This attribute indicates the current value of N401, that is negotiated
during Exchange Identification (XID). This is the maximum number
of octets that can be carried in the information field of an
information frame received from the PSTN modem. An incoming
information frame carrying data octets more than this size shall be
rejected by the IWF V.42. The value used for XID negotiations can
be provisioned via TdmaCs/* V42.