Asynchronous Transfer Mode (ATM)

Definition of Asynchronous Transfer Mode (ATM) in The Network Encyclopedia.

What is ATM (Asynchronous Transfer Mode) in computer networking?

A high-speed, broadband transmission data communication technology based on packet switching, which is used by telcos, long distance carriers, and campus-wide backbone networks to carry integrated data, voice, and video information. Asynchronous Transfer Mode (ATM) can be used as the underlying technology for Fiber Distributed Data Interface (FDDI), Synchronous Optical Network (SONET), and other high-speed networks. Plus, ATM can run on any media including coax, twisted-pair, or fiber-optic.

ATM is a connection-oriented protocol that can work with either permanent virtual circuits (PVCs) or switched virtual circuits (SVCs), depending on your wide area network (WAN) traffic needs. ATM networks use bandwidth at maximum efficiency, while maintaining guaranteed quality of service (QoS) for users and applications that require it. The two main benefits of ATM are its high transmission speeds and its flexible bandwidth-on-demand capability.

How it works

The «asynchronous» in ATM means ATM devices do not send and receive information at fixed speeds or using a timer, but instead negotiate transmission speeds based on hardware and information flow reliability. The “transfer mode” in ATM refers to the fixed-size cell structure used for packaging information. This cell-based transmission is in contrast to typical local area network (LAN) variable-length packet mechanisms, which means that ATM connections are predictable and can be managed so that no single data type or connection can monopolize the transmission path.

ATM technology originated in broadband ISDN (B-ISDN) technology and works primarily at layer 2 of the Open Systems Interconnection (OSI) reference model. ATM connects devices over a WAN using virtual channels (VCs) and virtual paths (VPs). Virtual channels consist of one or more physical ATM links connected in a series for transmitting data between remote stations. A VC exists only while data is being transmitted on it, and all cells in a given ATM transmission follow the same VC to ensure reliable data transmission. A virtual path is a collection of VCs having the same source and destination points that can be used to pool traffic being transmitted to a given destination.

ATM is a connection-oriented technology that requires the establishment of a specific network path between two points before data can be transported between them. Typically a subscriber would lease a T1 or T3 line to connect their customer premises equipment (CPE) to the telecommunication carrier’s ATM network, but frame relay or SONET can also be used to connect a site to an ATM network. The kind of CPE needed varies with the access method employed—for example, Channel Service Unit (CSU) for T1 line, frame relay access device (FRAD) or router for frame relay, and so on. Large corporate networks using an ATM backbone might use a switch-to-switch connection to the carrier’s network instead of CPE.

ATM uses fixed-size packets called “cells.” Each 53-byte ATM cell contains 48 bytes of data payload and 5 bytes of control and routing information in the header. The header provides addressing information for switching the packet to its destination. The payload section carries the actual information, which can be data, voice, or video. The payload is properly called the user information field. The reason for choosing 48 bytes as the payload size is to compromise between the optimal cell sizes for carrying voice information (32 bytes) and data information (64 bytes). The fixed size of an ATM cell makes ATM traffic simple and predictable, and makes it possible for ATM to operate at high speeds. Typical ATM speeds vary with transmission media and can include

  • 25 Mbps over unshielded twisted-pair (UTP) category 5 cabling
  • 155 Mbps over either UTP or fiber-optic cabling
  • 622 Mbps and 4.8 Gbps over fiber-optic cabling only

ATM also includes a mechanism for allocating bandwidth dynamically; that is, bandwidth is allocated only in required amounts and the required direction. As a result, when an ATM link is idle, it utilizes no bandwidth, which can result in considerable cost savings depending on the needs of your network.

ATM optimizes performance through different classes of service, which can be allocated through QoS settings. This is different from frame relay, which is a classless service. The four classes of ATM services that subscribers can specify depending on their needs are

  • Constant Bit Rate (CBR):
    This service level is suitable for applications that are sensitive to cell delay and cell loss, must have continual availability, and do not require much bandwidth (for example, voice traffic).


  • Variable Bit Rate/Realtime (VBR-RT):
    This level is suitable for applications that are sensitive to cell delay and cell loss and require a large amount of bandwidth (for example, videoconferencing).


  • Variable Bit Rate/Non-Realtime (VBR-NRT):
    This level is suitable for applications that require a large amount of bandwidth but can tolerate some cell delay and cell loss (for example, video playback).


  • Unspecified Bit Rate (UBR) or Available Bit Rate (ABR):
    This level is suitable for network services that do not have special bandwidth or cell latency/loss needs (for example, file transfer or e-mail).


ATM provides the following advantages:

  • High-speed, fast-switched integrated data, voice, and video communication that is not bound by the physical or architectural design constraints of traditional LAN networking technologies.
  • A standards-based solution formalized by the International Telecommunication Union (ITU) that allows ATM to easily replace existing telephony network infrastructures. ATM provides a global telephony standard, and over 70 percent of U.S. telcos have migrated their internal networks to ATM.
  • Interoperability with standard LAN/WAN technologies. ATM networks can be interconnected with Ethernet and token ring LANs using LAN emulation (LANE) services to provide TCP/IP over ATM.
  • QoS technologies that enable a single network connection to reliably carry voice, data, and video simultaneously and manage bandwidth on a per-connection basis depending on the priority of the service required.

Microsoft Windows 2000 supports direct connectivity to ATM networks with up to four ATM adapters in a single computer.