standby mode on serial interface

Rack1R5#sh ip int brie
Interface IP-Address OK? Method Status
 Protocol
FastEthernet0/0 132.1.5.5 YES manual up
 up
FastEthernet0/1 192.10.1.5 YES manual up
 up
Serial0/0/0 unassigned YES unset up
 up
Serial0/0/0.1 132.1.35.5 YES manual up
 up
Serial0/1/0 132.1.45.5 YES *manual standby mode *
   down

What is standby mode? How do I unset this or is it physical issue?

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Received on Sun Apr 15 2012 - 12:17:01 ART