
an external Smartjack NIU is present, the HDSL2
arming process will not interfere with NIU detection
of the loop up code.
All other in-band sequences are ignored in the
disarmed state.
The ESF Data Link sequence used to simultaneously
arm the loopback capability of all of the HDSL2
elements is the following 16-bit pattern ESF data link
sequence:
ESF Arm Sequence .................... 0001 0010 1111 1111
HDSL2 element arming and NIU loop up is performed
as described for the in-band arming sequence.
All other ESF patterns are ignored in the disarmed
state.
In the Armed State, the HDSL2 system element
continues to be transparent to the data flow. However,
the in-band data flow and ESF data link is monitored
for disarming and activation codes. An arming time
out value causes the automatic return of the HDSL2
element to the disarmed state.
Transition from Armed to Loop Up State: An
in-band control code sequence is used to command a
specific HDSL2 element to move from the armed state
into the loop up state. Each HDSL2 element has a
unique 16-bit activation control code sequence as
shown in the following example.
H2TU-C Activation Sequence .. 1101 0011 1101 0011
H2TU-R Activation Sequence .. 1100 0111 0100 0010
Section 61222024L6-5, Issue 1
for four repetitions
61222024L6-5A