Inactive TCP session - error 636 (firewall)

You need  to confgure TCP keepalive on master and media servers with a keep alive time of 15 minutes . The network admin won't change the parameter in the firewall becuase its again best pratice.

On a red hat host add the following to /etc/sysctl.conf:

net.ipv4.tcp_keepalive_time=900

Apply the setting with sysctl -p

The OS will then keep alive sessions by sending "ping" packages every 15 minutes therby preventing the firewall closing the sessions becuase of idle time. The keep alive is not just for Netbackup but for all application on the host.

Please see this tech note for configuring AIX hosts 

DOCUMENTATION: COMM_FAILURE as a consequence of reusing a transport that has been inactive across a firewall

http://www.veritas.com/docs/000005752

Hint: be aware of "unit per messure".