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COMSPHERE 3550 Series Data Service Units
4-10 February 1995 3550-A2-GB20-10
NOTE
To issue a Local Loopback
to a
tributary DSU
, the DSU must have a
DBM installed or a
Conflict w/ Environ
message will appear. The DBM is
required to call the tributary and abort
the Local Loopback.
If the NMS is to abort the loopback,
the tributary’s DBM must be in Idle or
Standby mode. If in Dial Backup
mode and the NMS issues the
Abort
command, the command will not be
recognized. The loopback will have to
be aborted from the DSU’s control
panel.
DTE Loopback
DTE Loopback (DTE) loops back the data path at the
DTE interface on a per-port basis without affecting the
operation of the remaining ports. This loopback is used to
verify that the DTE connection and the cable are good.
Digital Loopback
Digital Loopback (DL) allows manual testing of the
remote end of the circuit. Data coming in is immediately
transmitted back. For example, a Digital Loopback may
be required in order to complete an external bit error rate
test (BERT) from the remote DSU. The local DSU
receives test data, loops it back to the transmitter before
the DTE interface, and returns it to the network.
Digital Loopback can also be initiated by receiving a
V.54 pattern to go into Digital Loopback. Enable
configuration options V.54 Lpbk (V.54 Loopback) and
RespondRDL (Respond to Remote Digital Loopback).
Remote Digital Loopback
Remote Digital Loopback (RL), sometimes referred to
as RDL, typically supports testing using an external
device, like a protocol analyzer that is connected to the
local DSU’s DTE interface. A test message from the
external device is looped back from the receiver to the
transmitter in the remote DSU and returned to the local
DSU.
In Remote Digital Loopback, the local DSU (control or
tributary) puts the remote DSU into Digital Loopback. A
control DSU can originate Remote Digital Loopback in a
point-to-point or multipoint network, whereas a tributary
DSU can originate Remote Digital Loopback in a
point-to-point network only.
When a DSU (control or tributary) originates Remote
Digital Loopback, both the originating DSU and the
targeted DSU enter Test mode. No other test can be run at
the originating DSU or the targeted DSU until the Remote
Digital Loopback is aborted.
The DSU does not generate test results.
Bilateral Loopback
Bilateral Loopback (Bilat Lpbk) is a combination of
DTE and Digital Loopbacks operating simultaneously in
the same DSU. Both Bilateral and Digital Loopbacks are
selected from the General (Gen) option set.
NOTE
If
Bilat Lpbk
is enabled, requesting a
Remote Digital Loopback or Digital
Test will automatically initiate a DTE
Loopback, as well.
Digital Test
The Digital Test (DT) checks the functionality of a pair
of DSUs and the data circuit between them. This test can
also be run on a DBM over a backup connection to a
remote DBM for point-to-point testing.
The remote DSU is placed in Digital Loopback, then
the local DSU transmits the 511-bit test pattern over the
network to the remote DSU for an operator-specified
length of time. In a multipoint network (where only a
control DSU can originate a Digital Test), tributary DSUs
not involved in the test are placed in Test mode for the
duration of the test.
During the test, the TXD, RXD, and RTS LED
indicators show the states of the leads at the DTE
interface. At the conclusion of the test, the local DSU
releases the remote DSU from Digital Loopback.
To review the results of the test, select Displ (Display)
and press the
key to scroll through the results shown
in Table 4-3.
When the test is over, the top line displays Final
instead of Active.