by Dinesh Thakur Category: Network Technologies

Definition: Asynchronous Transfer Mode (ATM) also called cell relay (transferring data in cells of a fixed size) that is operates at the data link layer (Layer 2) of OSI Model over fiber or twisted-pair cable, a high-speed switched network technology based on ITU-T Broadband Integrated Services Digital Network (B-ISDN) standard, developed by the telecommunications industry to implement the next generation network. ATM was designed for use in WANs such as the public telephone system and corporate data networks, though it has also been applied to create super-fast LANs.

ATM can carry all kinds of traffic: voice, video and data simultaneously at speeds up to 155 megabits per second. It Convert voice, video data to packets and passing large packet data through the same medium. ATM is differing from TCP/IP because it use fixed channel routing protocol routes between two end points. A real-time low-latency application such as VoIP and video takes precedence on an ATM network.

What is Asynchronous Transfer Mode (ATM)?

Asynchronous Transfer Mode

ATM is a dedicated connection-oriented switching technology, in which switches create a virtual connection or virtual circuit between the sender and receiver of a call that permanent or switched for the duration of the call. It is a small-packet switched system or similar to circuit-switched network, which breaks down messages into very small, fixed length packets called cells generally organizes digital data into 53 bytes in length (48 bytes of data plus a 5-byte header).   ATM frame structure

An ATM header can have User-Network Interface (UNI) and Network-Node Interface (NNI) two formats.
User-Network Interface (UNI) used for communication between end systems.
Network-Node Interface (NNI) used for communication between switches.

Two type of connections are supported by ATM (Asynchronous Transfer Mode)

Point-to-point connections: It connects either unidirectional or bi-directional two end-systems.
Point-to-multipoint connections: It connects one unidirectional ATM to number of destination ATM.
It is different in packet sizes from Ethernet data or frames. ATM is a core protocol for SONET that is the backbone of ISDN. The advantage conferred by such small cells is that they can be switched entirely in hardware, using custom chips, which makes ATM switches very fast (and potentially very cheap).
The asynchronous part of the name refers to the fact that although ATM transmits a continuous stream of cells over a physical medium using digital signal technology, some cells may be left empty if no data is ready for them so that precise timings are not relevant. Every cell is encoding data with asynchronous time-division multiplexing (TDM) and it queued before being multiplexed over the transmission path.
Every cell are encodes data and processed within their time slot allocated to it. When cell time slot allocated is finished, the next cell starts same procedure. That’s why it's called asynchronous time-division multiplexing (TDM);
This is ATM's greatest strength, as it enables flexible management of the quality of service (QoS) so; an operator can offer different guaranteed service levels (at different prices) to different customers even over the same line. This ability will enable companies to rent virtual private networks based on ATM that behave like private leased lines but in reality share lines with other users.
Available ATM service: Generally four data bit rates are available for ATM services: constant bit rate (CBR), variable bit rate (VBR), available bit rate (ABR) and unspecified bit rate (UBR).



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