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Configuring ATM Services

2-6

ATM DXI Modes of Operation

The ATM Forum’s ATM DXI speciﬁcation deﬁnes three modes of operation. Each

mode speciﬁes a different set of operations and data transfer between the DTE, the

router, the DCE, and the DSU/CSU. The combination of the router link module

and the DSU/CSU provides a User-Network Interface to the ATM network.

The modes of operation include the following:

Mode 1A

Using this mode, the DTE encapsulates the DTE service data unit (SDU) into

a DXI frame and transmits it to the DCE. This mode operates with AAL 5

only. No AAL 5 functions are performed by the DTE; the DCE performs the

AAL 5 and ATM layer functions.

Mode 1B

This mode serves the same purpose as Mode 1A, but it offers additional

support for AAL 3/4 for at least one VPI/VCI.

Mode 2

This mode allows the DTE to perform the AAL 3/4 CPCS encapsulation for

the DTE SDU, then puts this PDU into an ATM DXI frame. The DSU/CSU

then modiﬁes the frame to support either AAL 3/4 or AAL 5.

The DSU/CSU performs AAL functions based on whether the virtual channel

connection is set for AAL 3/4 or AAL 5.

In addition to the three modes described by the ATM Forum, our implementation

of the ATM DXI speciﬁcation deﬁnes a fourth mode of operation: Mode 2

Proprietary.

Mode 2 Proprietary

This mode is a proprietary version of Mode 2 used prior to the development of

ATM Forum’s standard Mode 2. This version of Mode 2 has some DXI header

differences from the standard Mode 2 and it handles only AAL 3/4.

Note: Use this mode if your conﬁguration uses pre-ATM Forum DXI DSU/

CSU devices (for example, a Digital Link DL3200).

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Avaya Configuring ATM Services Manual do Utilizador Página 42