Library /sys$common/syshlp/nclhelp.hlb set, hdlc, link, preferred_window_size  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: 2             Value: 1-127

    Window size to be offered during negotiation for both receive
    and transmit frames. The value of this characteristic must be
    compatible with that for the sequence modulus characteristic.