问题：

[多选] Which two statements are true about broadband cable （HFC） systems？（）

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.