Beginner's Guide to Asynchronous Transfer Mode (ATM)

Chocolate labrador teleconferencing on laptop
Justin Paget / Getty Images

ATM is an acronym for Asynchronous Transfer Mode. It's a high-speed networking standard designed to support voice, video and data communications, and to improve utilization and quality of service (QoS) on high-traffic networks.

ATM is normally utilized by internet service providers on their 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 NGN (next generation network), this protocol is critical to the SONET/SDH backbone, the PSTN (public switched telephone network) and ISDN (Integrated Services Digital Network).

Note: ATM also stands for automated teller machine. If you're looking for that type of ATM network (to see where ATMs are located), you might find VISA's ATM Locator or Mastercard's ATM Locator to be helpful.

How ATM Networks Work

ATM differs from more common data link technologies like Ethernet in several ways.

For one, 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.

Additionally, instead of using variable-length packets like Ethernet and Internet Protocol does, ATM utilizes fixed-sized cells to encode data. These ATM cells are 53 bytes in length, that include 48 bytes of data and five bytes of header information.

Each cell is processed at their own time. When one is finished, the procedure then calls for the next cell to process. This is why it's called asynchronous; none of them go off at the same time relative to the other cells.

The connection can be preconfigured by the service provider to make a dedicated/permanent circuit or be switched/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 are 155 Mbps (OC-3) and 622 Mbps (OC-12).

Without routing and with fixed-size cells, networks can much more easily manage bandwidth under ATM than other technologies like 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 a mobile ATM or wireless ATM. This type of ATM network was designed to offer high-speed mobile communications.

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

VoATM

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 doesn't use the IP protocol and is more expensive to implement.

This type of voice traffic is encapsulated in AAL1/AAL2 ATM packets.