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Definition: ATM

(1) See also ATM machine and Adobe Type Manager.

(2) (ATMosphere) A measure of pressure. See atmosphere.

(3) "At the moment." See digispeak.

(4) (Asynchronous Transfer Mode) A local area and wide area network technology stemming from the telephone world that supports quality of service (QoS) for real-time voice and video. In the early 1990s, ATM was touted as the future multimedia transport starting at the desktop. A decade later, ATM was being replaced by Internet protocols (see IP and MPLS). Adapters for PCs were expensive, and standards for connecting legacy networks were confusing. When Gigabit Ethernet debuted at the end of the 1990s with a familiar technology and 10-fold speed increase, ATM's demise as a LAN was assured. However, ATM was deployed as a backbone technology in major telcos and ISPs.

Fixed-Size Cells and Logical Circuits
ATM transmits 53-byte cells that can be processed very quickly, and the small cell ensures that voice and video can be inserted into the stream for real-time transmission. ATM uses switches that establish a logical circuit from end to end. However, unlike the dedicated circuits of legacy telephone switches, unused bandwidth in ATM's circuits can be appropriated as needed. For example, idle bandwidth in a videoconference can be used to transfer data.

Five Service Levels
Constant Bit Rate (CBR) guarantees bandwidth. Real-time Variable Bit Rate (rt-VBR) supports multimedia, and non-real-time Variable Bit Rate (nrt-VBR) is used for bursty traffic. Available Bit Rate (ABR) adjusts bandwidth according to LAN congestion levels, and Unspecified Bit Rate (UBR) provides the best effort for non-critical data. See MPOA.

ATM in the Large Enterprise
ATM was used as a network backbone ("switch fabric") in some large enterprises. The edge device is an Ethernet switch, and the LANE (LAN Emulation) driver converts Ethernet frames into ATM cells. See LANE.

ATM in the Internet
ATM has been used by carriers and ISPs as the backbone between points of presence (POPs) and at interconnecting points (see NAP and MAE). Highly scalable, ATM supports transmission speeds up to 10 Gbps but can run as slow as 9.6 Kbps between ships at sea. When a new ATM switch is added, it is automatically updated using ATM's PNNI routing protocol.

Where ATM Fits In
ATM functions at layer 2 of the OSI model and converts its cells into SONET frames (see OC) or T-carrier frames (see DS).