Atm Wan Interfaces

ATM WAN architecture supports the following five interface types, which are discussed in greater detail in the following sections:

• ATM user-network interface (UNI)— This interface defines the interaction among the following:

- An ATM endpoint device and a private network switch

- An ATM endpoint device and a public network switch

- A private network switch and a public network switch

• ATM Network-Network Interface (NNI)— This interface supports signaling between ATM switches across NNI links.

• ATM data-exchange interface (DXI)— This interface supports communication with a DXI-enabled router via the router's serial interface.

• ATM private network node interface or private network-to-network interface (PNNI)— This interface enables differing switch vendors to interoperate across an ATM WAN. Vendors can opt to implement proprietary ATM signaling between their own switches. Cisco Systems, Inc. implements PNNI 1.0.

The ATM Forum released the full PNNI 1.0 specification in May 1996. PNNI 1.0 enables scalable, full function, dynamic, multivendor ATM networks by providing both PNNI routing and PNNI signaling. PNNI is based on UNI 3.0 signaling and static routes.

• ATM broadband ISDN (B-ISDN) intercarrier interface (B-ICI)— This interface describes public network-to-network interface points. For a global telecommunications network to exist, public ATM networks must be interconnected.


UNI signaling in ATM defines the protocol by which the ATM devices in the network dynamically set up SVCs.

The UNI specifications include physical layer, integrated local management interface (ILMI), traffic management, and signaling.


NNI signaling is part of the PNNI specification, which includes both signaling and routing.


The ATM Forum developed a standard known as the ATM DXI to make ATM features and functionality immediately available to the networking community. DXI can be used to enable UNI support between Cisco routers and ATM networks, as illustrated in Figure 18-3.

Figure 18-3. ATM DXI

The ATM data service unit (ADSU) receives data from the router in ATM DXI format over a high-speed serial interface (HSSI). The DSU converts the data into ATM cells and transfers them to the ATM network over a DS-3/E3 line.

ATM DXI is available in several modes for DXI header format:

• Mode 1a— Supports AAL5 only, a 9,232 octet maximum, a 2-byte DXI header, and a 2-byte frame check sequence (FCS). Mode 1a supports up to 1,023 virtual circuits.

• Mode 1b— Supports AAL3/4 and AAL5, a 9,224 octet maximum, a 2-byte DXI header, and a 2-byte FCS. Mode 1b AAL5 support is the same as Mode 1a. AAL3/4 is supported on one virtual circuit. Mode 1b supports up to 1,023 virtual circuits.

• Mode 2— Supports AAL3/4 and AAL5, a 65,535-octet maximum, a 4-byte DXI header, and a 4-byte FCS. DXI Mode 2 double-encapsulates AAL5 frames and supports up to 16,777,215 virtual circuits.


Cisco uses Mode 1a for the DXI header format.

On the router, data from upper-layer protocols is encapsulated into ATM DXI frame format, as illustrated in Figure 18-4.

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