Which of the following statements about IPSec and IKE is correct is ____ .( Choose one or more )

When the user uses the phone, after picking up the phone handle and inputting the called number, which of the following signaling is the ring back tone from the receiver?

Which of the following descriptions about L2TP tunnel ( Tunnel ) and L2TP session ( Session ) are correct is ____ .( Choose one or more )

OLT issue register cancellation message 5 ONU issuing register_ACK message
3.The following statements about VLAN planning in the EPCNI network are correct: _____ ( choose one or more )

A PBX of a company in Beijing was originally connected to the operator's telephone network through an analog line, using a total of 18 analog lines.In order to reduce maintenance complexity, it is the most economical choice to apply for a digital line from the operator .