Why can cable not use PPPoE or PPPoA connection like DSL does?
The protocol between the cable modem and the Cable Modem Termination System (CMTS) is defined in the DOCSIS. You can search on this on the Internet.
This is a collections of standards used by cable providers and vendors.
Data are transmitted coded into RF signals using the RF channels 50 MHz to 860 MHz downstream and 5 MHz to 42 MHz upstream.
This protocol is quite different from either PPP over Ethernet or PPP over ATM.
PPPoE and PPPoA are used for last mile connections over the traditional public telephone lines.
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