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TESTS
DS1 LOOPBACK
FUNCTIONS
DS1 Line Loopback
Function
AIS
EQUIPMENT
4-4
The DS1 Line Loopback and DS1 Test Loopback functions allow the sig-
nal received from the network to be looped through the CSU's circuitry
and transmitted back toward the network. These functions in themselves
are not tests, but are used in conjunction with other DS1 diagnostic tests.
The DS1 Network Interface Loopback function allows the signal received
from the network to be looped back toward the network at the Telco-pro-
vided DS1 Interface Connector. It is not normally used alone, but used in
conjunction with external test equipment or with DS1 Self-Test.
The DS1 Line Loopback (LLB) function, illustrated in Figure 4-2, allows
the signal received from the network to be looped through a minimum of
the CSU's circuitry and transmitted back toward the network. Only the
CSU's critical circuitry (the Line Build-Out, equalization, and timing re-
generation circuits) is utilized, with bipolar violations uncorrected. This
tests everything on the network side up to the CSU's critical
While the LLB function is active, the CSU transmits an Alarm Indica-
tion Signal (AIS) toward the equipment.
The LLB function can be activated and deactivated locally via the Super-
visory terminal, or remotely via loopback codes as defined
PUB 54016 (for Extended Superframe Format) or AT&T PUB 62411 (for
D4 Superframe Format).
To allow remote activation and deactivation, the LLB function must be
enabled via the Supervisory terminal or option switches S5-6 and S6-8.
The LLB function cannot be used alone, but must be used in conjunction
with one of the following tests: DS1 Line Loop, DS1 Remote Test, or DS1
Remote Test with Self-Test.
CSU
FIGURE 4-2. DS1 LINE LOOPBACK FUNCTION
T1
NETWORK
circuitry.
in
EQUIPMENT
CSU
AT&T
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