A.Cable modems only operate at Layer 1 of the OSI model.
B.Cable modems operate at Layers 1 and 2 of the OSI model.
C.Cable modems operate at Layers 1， 2， and 3 of the OSI model.
D.A function of the cable modem termination system （CMTS） is to convert the modulated signal from the cable modem into a digital signal.
E.A function of the cable modem termination system is to convert the digital data stream from the end user host into a modulated RF signal for transmission onto the cable system.

A.Dead Peer Detection （DPD）
B.CDP
C.isakmp keepalives
D.GRE keepalive mechanism
E.The hello mechanism of the routing protocol across the IPsec tunnel