Avaya Configuring ATM and MPLS Services Instrukcja Użytkownika Pobierz pdf (Strona 45)

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Understanding ATM, MPOA, ATM Router Redundancy, and OAM

117374-C Rev 00

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• For SVCs, the stations themselves negotiate the multiplexing method by

sending B-ISDN signaling messages. These messages include “low-layer

compatibility” information that allows negotiation of AAL5 and the carried

(encapsulated) protocol.

Selecting LLC/SNAP Encapsulation

When the same virtual circuit carries several protocols, select LLC/SNAP

encapsulation. LLC/SNAP encapsulation attaches an LLC/SNAP header before

the PDU. This header includes information that the receiving ATM station needs

to properly process the incoming PDU. For bridged PDUs, this header also

includes the type of the bridged media.

Selecting NULL Encapsulation (VC-Based Multiplexing)

In NULL encapsulation, the carried network protocol is identified implicitly by

the virtual circuit connecting the two ATM stations. Because each protocol must

travel over a separate virtual circuit, there is no need to include explicit

multiplexing information in the payload of the PDU. For this reason, the

bandwidth requirements and processing overhead remain minimal.

You can either manually configure the carried protocol or let the signaling

procedures negotiate it dynamically during call establishment.

NULL encapsulated cells do not receive a header in a routed environment. In a

bridged environment, the content of the PDU itself includes the necessary

information for bridging the multiplexed protocols.

Note:

Routed and bridged PDUs are always encapsulated within the payload

field of the AAL5 CPCS PDU, regardless of the selected multiplexing method.

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