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What Is Asynchronous Transfer Mode?

What Is ATM?

Preview: Learn more about Asynchronous Transfer Mode (ATM) and how it combined the advantages of circuit switching and packet switching.

Asynchronous Transfer Mode (ATM) is a high-speed networking technology that transports information using small, fixed-length packets known as cells. Developed during the late 1980s and early 1990s, ATM was designed to carry voice, video, and computer data over a single integrated network while providing predictable performance and guaranteed quality of service. Although it has largely been superseded by Ethernet and Internet Protocol (IP)-based technologies, ATM introduced many concepts that continue to influence modern communication networks.

During the 1980s, communication networks faced a growing challenge. Telephone networks had been designed primarily for voice traffic using circuit switching, while computer networks employed packet switching to transport data. As multimedia applications began to emerge, engineers sought a single networking technology capable of carrying voice, video, and data efficiently without requiring separate communication infrastructures. ATM was developed to meet this objective and became the foundation of the Broadband Integrated Services Digital Network (B-ISDN) concept.

Unlike conventional packet-switched networks, which permit packets of many different sizes, ATM uses a fixed-length 53-byte cell. Each cell consists of a 5-byte header containing routing and control information followed by a 48-byte payload carrying user data. The use of fixed-length cells greatly simplifies switching hardware because every cell is processed in exactly the same manner, allowing very high switching speeds with predictable delay.

The name Asynchronous Transfer Mode reflects the way in which the cells are transmitted. In Synchronous Transfer Mode (STM) systems, transmission capacity is allocated to users at regular, predetermined intervals whether or not they have data to send. ATM, by contrast, transmits cells only when information is available. Capacity is therefore allocated dynamically, allowing the network to use available bandwidth much more efficiently while still providing predictable quality of service.

ATM is often described as a connection-oriented packet-switching technology. Before user data can be transmitted, the network establishes a virtual circuit between the communicating devices. Every ATM cell belonging to that connection follows the same predetermined path through the network and carries a Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) rather than a complete destination address. Because intermediate switches need examine only these short identifiers, ATM switches can forward cells extremely rapidly.

One of ATM's greatest strengths was its support for Quality of Service (QoS). Different applications have different communication requirements. Voice and video require low delay and minimal delay variation, whereas computer file transfers are generally more tolerant of latency but require very low error rates. ATM was designed to support multiple classes of service simultaneously, allowing the network to reserve bandwidth, limit delay, and guarantee performance according to the needs of each application.

ATM found widespread use in telecommunications networks, broadband backbones, digital subscriber line (DSL) systems, enterprise networks, and early broadband Internet services. During the 1990s, many telephone companies employed ATM as the core transport technology for integrating voice, video, and data traffic. It also became common in university campuses, corporate networks, and broadband access systems because of its ability to provide predictable performance under heavy traffic loads.

Despite its technical strengths, ATM also had significant disadvantages. The fixed 53-byte cell size introduced relatively high overhead, particularly when carrying large data transfers. The equipment was more complex and expensive than competing Ethernet technology, and the rapid growth of the Internet favoured the simpler, more flexible IP protocol suite. Improvements in Ethernet speeds, Internet routing, and Multiprotocol Label Switching (MPLS) eventually provided many of ATM's advantages with lower cost and greater compatibility with existing data networks.

Many concepts pioneered by ATM continue to influence modern networking. Its emphasis on traffic engineering, virtual circuits, quality of service, traffic shaping, and high-speed switching directly influenced the development of MPLS and other modern networking technologies. Although ATM itself has largely disappeared from new installations, many of its underlying principles remain central to broadband network design.

It is important to distinguish ATM from conventional IP networking. Internet Protocol forwards packets independently using destination addresses, allowing different packets belonging to the same communication session to follow different routes. ATM establishes a virtual connection before transmission begins, and all cells belonging to that connection normally follow the same path through the network. ATM therefore combines characteristics of both circuit-switched and packet-switched communication.

Today, Asynchronous Transfer Mode has largely been replaced by high-speed Ethernet, MPLS, and all-IP networking. Nevertheless, ATM occupies an important place in the history of communications engineering because it represented one of the first practical attempts to integrate voice, video, and data within a single broadband network while providing guaranteed quality of service.

Asynchronous Transfer Mode therefore represents an important milestone in the evolution of digital networking. Although its widespread deployment was ultimately overtaken by simpler packet-based technologies, many of the ideas first developed for ATM continue to underpin modern broadband communication systems, demonstrating how innovative engineering concepts often outlive the technologies in which they first appeared.