Used to configure the type of CS_PDU encapsulation supported on
the ATM DXI PVC based on the ATM Forum's ATM DXI specification.
This configuration is on a per Direct VC basis. The supported
values are AAL3/4, AAL5 and Null (no CS_PDU encapsulation). Null
is a propriatary configurable value when no CS_PDU encapsulation
is desired.