Chapter 6 Broadband
VMG1312-B Series User’s Guide
101
Encapsulation
Be su r e t o use t h e encap sulat ion m et hod r equ ir ed by you r I SP. The Device can wor k in br idge m ode
or rout ing m ode. When t he Device is in routing m ode, it supports t he following m ethods.
IP over Ethernet
I P over Et hernet ( I PoE) is an alternat ive t o PPPoE. I P packet s are being delivered across an
Et hernet net work, wit hout using PPP encapsulation. They are routed bet ween t he Ethernet interface
and t he WAN interface and t hen form at t ed so that they can be understood in a bridged
environm ent . For inst ance, it encapsulat es routed Et hernet fram es into bridged Ethernet cells.
PPP over ATM (PPPoA)
PPPoA st ands for Point to Point Prot ocol over ATM Adapt at ion Layer 5 (AAL5) . A PPPoA connect ion
funct ions like a dial- up I nternet connection. The Device encapsulat es the PPP session based on
RFC1483 and sends it through an ATM PVC (Perm anent Virt ual Circuit) to the I nternet Service
Provider ’s ( I SP) DSLAM ( digit al access m ultiplexer) . Please refer to RFC 2364 for m ore inform ation
on PPPoA. Refer to RFC 1661 for m ore inform at ion on PPP.
PPP over Ethernet (PPPoE)
Point-t o- Point Prot ocol over Et hernet (PPPoE) provides access cont r ol and billing functionalit y in a
m anner sim ilar t o dial- up services using PPP. PPPoE is an I ETF st andard (RFC 2516) specifying how
a personal com puter (PC) interact s with a broadband m odem ( DSL, cable, wireless, etc.)
connection.
For the service provider, PPPoE offers an access and aut hent ication m ethod that works with existing
access cont rol syst em s ( for exam ple RADI US).
One of t he benefit s of PPPoE is t he abilit y to let you access one of m ultiple net work services, a
funct ion known as dynam ic service selection. This enables the service pr ovider t o easily create and
offer new I P services for individuals.
Operationally, PPPoE saves significant effort for bot h you and the I SP or carrier, as it requires no
specific configurat ion of t he br oadband m odem at the custom er sit e.
By im plem ent ing PPPoE directly on t he Device ( rat her t han individual com puters), t he com puters on
the LAN do not need PPPoE soft ware inst alled, since the Device does that part of the t ask.
Further m ore, wit h NAT, all of the LANs’ com puters will have access.
RFC 1483
RFC 1483 describes t w o m ethods for Multipr otocol Encapsulation over ATM Adapt at ion Layer 5
( AAL5). The first m ethod allows m ultiplexing of m ult iple prot ocols over a single ATM virtual circuit
( LLC- based m ultiplexing) and t he second m ethod assum es that each protocol is carried over a
separat e ATM virt ual circuit (VC- based m ult iplexing) . Please refer to RFC 1483 for m ore detailed
inform at ion.
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