Mahesh -

You need to look in your TSM server Activity Log to see why the
session was disconnected.  You may be constrained on drive resources
such that the very long running, large file backup is pre-empted by a
higher priority process.  Check networking to see if, possibly,
routing might go over a faster path, or if parameters like
TCPwindowsize might be tuned, to reduce the exposure.  Also consider
boosting your IdleTimeOut to 60.

  Richard Sims