Beginner's Guide to Asynchronous Transfer Mode (ATM)

This high-speed network standard supports voice and data

Asynchronous Transfer Mode (ATM) is a high-speed networking standard that supports voice, video, and data communications. It also improves the utilization and quality of service (QoS) of high-traffic networks. Internet service providers use ATM on private long-distance networks. ATM operates at the data link layer—Layer 2 in the OSI model—over either fiber or twisted-pair cable.

Although it's fading in favor of the next-generation network (NGN), the ATM protocol is critical to the Synchronous Optical Network (SONET) that forms the backbone of many internet service providers, the public switched telephone network (PSTN), and the Integrated Services Digital Network (ISDN).

How ATM Networks Work

ATM differs from other data link technologies such as Ethernet in several ways. ATM uses zero routing. Instead of using software, dedicated hardware devices known as ATM switches establish point-to-point connections between endpoints, and data flows directly from source to destination.

Ethernet and Internet Protocol (IP) use variable-length packets. ATM uses fixed-sized cells to encode data. These ATM cells are 53 bytes in length, which consists of 48 bytes of data and five bytes of header information. Each cell is processed at its own time. When one is finished, the procedure calls for the next cell to process. This is why it's called asynchronous; none of the cells go off at the same time relative to the other cells.

The ATM connection can be preconfigured by the service provider to make a dedicated or permanent circuit, or it can be switched or set up on demand and then terminated at the end of its use.

Four data bit rates are usually available for ATM services: Available Bit Rate, Constant Bit Rate, Unspecified Bit Rate, and Variable Bit Rate (VBR).

The performance of ATM is often expressed in the form of OC (Optical Carrier) levels, written as OC-xxx. Performance levels as high as 10 Gbps (OC-192) are technically feasible with ATM. However, more common for ATM is 155 Mbps (OC-3) and 622 Mbps (OC-12).

Without routing and with fixed-size cells, networks can manage bandwidth under ATM easier than under technologies such as Ethernet. The high cost of ATM relative to Ethernet is one factor that has limited its adoption to the backbone and other high-performance, specialized networks.

Wireless ATM

A wireless network with an ATM core is called wireless ATM (WATM). This type of ATM network offers high-speed mobile communications. It came about following the success of wired ATM technology in response to demands for wireless service everywhere. WATM supports data, voice, and video with guaranteed QoS.

Similar to other wireless technologies, the ATM cells broadcast from a base station and transmit to mobile terminals, where an ATM switch performs the mobility functions.


Another data protocol that sends voice, video, and data packets through the ATM network is called Voice over Asynchronous Transfer Mode (VoATM). It's similar to VoIP but does not use the IP protocol and is expensive to implement. However, it provides high-speed transport for networks and is beneficial for companies that have an ATM network in place.

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