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Configuring the T1 Physical Link
Monitoring T1 Physical Link
T1 devices have a built-in facility designed for troubleshooting the physical link, called
loopback. The T1 link can also be monitored using show commands to display the
current configuration of the link, any alarms on the link, link statistics, and link errors.
Loopback
The T1 device can be set up to loopback, that is, return a transmitted signal back to the
sender so it can be compared with the original. There are several different types of
loopback available to test different parts of the device and the line, specified in the T1
standards.
Local T1 Port
Remote T1 Port
Data out
Data in
Data in
Data out
Framer
Framer
Data with framing
XM_010
Figure 2-1: Normal operation of T1 link
During normal operation of a T1 link, as the local data stream enters the framer, the
appropriate framing bits are inserted into the data, and the framed signal is transmitted
to the remote end. At the remote end, the process is reversed as the framing bits are
discarded and the data stream is passed to the remote system.
Loopback can be enabled on the near-end or the far-end of the link. The near-end
loopback modes are controlled directly by the hardware on the near-end. Far-end
loopback modes require the cooperation of the far-end hardware. A message is sent to
the far-end to cause it to enter a far-end loopback mode. When loopback is enabled on a
T1 port, the green port LED will blink.
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WM-4T1i Module Installation and User Guide
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