This attribute indicates the current value of k, that is negotiated
during Exchange Identification (XID). This is the size of flow
control sliding window used for the information frames that the
V.42 transmits to the PSTN modem. This size governs the
maximum number of information frames that the IWF V.42 error-
correcting entity can have outstanding (that is, unacknowledged).
The value used for XID negotiations can be provisioned via
TdmaCs/* V42.