The encoding value indicates the serial encoding mechanism
that is the nominal design target of the particular
transceiver. The value shall be contained in the serial data.
For XFPs this object must return notSupported, and user should
refer to these MIB objects
nbsXfpSerialEncodingRZ
nbsXfpSerialEncodingNRZ
nbsXfpSerialEncodingSonetScrm
nbsXfpSerialEncoding8B10B
nbsXfpSerialEncoding64B