This enables or disables the Physical Layer Convergence Protocol (PLCP) on this ATM port; when disabled, the port instead uses direct ATM cell mapping with HEC-based cell delineation, per the description. PLCP versus direct mapping is a framing-level choice that must match what the carrier circuit or connected equipment expects on this specific port, since the two schemes locate ATM cell boundaries within the DS1/DS3 payload completely differently. A DS3 circuit provisioned by the carrier for direct ATM cell mapping would need pportAtmPlcp set to disable on this port; leaving PLCP enabled against such a circuit would prevent the port from correctly delineating cells at all.