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Chapter 4 WAN Setup
P-660R-F1 Series User’s Guide
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4.1.1.3 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.
4.1.1.4 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 the RFC for more detailed
information.
4.1.2 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.
4.1.2.1 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.
4.1.2.2 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.
4.1.3 Encapsulation and Multiplexing Scenarios
For Internet access you should use the encapsulation and multiplexing methods used by your ISP.
Consult your telephone company for information on encapsulation and multiplexing methods for
LAN-to-LAN applications, for example between a branch office and corporate headquarters. There
must be prior agreement on encapsulation and multiplexing methods because they cannot be
automatically determined. What method(s) you use also depends on how many VCs you have and
how many different network protocols you need. The extra overhead that ENET ENCAP
encapsulation entails makes it a poor choice in a LAN-to-LAN application. Here are some examples
of more suitable combinations in such an application.
4.1.3.1 Scenario 1: One VC, Multiple Protocols
PPPoA
(RFC-2364) encapsulation with
VC-based
multiplexing is the best combination because no
extra protocol identifying headers are needed. The
PPP
protocol already contains this information.
Summary of Contents for P-660R-F1 series
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