How to access the value of 'FrameTime' in the SRDU QPSK transmitter demo model...?

Hello,

    I'm testing the 'QPSK transmitter for USRP hardware' demo...I need to use the message source block in bit's generator in another model...but when I do this, simulink is not able to identify the variable 'FrameTime' under the 'Sample time' parameter of the Bernoulli Binary Generator block...The error is "error evaluating parameter Ts"..I know that FrameTime=FrameSize*ScanTime...Can someone explain me what ScanTime is..? Is it the simulation stop time or something else..? how to access the value of FrameTime in the workspace..? why is it that  the sdruqpsktx model works without any value being assigned to FrameTime, but when the the block is copied to another model errors turn up..? some answers will be much appreciated.

Thank you,

Naveen