Cisco Cisco ONS 15600 SDH, Troubleshooting Manual

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Cisco ONS 15600 SDH Troubleshooting Guide, R8.0
July 2010
Chapter 1 General Troubleshooting
1.1.2 Payload Loopbacks
1.1.2 Payload Loopbacks
The payload loopback is similar to a facility loopback but occurs on STM-64 cards. Another difference
is that a payload loopback terminates and regenerates section and line overhead, while a facility
loopback passes section and line overhead through, untouched. The STM-16 card executes a facility
loopback by looping the signal back just before the framer chip. The STM-64 card cannot do this because
of the differences in the design. To execute a loopback on an STM-64 card, the loopback signal passes
through the framer chip and then terminates and regenerates line and section overhead. Since STM-64
card line and section overhead is terminated and regenerated, this type of loopback is called a payload
loopback.
1.1.3 Terminal Loopbacks
The following sections give general information about ASAP card terminal loopback operations.
1.1.3.1 General Behavior
A terminal loopback tests a circuit path as it passes through the SSXC card and loops back from the card
with the loopback.
Figure 1-2 shows a terminal loopback on an ASAP card. The test-set traffic enters the
optical or Ethernet port and travels through the cross-connect card to the optical port. A terminal
loopback turns the signal around before it reaches the LIU and sends it back through the SSXC card to
the card. This test verifies that the SSXC card and terminal circuit paths are valid, but does not test the
LIU on the optical card.
Note CTC sometimes calls a terminal loopback a terminal (inward) loopback. This is done to clarify the
direction that the loopback signal travels, that is, inward toward the facility.
Figure 1-2 Terminal Loopback Path on an ASAP Card
1.1.3.2 Card Behavior
ONS 15600 SDH terminal port loopbacks can either terminate or bridge the signal. (Some
ONS 15600 SDH cards bridge the loopback signal, while others terminate it.)
If a port terminates a terminal loopback signal, the signal only loops back to the originating port and is
not transmitted downstream. If the port bridges a loopback signal, the signal loops back to the originating
port and is also transmitted downstream.
An STM-N terminal loopback example is shown in Figure 1-3 .
159637
STM-N
Test Set
XC DS-N/
E-N