Data Exchange Interface
Early ATM services and products were directed at LAN interconnection, which means the ATM end device was most likely a router. Routers operate at the OSI Network Layer and forward packets based on the Level 3 address. Normally routers are not involved in actually transmitting bits over the digital access facility; this is the function of a network termination unit (NTU). In the cell relay environment, the router is best suited to handle frames, and the NTU is best suited for cells.
The separation of these functions first appeared in the early days of switched multimegabit data service (SMDS). At the time, there was no standard interface between the router and NTU. Several router and NTU vendors made proprietary alliances to exchange SMDS frames between the two devices. Since the solution required users to have the “right” equipment, a standard router-NTU interface, called the Data Exchange Interface (DXI), was developed by the SMDS Interest Group (SIG). The ATM Forum created an ATM DXI for the same reason. The ATM DXI describes an open, standard interface between DTE (e.g., a router) and DCE (e.g., an NTU). DXI is based on High-level Data Link Control (HDLC), so it is both simple and inexpensive—always powerful technical and economic incentives.
The DXI Data Link Layer describes a framing format to transport AAL PDUs. The DXI frame comprises a Flag (e.g., 01111110), Header field (containing an address corresponding to the ATM virtual connection identifier), DTE Service Data Unit field (containing an ATM CS-PDU or higher-layer information), FCS (bit error detection), and another Flag.
The DXI Data Link Layer can operate in one of the following modes.
- Mode 1 supports up to 1023 virtual connections (two-octet Header) and a 16-bit FCS. Mode 1a supports only AAL5 cells; Mode 1b supports AAL3/4 on at least one virtual connection and AAL5 on the other connections. Mode 1b might be used, for example, to define one virtual connection for connectionless AAL3/4 traffic (such as SMDS), while using the other virtual connections for connection-oriented services. The visual shows the protocol architecture for Mode 1 operation. Only Mode 1a operation (AAL5) has been used widely.
- Mode 2 supports up to 16,777,215 (224-1) virtual connections (four-octet Header) and a 32-bit FCS. The DTE encapsulates the higher-layer data into an AAL3/4 frame, which is sent to the NTU in a DXI frame. If the Header refers to an AAL3/4 virtual connection, the DCE creates AAL3/4 cells; if the Header refers to an AAL5 virtual connection, the NTU converts the AAL3/4 frame to an AAL5 frame and then creates AAL5 cells.
The DXI Physical Layer can use ITU-T Recommendation V.35, EIA-530, EIA-449, or the EIA-612/613 High-Speed Serial Interface (HSSI).
The DXI also specifies the ATM DXI Local Management Interface (LMI). This protocol operates in conjunction with the DXI and supports the exchange of ATM UNI, DXI, and/or AAL-specific information between the DTE and DCE. The ATM DXI LMI is based on the Simple Network Management Protocol (SNMP).