Furthermore, the PPP interface may be operating over a lower layer hardware interface (such as an RS-232 port). It is important to capture the relationship between the PPP interface and the lower- layer interface over which it operates. This MIB presumes that the lower-layer interface has an ifEntry associated with it. The lower- layer ifEntry is identified via the pppLinkStatusPhysicalIndex object, which contains the value of ifIndex for the lower-layer ifEntry.