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 information frames transmitted
from the PSTN modem. This size governs the maximum number of
information frames that the peer of 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.