What is PPP, PPPoA and PPPoE?

By David W Christie

PPP (Point to Point Protocol)

PPP was designed to provide a managed data-link connection between two devices, generally over some form of Serial Data Link.

Prior to the development of PPP a lot of serial links used the HDLC (High Level Data-link Control) protocol. This was a no frills protocol with no other purpose other than to provide an encapsulation at the data-link layer.

In contrast PPP comes with a number of options including providing encryption, authentication and data compression. It is the authentication option that was identified as being an ideal method of authenticating a user or client device by a network that had a service to provide. Being able to identify and authenticate a user by means of username and password combination became the common method in use by Service Providers.

PPP really came in to its own with the advent of Dial-Up Internet services when the authentication feature was used to authenticate a dial-up user to the network. A network RADIUS (Remote Access Dial In User Service) server would be positioned in the network to perform the role of the authenticator. These days PPP is used on many connections including ADSL, Cable and also for authentication ISDN lines. PPP can be used with ATM as PPPoA or Ethernet as PPPoE. Next Generation Networks being designed and built to deliver voice, data and multimedia will, in the main continue the use of PPP.

PPPoA (Point-to-Point Protocol over ATM)

When dial-up services were eventually superseded by ADSL broadband services, PPPoA became the method by which an ADSL user identified themselves to the network. PPPoA quickly became the de-facto standard for authentication in an ADSL network.

Some broadband services are delivered over an Ethernet connection rather than over a telephone line. The user still needs to be authenticated, so PPP was adapted to be run over Ethernet in a similar manner to how it was used over an ATM connection. This became known as PPPoE (Point-to-Point Protocol over Ethernet) and it is this method that is proposed as the access authentication method for many Next Generation Network Services.

PPPoE (Point-to-Point Protocol over Ethernet)

PPPoE is often bundled with end user operating systems and software and supports a simple and reliable method of end user authentication. In order to make broadband services inexpensive to the end user, it is important that a simple method is used to identify and authenticate the end user devices. Using this method will even mean that existing authenticating RADIUS servers can be used for the new services.

The use of PPPoE has a number of distinct advantages over alternative access methods:

• Simple access control and authentication
• Accounting of users for billing purposes is easy
• Broadcast traffic is kept to a minimum, as PPP is not a broadcast protocol
• PPP connections scale very well to allow thousands or tens of thousands of users

So PPPoE will continue to be used for some time to come, even in the developing Next Generation Networks. PPPoA will largely disappear as the standard ADSL services are migrated over to fibre and Ethernet. Service providers evolving Next Generation Networks do not have to go back to the drawing board to come up with a new authentication and accounting system.

This article on PPPoA and PPPoE was written by David Christie, MD at Network Systems Training (UK) Ltd.

More Networking Protocols and Standards:
• IPv6 Anycast Addresses
• IGRP (Interior Gateway Routing Protocol)
• Shortest Path Bridging (SPB) Protocol
• WAN Network Protocols - DSL, SONET, HDLC, DWDM, DLSW+
• IPv6 Address Format
• IPv6 Packet Fragmentation
• Comparing IPv4 and IPv6 at a Glance
• RADIUS Protocol
• TCP/IP Features
• Protocol Suites

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