This attribute is used to configure the type facilities data link (FDL).
The ANSI403 configures the port to use the FDL exchange described in
the ANSI T1 403 specification. ATT54016 is used to configure the port
for the for the FDL exchange listed in the AT&T 54016 document. NONE
is used when no FDL is desired. The FDL can only be used when the line
type is ESF.