Start encrypting all downstream/upstream and downstream
directions traffic destined to and originated from a
specific LLID (ONU in current implementation). ONU
registration encryption key is used as an initial value.
If the encryption key was updated after ONU
registration, the last updated key is used.
off(1): - Stop encrypting.
downlink(2): - Start encrypting all downstream
directions traffic.
updownlink(3): - Start encrypting all downstream/upstream
directions traffic.