For each blank, choose the best answer from the four choices and write down on the answer sheet.
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.