The type of physical media in use. One of the
following values:
other undefined
unknown true state not known
pmdMissing PMD device not attached
utp4 4-pair unshielded twisted pair
stp2 2-pair shielded twisted pair
fibre 802.12 fibre optic cabling
This object may be 'unknown' if the implementation is
not capable of identifying the PMD media type, or
whether or not the PMD is even present.