
Understanding ATM DXI Concepts
117375-A Rev. A 2-7
ATM DXI Modes of Operation
The ATM Forum’s ATM DXI specification defines three modes of operation, each
with a different set of operations and data transfer between the data terminal
equipment (DTE; that is, the router) and the data communications equipment
(DCE; that is, the DSU/CSU). The combination of the router link module and the
DSU/CSU provides a User-to-Network Interface (UNI) to the ATM network.
The modes of operation are as follows:
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 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.
Mode 2 Proprietary
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.
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.
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