Frame User-to-Network Interface

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FUNI Frame Structure

The ATM DXI provides a protocol interface between a non-cell-based router and a special purpose, on premise ATM DSU. The DXI permits the use of existing, non-ATM equipment, but it requires a significant expenditure in the form of the ATM DSU.

To obviate the need for an expensive ATM-compatible DSU, the ATM Forum has standardized the Frame User-to-Network Interface (FUNI). The FUNI is conceptually similar to SMDS’s DXI Subscriber Network Interface (SNI); both allow use of inexpensive, simple DSUs and provide much better throughput, particularly at lower speeds because they eliminate much of the overhead associated with cell-based access. So, FUNI access to an ATM network requires no cell-based hardware at the customer premises, significantly reducing access cost and providing more usable bandwidth on the access link. Any two devices attached to an ATM network can communicate with each other, even if one uses a cell-based access method (e.g., a UNI) while the other uses framed access (e.g., a FUNI).

Because FUNI uses frames and not cells across the access link and operates at low speeds (DS-1/DS-3), the FUNI specification states that only non-real-time service categories are to be supported (e.g., unspecified bit rate (UBR) and variable bit rate (VBR-NRT)).

Version 2.0 of the FUNI specification is backwards compatible with version 1.0 for data transfer as no changes were made in the user plane. Version 2.0 provides control and management enhancements by clarifying and defining FUNI-based support for signaling, ILMI, and operations, administration, and maintenance (OAM) flows.

ATM DXI and the FUNI protocols are not identical, even though their similarities often cause the two terms to be used as if they were interchangable. While FUNI is based on the DXI protocol, FUNI and ATM DXI are used in different scenarios and are not interoperable.