Fatal NI connect error 12170.

Dear All,
i have found below mentioned  error from Alert log, so want to know
why such errors occured? what they indicate ?
what are the steps we can do to avoid such kind of errors?

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 Fatal NI connect error 12170.

  VERSION INFORMATION:
    TNS for 32-bit Windows: Version 11.1.0.6.0 - Production
    Oracle Bequeath NT Protocol Adapter for 32-bit Windows: Version 11.1.0.6.0 - Production
    Windows NT TCP/IP NT Protocol Adapter for 32-bit Windows: Version 11.1.0.6.0 - Production
  Time: 10-JAN-2011 19:55:30
  Tracing not turned on.
  Tns error struct:
    ns main err code: 12535
    
TNS-12535: TNS:operation timed out
    ns secondary err code: 12560
    nt main err code: 505
    
TNS-00505: Operation timed out