Network protocol intended for applications ranging from global telco networks to private local area computer networks.
Asynchronous Transfer Mode (ATM) is a connection-oriented network transport technology that allows for high-speed transmissions of a variety of traffic types (voice, data and video) over one and the same network. These capabilities, along with ATM’s Quality-of-Service to avoid congestion by any individual traffic type, have made it the network backbone technology of choice for many service providers.

AT&T offers ATM as a service to support corporate networks at speeds from 1.544 Mbps (T1) to 155 Mbps (OC3), in point-to-point and point-to-multipoint topologies.

Features and Components
The common components of AT&T’s ATM service include the following:
• Local loop, which connects the customer location to the closest point of entry to AT&T’s network.
• ATM port, which connects the local loop to our ATM network.
• Permanent Virtual Circuit (PVC), a logical connection between two sites to be defined at a bandwidth rate between 1.544 Mbps and 155 Mbps.

Quality of Service (QoS), selected in increments of 64 Kbps or 1 Mbps. QoS are determined based on the customer’s applications and latency requirements. Options available include the following:
• Constant Bit Rate (CBR), which specifies a fixed bit rate so that data is sent in a steady stream (analogous to a leased line).
• Variable Bit Rate (VBR), which provides a specified throughput capacity but data is not sent evenly (a popular choice for voice and video-conferencing data).
• Available Bit Rate (ABR), which provides a guaranteed minimum capacity but allows data to be bursted at higher capacities when the network is free.
• Unspecified Bit Rate (UBR), which does not guarantee any throughput levels (for delay-tolerant applications, such as file transfer)

Benefits
ATM provides a flexible and scalable solution to the increasing need for quality of service in networks where multiple information types (such as data, voice, and real-time video and audio) are supported. The technology enables:
• high-speed communication
• Fast, hardware-based switching
• A single, universal, interoperable network transport
• A single network connection that can reliably mix voice, video, and data
• Flexible and efficient allocation of network bandwidth

How Does It Work?
AT&T provides guidance to help select the proper bandwidth and QoS parameters for the particular application you are looking to support. Local access facilities are then provisioned and logical connections made between ATM-enabled CPE on either end of the links.  Locations supported by ATM may inter-work with locations supported by Ethernet Services within one and the same corporate data network, by mapping PVC’s between them.