What is ATM?

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ATM is the abbreviation of Asynchronous Transfer Mode. It is using time-division multiplexing (TDM) for data communications. It is capable of transferring voice, video, and data through private and public networks. ATM is working on a cell-based architecture in place of frame-based architecture. The size of the Asynchronous Transfer Mode cell is fixed to 53 bytes. The Asynchronous Transfer Mode header is 5-byte which is included in the 53-byte cell. The Asynchronous Transfer Mode cells are two different types: User-network interface (UNI) and the network-network interface (NNI). The small and fixed size of the Asynchronous Transfer Mode cells is perfect real-time traffic like voice and video.

The small and fixed-size cell of Asynchronous Transfer Mode helps easy switching of data between nodes without any further delay. Asynchronous Transfer Mode is the key to the internet bandwidth problem. It creates fixed routes between source and destination before data transfer starts. This makes the record of data usage including billing easier.  All subsequent cells follow the same path to the destination.


The Asynchronous Transfer Mode supports both constant and variable rate traffic therefore it can transport multiple types of traffic such as voice, video, data files etc. Asynchronous Transfer Mode provides committed circuits with assured bandwidths and differing requirements to be set up simultaneously. The parameters delay, jitter and error rate on the application is to decide about the Quality of Service (QoS). In the Asynchronous Transfer Mode, we can define two priorities for traffic; High priority and Low priority. Traffic with Low priority will be discarded if there are congestion problems.

Asynchronous Transfer Mode provides both PVCs and SVCs pipes, but usually, PVCs are mostly used with WANs. Asynchronous Transfer Mode also allows multiple PVCs and SVCs on a single leased-line connection to the network edge. Asynchronous Transfer Mode provides extremely scalable and to support link speeds of T1/E1 including Optical Carrere for faster speed.

Layers in ATM

The ATM uses a logical model to describe the functionality that it supports. ATM functionality corresponds to the physical layer and part of the data link layer of the OSI reference model. The ATM reference model is containing the following layers:

Physical Layer

Asynchronous Transfer Mode physical layer manages the medium-dependent transmission. The physical layer is further divided into sub-layers called the physical medium-dependent (PMD) sublayer and the transmission convergence (TC) sublayer. The ATM physical layer has four functions:

  • Conversion of cells into a bitstream.
  • Control the transmission and reception of bits in the physical medium
  • It is responsible to trace the Asynchronous Transfer Mode cell boundaries
  • Cells are packaged into the appropriate types of frames for the physical medium

ATM Adaption Layer (AAL)

The Asynchronous Transfer Mode Adaption Layer (AAL) isolates the higher-layer protocols from the details of the ATM processes. The layer also prepares the system for conversion of user data into ATM cells and segments user data into 48-byte cell payloads.

ATM Layer

ATM Layer transmits a user data, switch user data, control congestion in switching, cell header processing including sequential delivery.

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