The EC protocol window size in units of frames in the
receive direction.
This is the maximum number of information frames sent from
remote modem that may be outstanding without receiving any
acknowledgement from the local modem.
If the EC protocol is LAPM, the Rx and Tx windows are
separately negotiated and may have different values. If
MNP, both windows are negotiated together and have the
same value for this object and cctHistoryTxECWindowSize.