This attribute indicates the current value of parameter N401 for
V.42 which is negotiated during Exchange Information (XID). This
is the maximum number of octets that can be carried in the
information field of an information frame that the IWF V.42
transmits to the PSTN modem. The IWF V.42 puts this or even
smaller size of data into the outgoing information frame. The value
used for XID negotiations can be provisioned via TdmaCs/* V42.