What Is Asynchronous Transfer Mode?

Date First Published: 12th May 2022

Topic: Computer Networking

Subtopic: Data Transmission Technologies

Article Type: Computer Terms & Definitions

Difficulty: Advanced

Difficulty Level: 9/10

CONTENTS

Bit Rate Choices

Learn more about what asynchronous transfer mode is in this article.

Asynchronous transfer mode, often abbreviated to ATM, and not to be confused with an automated teller machine, is a connection-switching technology that makes use of fixed-length cells of 53 bytes long in data packets. 5 bytes are allocated for the address header and 48 bytes are allocated for the data, which can be processed through a hardware device, called an ATM switch, and transmit data at speeds of up to 600 Mb/s.

ATM was defined by ANSI and ITU-T in 1980 and was developed to fulfil the requirements of the Broadband Integrated Services Digital Network. It operates at the data link layer (Layer 2) of the OSI model over a twisted pair or fibre cable. ATM does not use routing. Instead, ATM switches establish point-to-point connections between devices and data directly flows from source to destination.

Due to the small cells, ATM is capable of transmitting large amounts of data over a network whilst ensuring that no single transmission uses up all of the bandwidth and it enables ISPs to allocate a limited amount of bandwidth to each customer. It is different from Ethernet, which makes use of variable-length cells in data packets. ATM is used by long-distance carriers and campus-wide backbone networks to carry integrated data, video, and voice.

Note:

The term 'asynchronous' means that the data is not synchronised when it is sent or received. This means that synchronisation occurs at irregular intervals rather than in a steady stream. It can start and stop at any time and contains a start bit and stop bit, which is helpful for the receiving end to know when it has received all of the transmitted data. This means that ATM devices handle transmission speeds based on hardware and information flow reliably.

Bit Rate Choices

ATM services have four different bit rate choices:

Available bit rate - A minimum capacity is guaranteed, but it is possible for data to be moved to higher capacities when the network traffic is low.
Constant bit rate - A fixed bit rate is specified in order for data to be sent in a steady stream. This is similar to a leased line.
Unspecified bit rate - No data transfer rate is guaranteed and it is used for applications, such as file transferring, where delays can be tolerated.
Variable bit rate - A specified data transfer rate is provided. However, data is not sent evenly. It is recommended for real-time applications, such as VoIP and streaming.