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1.1.3 Hairpin Circuits
1.1.3 Hairpin Circuits
A hairpin circuit brings traffic in and out on an electrical port rather than sending the traffic onto the
optical card. A hairpin loops back only the specific VC3 or VC4 circuit and does not cause an entire
optical port to loop back, thus preventing a drop of all traffic on the optical port. The hairpin allows you
to test a specific VC circuit on nodes running live traffic.
E1-N-14 card.
Figure 1-9
Test Set
1.1.4 Cross-Connect Loopbacks
A cross-connect (XC) loopback tests an STM-N circuit path as it passes through the cross-connect card
and loops back to the port being tested without affecting other traffic on the optical port. Cross-connect
loopbacks are less invasive than terminal or facility loopbacks. Facility and terminal loopback testing
and circuit verification often involve taking down the whole line; however, a cross-connect loopback
allows you to create a loopback on any embedded channel at supported payloads of VC3 granularity and
higher. For example, you can loop back a single STM-1, STM-4, STM-16, etc. on an optical facility
(line) without interrupting the other synchronous transport signal (STS) circuits.
This test can be conducted locally or remotely through the CTC interface without on-site personnel. It
takes place only on an STM-N card and tests the traffic path on that VC (or higher) circuit through the
port and cross-connect card. The signal path is similar to a facility loopback.
The XC loopback breaks down the existing path and creates a new cross-connect—a hairpin—while the
source of the original path is set to inject a line-side
signal path and AIS injection are shown in
Cisco ONS 15454 SDH Troubleshooting Guide, R5.0
1-8
Hairpin Circuit Path on an E1-N-14 Card
E1N
XC
STM-N
Figure 1-9
shows the hairpin circuit path on an
"MS-AIS" condition, page
Figure
1-10.
Chapter 1
General Troubleshooting
2-190. The loopback
July 2005
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