Testing
5-2 7510-A2-GB20-10
November 1997
Loopbacks can be started from a variety of points in the network, as shown in
Table 5-1.
Table 5-1. Device Messages
Loopback Type Initiated By Notes
Bilateral Loopback User
Remote unit sending
V.54 sequence
When enabled, running a DSU loopback
also automatically starts a local
loopback. Refer to Table A-3, Data Port
Options, to enable.
CSU Loopback User (Network tests)
DDS Network, by
loop current reversal
When initiated by the network, CSU
Loopback cannot be disabled by the
user.
DSU Loopback
(Digital) User Refer to Table A-3, Data Port Options.
Local Loopback User
DTE via CT141
Control via CT141 can be disabled.
Refer to Table A-3, Data Port Options.
Network-initiated
56 kbps DSU
Loopback (Non-
latching loopback)
DDS Network Cannot be disabled by user.
Network-initiated
64 kbps CC DSU
Loopback
(Latching loopback)
DDS Network Includes optional data scrambling and
uses 25-second timer to detect the
network sequence. Can be disabled by
user.
Remote Digital
Loopback Remote unit sending
V.54 sequence Same as a DSU Loopback but initiated
by a remote unit via V.54 sequence.
Can be disabled locally. Refer to
Table A-2, Network Interface Options.
V.54 Sequences to
remote unit User
DTE via CT140
Control via CT140 can be disabled.
Refer to Table A-3, Data Port Options.