The state of the interface. When statistics are generated for EMS, `linkState` will be displayed as an integer value.
Possible values:
0 - null
1 - admnDisabled
2 - admnEnabledPortDown
3 - admnEnabledPortUp
4 - admnDisabledNoLicense
5 - admnEnabledPortDownInvalidSfpWrongSpeed
6 - admnEnabledPortDownInvalidSfpNonSonus
7 - unknown