User`s guide
Performing Loopback Modes and Pattern Tests
6-4
7800-A2-GB26-80
May 1999
Using Loopback Modes
The following sections discuss the various type of loopback modes available and
describes how to use each. Although only DCE Manager supported devices are
used in the following examples, the loopback modes and pattern tests are
defined in MIBs.
Line Loopback
The Line Loopback (LLB) mode loops the information received on the Network T1
interface back to the network as close to the network interface as possible. When
used with a pattern test, this loopback mode is useful for determining whether the
problem is with the sending device or the T1 facility. The figure below
illustrates
how you can use the LLB in conjunction with a pattern test (see
Using Pattern
Tests
on page 6-11) to isolate a problem
.
Note that while an interface is in loopback mode, the other interfaces send all 1’s.
In the following figure, Device A is sending a known QRSS pattern out the
Network T1 interface. (Device A sends a QRSS pattern when you select Send
QRSS Pattern.)
Device B is in LLB mode returning the pattern. (Device B is put in LLB mode
locally or by sending it an LLB Up Code from Device A.)
Device A monitors the returned information for errors. (Device A is put in monitor
mode by selecting Monitor QRSS Pattern.) If Device A detects an error, it reports
the error in the DCE Manager test dialog as In Sync With Bit Errors (device is
marginal) or Not in Sync (device is down).
If Device A detects an error, the likelihood is that the problem is in the T1 facility
and not within any internal circuitry of Device A because it was able to send and
monitor the pattern data. If there are no errors reported, you can run the Payload
Loopback test described in
Payload Loopback
, on page 6-5
,
to determine
whether the problem is caused by the CSU circuitry of the remote device
(Device B in our example).
3100 Series Device
Sending/Monitoring
a QRSS Test Pattern
3100 Series
Device in
Loopback
DEVICE A
DEVICE B
T1
NETWORK
496-15095
Loopback Mode (LLB) Combined with a Pattern Test