Number of datagrams in the transmission window (LBT-RM only).

- n (Int32)
- Statistics set index

Number of datagrams in the transmission window.

Each source transport session maintains a transmission window buffer (the
size of which is set by transport_lbtrm_transmission_window_size, default
24MB). When the source transport receives a NAK, the corresponding message for
retransmission must be found in this transmission window.
This method returns a value from the nth set of statistics
encapsulated within this object.