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ATM Abbrev. for asynchronous transfer mode.
1. A form of switching system designed to minimize the magnitude of switching times. In most switching systems an entire packet must be received and checked for accuracy before the software within the switch will initiate the onward transmission of the packet to the next switch or to the final destination. By using a simple fixed structure for the data, ATM can reduce the time needed to recognize and process packets as they move through a switch.

In common with other systems, data to be transmitted in ATM is subdivided into a number of small units that are transmitted in sequence and reassembled at the receiving end. ATM uses a very small fixed-length cell, with each cell having an identical format. The format of the cell guarantees that the addressing information for each cell is always found in a fixed position and format. This allows the actual switching operation for each cell to be implemented in table-driven firmware, which in turn allows a very short switching time for each cell so that very large numbers of cells can be switched each second.

An ATM cell is made up of a header, holding 5 bytes, and a body, holding 48 bytes. The header holds all the addressing, routing, and control information and is transmitted as the first part of the cell, followed immediately by the body, which contains the user's data. The accuracy of the header is checked by a single parity bit forming part of the header. As soon as the header is received and verified as having the correct parity, the software in the switch will immediately initiate the onward transmission of the complete cell, attaching it to a queue of outgoing cells on the appropriate connection. The overall effect is that the onward transmission of the cell may well overlap the receipt of the body of the cell, being started shortly after the receipt of 5 bytes (i.e. the end of the header) rather than after 53 bytes (i.e. the receipt of the entire cell). The switching algorithms used are also very simple, relying on the creation of a virtual circuit before the actual transmission of user data, with the identity of the virtual circuit embedded in the header and serving to define the switching action by what is in effect a table lookup. This simple algorithm can be implemented largely as firmware. ATM can operate at rates up to 10 Gbps and is a key component of broadband ISDN.

2. Abbrev. for automated teller machine. A computer-controlled device that dispenses cash, and may provide other services, to customers who identify themselves with a pin.

3. Abbrev. for Adobe Type Manager. A program by Adobe Inc. used to install and organize PostScript fonts. In earlier versions of Windows (up to 2000) ATM was required to view these fonts on the screen.

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ATM • abbr. ∎  Telecommunications asynchronous transfer mode. ∎  automated (or automatic) teller machine.

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atm Physics • abbr. atmosphere(s), as a unit of pressure.

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atm symbol for atmosphere (unit of pressure)