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I am having trouble understanding the need for WAN frames, such as PPP when using the SONET Protocol or T-Carrier System. Since SONET uses STS-1 frames and T-Carrier uses T1 frames, why is there a need to further encapsulate.

One user, Zac67, mentioned on this thread PPP, Ethernet and PPPoE differences that PPP is primarily need to authentication. Does that mean that PPP is optional when authentication is not needed? If so, why do DSL providers require PPPoE whereas Cable providers do not?

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  • STS-1 and T1 frames would be on the CSU side, and do not get transmitted to a a device, which would be on the DSU side. In other word, the STS-1 and T1 frames are CSU-to-CSU, but something like PPP would be router-to-router.
    – Ron Maupin ♦
    Commented Jan 20, 2021 at 21:11
  • Thanks @RonMaupin. Any idea why PPPoE is required for DSL and not cable?
    – Train
    Commented Feb 14, 2021 at 1:14
  • Cable uses DOCSIS for both the physical and data-link layers. DSL originally used PPPoA, then PPPoE Juniper has a whitepaper: "Until recently, PPP was the only transport mechanism allowed by the DSL Forum. The DSL Forum now also allows using IP over Ethernet (IPoE), which is based on DHCP. However, PPP remains the more mature and robust method for providing many broadband services."
    – Ron Maupin ♦
    Commented Feb 14, 2021 at 3:58

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