Planet ADE-3100, Руководство пользователя

Страница: 45 / 49

40
Appendix B. Encapsulation Mode
Bridged mode
（
RFC-1483
）
RFC-1483 provides the simplest method of connecting end station over an ATM network. User data in the
form of Ethernet frames is encapsulated into AAL5-PDU for transport over ATM. RFC-1483 provides no
authentication and configuration such as would be provided by PPP. RFC-1483 implementation supports VC
multiplexing and LLC/SNAP encapsulation in both routed and bridged configurations.
Classical IP over ATM-IPOA
（
RFC1577
）
User data in the form of IP packets is encapsulated into AAL-5 PDUs for transport over ATM. The fact that the
user data is routed at an IP layer instead of bridged MAC layer allows the source and destination to be on
different subnets. A notable drawback of IPoA is the lack of authentication and configuration such as would be
provided by PPP.
PPP over ATM-PPPoA
（
RFC-2364
）
The use of PPPoA is similar to IPoA. However, a PPP session is established to the remote access server
(RAS). The PPP packets are encapsulated according to RFC-2364 for transmission over an ATM link. On the
receive side, the de-encapsulation is performed. The PPP session is terminated and the IP packets can be
delivered to the end user over Ethernet or other medium.
PPP over Ethernet-PPPoE
（
RFC-2516
）
The PPP over Ethernet
（
PPPoE
）
encapsulation is used to transport PPP traffic from a PC to a DSL device
over Ethernet and then over the DSL link using RFC-1483 encapsulation. There may be multiple PPP
sessions, each terminated in a PC or in the CPE device and in a PPP aggregator on the CO side.
l
The PPPoE Client terminates PPPoE session within the CPE device, this configuration enables PPPoE
session without a need for additional software.
l
The PPP traffic for a Relay Agent is not terminated in the DSL device, rather it is relayed over the DSL
link to a PPP aggregator in the CO PPPoE relay agent determines on which session locally originated
PPPoE traffic belongs. The relay agent forwards that traffic, without any unnecessary processing to the
correct destination only. Similarly, received data is immediately relayed only to the appropriate client PC.
Not only does this approach enhance performance by eliminating additional process, it also provides a
critical security feature, so it prevents for example corporate bound data from being exposed to the
Internet.