Chapter 8 Broadband
VMG/EMG/AM/DM/GM Series User’s Guide
207
PPP over ATM (PPPoA)
PPPoA stands for Point to Point Protocol over ATM Adaptation Layer 5 (AAL5). A PPPoA connection
functions like a dial-up Internet connection. The Zyxel Device encapsulates the PPP session based on
RFC1483 and sends it through an ATM PVC (Permanent Virtual Circuit) to the Internet Service Provider’s
(ISP) DSLAM (digital access multiplexer). Please refer to RFC 2364 for more information on PPPoA. Refer
to RFC 1661 for more information on PPP.
PPP over Ethernet (PPPoE)
Point-to-Point Protocol over Ethernet (PPPoE) provides access control and billing functionality in a
manner similar to dial-up services using PPP. PPPoE is an IETF standard (RFC 2516) specifying how a
personal computer (PC) interacts with a broadband modem (DSL, cable, wireless, etc.) connection.
For the service provider, PPPoE offers an access and authentication method that works with existing
access control systems (for example RADIUS).
One of the benefits of PPPoE is the ability to let you access one of multiple network services, a function
known as dynamic service selection. This enables the service provider to easily create and offer new IP
services for individuals.
Operationally, PPPoE saves significant effort for both you and the ISP or carrier, as it requires no specific
configuration of the broadband modem at the customer site.
By implementing PPPoE directly on the Zyxel Device (rather than individual computers), the computers
on the LAN do not need PPPoE software installed, since the Zyxel Device does that part of the task.
Furthermore, with NAT, all of the LANs’ computers will have access.
RFC 1483
RFC 1483 describes two methods for Multiprotocol Encapsulation over ATM Adaptation Layer 5 (AAL5).
The first method allows multiplexing of multiple protocols over a single ATM virtual circuit (LLC-based
multiplexing) and the second method assumes that each protocol is carried over a separate ATM virtual
circuit (VC-based multiplexing). Please refer to RFC 1483 for more detailed information.
Multiplexing
There are two conventions to identify what protocols the virtual circuit (VC) is carrying. Be sure to use the
multiplexing method required by your ISP.
VC-based Multiplexing
In this case, by prior mutual agreement, each protocol is assigned to a specific virtual circuit; for
example, VC1 carries IP, etc. VC-based multiplexing may be dominant in environments where dynamic
creation of large numbers of ATM VCs is fast and economical.
LLC-based Multiplexing
In this case one VC carries multiple protocols with protocol identifying information being contained in
each packet header. Despite the extra bandwidth and processing overhead, this method may be
advantageous if it is not practical to have a separate VC for each carried protocol, for example, if
charging heavily depends on the number of simultaneous VCs.
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