Section 311-580-20B
030-101527 Rev. B
6
0407IBRB
enabled, is activated. Figure 6 shows a simplified view of the cir-
cuit during loopback .
4.1
Facility Loopback
During Facility Loopback, the LB LED is solid yellow and the
receive T1 bit stream from the facility enters the NIU on the
FAC IN port. The signal is then routed through the loopback
path to the opposite path and is passed back to the network via
the FAC OUT port. During Network-side Loopback, AIS is sent
toward the CPE.
4.2
CPE Loopback
CPE Loopback is available via the NIU craft port STATUS
menu. By using the <Ctrl> and <C> keys simultaneously, the
NIU can activate the CPE loopback. During CPE Loopback, the
signal from the CPE is routed through the loopback path back
to the CPE. When CPE Loopback is activated, the LB LED is
solid yellow.
4.3
Dual Loopback
Dual Loopback is available to simultaneously loop back signals
to the facility and then back to the CPE. Dual Loopback can be
activated via inband codes, via the MLB front panel switch, and
via the NIU craft port STATUS menu. When Dual Loopback is
activated, the LB LED is blinking yellow.
4.4
Loopback Activation/Deactivation
Loopback can be activated/deactivated by one of four ways:
1.
Remotely, on-demand, via control codes (inband or ESF
Data Link) in the DS1 signal from a network test location,
2.
Locally, on demand, via the front-panel MAN LB push but-
ton switch,
3.
Locally, on demand, via the front-panel RS-232 craft termi-
nal interface port, or
4.
Automatically, when the NIU detects a loss of signal from
the CPE and CPE LOS response is optioned for LB (see
Part 5).
4.5
Remote Loopback Activation/Deactivation
Loopback activation/deactivation is done from a remote test
center, such as a Hi-Cap Center or Central Office, by sending in-
band or ESF Data Link loopback control codes toward the NIU
for the proper duration. These control code sequences are listed
in Table 1.
Function
Code
Type
DS1/T1 Activate
11000
Unframed, SF or ESF
(Loop Up)
0001 0010 1111 1111
ESF DATA LINK
1
DS1/T1
11100
Unframed, SF or ESF
Deactivate
(Loop Down)
0010 0100 1111 1111
or
0011 1000 1111 1111
ESF DATA LINK
1
Addressed LB
Activate
1101 0011 1101 0011
(D3D3)
SF or ESF
Note 1: Right-most bit sent first.
Table 1. Loopback Control Codes
4.5.1
Inband Codes
Inband codes are repetitive patterns at the 1.544Mb rate and
may be unframed, SF framed or ESF framed. The Inband code
pattern must be sent for a minimum of five seconds. The unit will
reject the pattern if it is less than five seconds in duration or if
the error rate is greater than 10
-3
.
4.5.2
ESF Codes
ESF codes are bit-oriented messages sent in the ESF Data Link
and must be sent for a minimum of four consecutive repetitions
(approximately 16ms). The unit will reject the pattern if it is less
than four consecutive repetitions in duration or if the error rate
is greater than 10
-3
.
4.6
Addressed Loopback Mode
4.6.1
When installed in SONET/Mux applications, the NIU
can be configured for addressed loopbacks. This allows the craft
to test locations at the service interface (SI) then subsequently
loop the NIU adjacent to the SONET or Mux Interface. The ad-
dressed loopback mode is an option that can be set remotely via
TL1 or 16-bit commands or locally via the craft interface port.
5
15
CT1
CR1
7
13
41
47
55
49
CT
CR
FAC
IN
FAC
OUT
CPE
IN
CPE
OUT
T
R
T1
R1
Figure 6.
Transmission Path During Loopback Operation
FAC
MON
TO/FROM
TELCO
FACILITY
CPE
MON
FLOS
LOS DETECTOR
(CPE-to-FAC PATH - Z-A)
LOS DETECTOR
(FAC-to-CPE PATH - A-Z)
AIS
GENERATOR
Note:
Switches shown are for illustrative purposes
only - options are provisioned electronically
THRU
REGEN
FAC-to-CPE
REGEN
TO/FROM
CUSTOMER
INTERFACE
(CPE)
CLOS
CONTROL CODE
DETECTORS
X
CPE (Dual)
Loopback
Only
CPE-to-FAC
REGEN
DIS
EN
Provisionable/
Self-Aligning
LBO
22.5 dB
15 dB
0 dB
7.5 dB
AB15 dB
AB22.5 dB
LBO
15
7.5
0
