Avaya Configuring ATM Services Manual de usuario descargar pdf (Pagina 41)

Understanding ATM, ATM Router Redundancy, and OAM

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Selecting a Data Encapsulation Method

Generally speaking, the designers of these data encapsulation methods envisioned

that NULL encapsulation would dominate in environments where the dynamic

creation of large numbers of ATM VCs is fast and economical. These conditions

usually exist in private ATM networks.

LLC/SNAP encapsulation is an alternative for environments in which it is not

practical to have a separate VC for each carried protocol (for example, if the ATM

network supports only PVCs, or if billing depends heavily on the number of

simultaneous virtual circuits).

The choice of multiplexing methods that two ATM stations use to exchange

connectionless network traffic depends on the type of virtual circuit involved:

• For PVCs, you select the multiplexing method when you manually configure

the connection.

• 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.

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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Avaya Configuring ATM Services Manual de usuario Pagina 41