The value of ifIndex (in the ifTable; see the interface
section of MIB-2/RFC 1213) which corresponds to this channel.
When more than one row of the ifTable is relevant, this is
the index of the row representing the topmost layer in the
interface hierarchy. A value of zero indicates that no
interface is associated with this channel.
Additional information:
IIO-based channels provide their own values for this
object.