Planet ADE-4110, Руководство пользователя

Страница: 64 / 67

58
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 re-
layed 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. Simi-
larly, 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.