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ATM Address - an explanation

Defined in UNI Specification as 3 formats, each having 20 bytes in length, including country, area and end-system identifiers.ATM connection services are implemented using permanent virtual connections (PVCs) and switched virtual connections (SVCs). In the case of a PVC, the VPI/VCI values at each switching point in the connection must be manually configured. While this can be a tedious process, it only needs to be done once, because once the connection is set up, it remains up permanently. PVCs are a good choice for connections that are always in use or are in frequent, high demand. However, they require labor-intensive configuration, they are not very scalable, and they are not a good solution for infrequent or short-lived connections.

SVCs are the solution for the requirements of on-demand connections. They are set up as needed and torn down when no longer needed. To achieve this dynamic behavior, SVCs use signaling: End systems request connectivity to other end systems on an as needed basis and, provided that certain criteria are met, the connection is set up at the time of request. These connections are then dynamically torn down if the connections are not being used, freeing network bandwidth, and can be brought up again when needed.

The ITU-T long ago settled on telephone number-like addresses, called E.164 addresses or E.164 numbers, for use in public ATM (B-ISDN) networks. Since telephone numbers are a public (and expensive) resource, the ATM Forum set about developing a private network addressing scheme. The ATM Forum considered two models for private ATM addresses: a peer model, which treats the ATM layer as a peer of existing network layers, and a subnetwork, or overlay, model, which decouples the ATM layer from any existing protocol and defines for itself an entirely new addressing structure.,br>
The ATM Forum settled on the overlay model and defined an ATM address format based on the semantics of an OSI Network Service Access Point (NSAP) address. This 20-byte private ATM address is called an ATM End System Address (AESA), or ATM NSAP address (though it is technically not a real NSAP address). It is specifically designed for use with private ATM networks, while public networks typically continue to use E.164 addresses.