Addressing endpoints in an ATM network is part of the UNI signaling. While ATM switches route based on connections (identified by the VPI/VCI), once the cell arrives at the destination network, there has to be an address to get it to the proper node.

Endpoint addresses are carried in the payload of an ATM cell as part of the signaling message. There are three different address formats presently defined for use at the private UNI and an additional format for the public UNI:

• Data Country Code (DCC) (private)
• International Code Designator (ICD) (private)
• E.164 ATM private format (private)
• E.164 ATM public format (public)

Figure 9.6 shows the four address formats used in ATM. The Authority and Format Identifier (AFI) defines the authority responsible for address registration and the format used. The authority can be an ATM equipment manufacturer, service provider, telephone company, or administrator of a private network. The DCC is a 2-byte field that identifies the country in which the address is registered.

The Domain Specific Identifier (ESI) is a 6-byte field that identifies an end system within an area. The selector field (SEL) is a 1-byte field used by the end system to select an endpoint within an end system. The ICD is a 2-byte field that identifies an international organization. Codes are maintained by the British Standards Institute. The E.164 field is an 8-byte field that uses the same addressing as defined for ISDN; it is used to identify ISDN numbers.

When a cell is transmitted, the subscriber equipment provides the ESI and SEL values which identify the end system and endpoint originating the cell. The network then fills in the rest of the address information when the cell is passed to the network over the UNI. The addreses are registered with the network for future connections.

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Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 372-386.