单选题
______in the strength of the carrier signal is varied to represent binary 1 or 0． ______is a system that can map a name to an address and conversely an address to an name． ______is a three-way handshaking authentication protocol that it's packets are encapsulated in the point-to-point protocol ______is an approach that enables an ATM Switch to behave like a LAN switch． ______is an guided media which is made of glass or plastic and transmits signals in the form of light．