Chapter 6 Broadband
VMG8825-B Series User’s Guide
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6.5 Technical Reference
The following section contains additional technical information about the VMG features described in
this chapter.
Encapsulation
Be sure to use the encapsulation method required by your ISP. The VMG can work in bridge mode or
routing mode. When the VMG is in routing mode, it supports the following methods.
IP over Ethernet
IP over Ethernet (IPoE) is an alternative to PPPoE. IP packets are being delivered across an Ethernet
network, without using PPP encapsulation. They are routed between the Ethernet interface and the
WAN interface and then formatted so that they can be understood in a bridged environment. For
instance, it encapsulates routed Ethernet frames into bridged Ethernet cells.
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 VMG 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 VMG (rather than individual computers), the computers on the
LAN do not need PPPoE software installed, since the VMG 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.