Specifications

Troubleshooting
7-9
Service Loops
Service loops are part of the DDS network's diagnostic capabilities. They are used to
isolate trouble conditions in the local serving loop and the customer's data
communications equipment. The service loops provided by the Excalibur Multirate DAP
are described in the following sections. Refer to AT&T Publication 62310 for more
complete information.
CSU Loopback Test
The CSU Loopback Test is a mandatory network loopback test enabled by a reversal of
the local cable simplex polarity. The DDS line receive data is looped to DDS line
transmit data. At the DTE interfaces, DCD is off and receive data is clamped to a Mark.
(See Figure 7-11.)
TRANSMITTER
RECEIVER
LOCAL DAP
MARKING
GEN
LINES
TX
RX
Figure 7-11. CSU Loopback Test
Non-Latching Loopback Test
The Non-Latching Loopback Test is an optional network loopback test. This test is
enabled by the reception of a minimum of four specific loopback codes. It is maintained
as long as every other byte contains a loopback code, until five successive bytes are
received without the loopback indication. This test is operational in DDS and DDSNI
modes. It is operational in DDS-SC mode only when the line rate is 56 Kbps. In DDS
mode, DDS line receive data is looped to DDS line transmit data. In DDS-SC mode, line
receive data is also looped to line transmit data, except that certain code bytes are mapped
to produce specific return bytes as specified by AT&T Publication 62310. At the DTE
interfaces in both modes of operation, DCD is off and receive data is clamped to a Mark.
(See Figure 7-12.) CSU, DSU, and OCU loopbacks are supported.