What Asynchronous Transfer Mode Does

Asynchronous Transfer Mode (ATM) is a way of moving data through a network by breaking it into small, fixed-size chunks called cells and sending them one at a time, rather than waiting to bundle everything together. Each cell is 53 bytes — 48 bytes of actual data plus 5 bytes of routing information. The "asynchronous" part means the cells don't have to arrive on any particular schedule; the network sends them whenever they're ready, and the receiving end reassembles them in the right order.

ATM was designed in the 1980s and 1990s to handle voice, video, and data all on the same network at the same time, with predictable delays. It may provide that a video call wouldn't stutter because someone downloaded a file. For that reason, telephone companies and large institutions adopted it heavily. Today, ATM is mostly gone from home networks — Ethernet and fiber have replaced it — but understanding how it worked explains why modern networks are built the way they are.

Key Takeaways

  • ATM breaks all data into identical 53-byte cells and sends them individually, which lets the network prioritize different types of traffic.
  • The fixed cell size meant ATM could may provide that voice or video would not be delayed by large file transfers, a problem earlier networks had.
  • ATM required dedicated hardware and was expensive to install and maintain, which is why it never became common in homes.
  • Modern networks like Ethernet accomplish similar traffic control without ATM's complexity, which is why ATM largely disappeared after the 2000s.

How Cells Move Through an ATM Network

When you send data over ATM, the network breaks it into 53-byte cells at the source. If your message is larger than 48 bytes, it gets split across multiple cells. Each cell gets a header — the 5-byte routing information — that tells the network where the cell should go. The header includes a Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI), which work like a combination lock: they tell each switch in the network exactly which outgoing port to send that cell to.

The cells then travel through the network switch by switch. Each switch reads the header, looks up the VPI and VCI in a table, and forwards the cell to the next switch. Because every cell is the same size, the switches can process them at a steady, predictable rate. At the destination, the receiving equipment reads the cells in order and reassembles them back into the original message. If a cell arrives out of order or gets lost, the receiving end can request that it be sent again.

This approach is different from Ethernet, which sends variable-sized packets. An Ethernet packet can be anywhere from 64 bytes to 1,500 bytes (or larger with jumbo frames). That flexibility is useful for most purposes, but it means a large file transfer can tie up a network link for longer, delaying a voice call that arrives while the file is still being sent.

Why ATM may provide Quality of Service

The main reason ATM existed was to solve a real problem: older networks could not promise that a voice call or video stream would arrive without delays. If someone started downloading a large file on the same network, the voice call would stutter or drop. ATM fixed this by letting the network reserve bandwidth for specific types of traffic before the data started flowing.

When you set up an ATM connection, you told the network what kind of traffic you were sending — voice, video, or data — and how much bandwidth you needed. The network would check whether it had enough capacity and, if it did, reserve a path for your cells. Once the path was reserved, the network may provide that your cells would arrive with a predictable delay. Other traffic could not use that reserved bandwidth, so your voice call would never be delayed by someone else's file read.

This may provide is called Quality of Service (QoS). Modern networks like Ethernet can also provide QoS, but they do it differently — by marking packets with priority tags and having switches honor those tags. ATM built QoS into the network from the ground up, which made it more reliable but also more rigid and expensive.

The Hardware ATM Required

To use ATM, you needed specialized equipment at every point in the network. Your computer needed an ATM network card instead of an Ethernet card. Your switches had to understand ATM and be able to read VPI and VCI headers. Your router had to speak ATM. If you wanted to connect an ATM network to an Ethernet network, you needed a bridge or gateway that could translate between the two — a piece of equipment that understood both languages.

This specialization was expensive. An ATM network card cost hundreds of dollars in the 1990s. ATM switches cost tens of thousands. A small business or home user could not afford to build an ATM network, so ATM stayed in telephone company backbones, university networks, and large corporations. Ethernet cards and switches were cheaper and good enough for most purposes, so they won the market.

By the early 2000s, Ethernet had become fast enough that the performance difference between ATM and Ethernet had shrunk to almost nothing. At the same time, Ethernet switches got smarter and could provide QoS without ATM's complexity. There was no longer a reason to pay for ATM hardware, so it disappeared from most networks. A few telephone companies still use ATM in their backbones, but it is no longer a technology you will encounter in a home or small business network.

How ATM Differs From Ethernet and Modern Networks

Ethernet, which is what your home network uses, sends variable-sized packets. A packet can be as small as 64 bytes or as large as 1,500 bytes (or more). This flexibility means Ethernet can adapt to different types of data — a short acknowledgment message can be sent quickly, and a large file can be sent in fewer, larger chunks. The tradeoff is that a large packet can delay smaller packets behind it.

ATM's fixed cell size meant the network could always process data at a predictable rate, but it also meant that small messages got padded with empty space to reach 53 bytes. If you were sending a one-byte acknowledgment, it still took up a full 53-byte cell. That wasted space was one reason ATM was less efficient than Ethernet for typical office work.

Modern networks solve the QoS problem without ATM by using priority tags. When a voice call or video stream starts, the sending device marks those packets with a high priority. Switches and routers read the priority tag and send high-priority packets first, even if they arrived after lower-priority packets. This approach is simpler, cheaper, and flexible enough for most real-world needs. It is why Ethernet won and ATM disappeared.

Where ATM Still Exists Today

ATM is almost entirely gone from networks you will see. Some telephone companies still use ATM in their long-distance backbones — the high-speed links that carry traffic between cities — because they have invested billions in ATM infrastructure and it still works. A few universities and research institutions still have ATM networks running, usually because they were built decades ago and have not been replaced.

If you are setting up a home network or a small business network, you will not encounter ATM. Every network card, switch, and router sold today uses Ethernet or Wi-Fi. If you are reading about ATM, it is likely because you are learning about network history or because you encountered the term in an old textbook or documentation. Understanding how ATM worked helps explain why modern networks are built the way they are — why we have QoS, why we use variable-sized packets, and why we do not need specialized hardware for every type of traffic.

Frequently Asked Questions

Is ATM still used in home networks?

No. ATM disappeared from home and small business networks in the early 2000s. Every home network today uses Ethernet or Wi-Fi. You will not find ATM network cards, switches, or routers for sale anymore.

Why was ATM better than Ethernet for voice and video?

ATM may provide that cells would arrive with a predictable delay because the network reserved bandwidth before traffic started flowing. Ethernet did not have this may provide, so a large file read could delay a voice call. Modern Ethernet solves this with priority tags, which is simpler and cheaper than ATM's approach.

What does the 53-byte cell size mean?

ATM broke all data into cells of exactly 53 bytes: 48 bytes of actual data and 5 bytes of routing information. This fixed size let the network process cells at a steady rate, but it also meant small messages wasted space because they still took up a full 53-byte cell.

Can I still buy ATM equipment?

You can find used ATM equipment on the secondhand market, but new ATM hardware is no longer manufactured. There is no reason to buy it for a new network — Ethernet is cheaper, faster, and easier to set up.

How did ATM's VPI and VCI work?

The VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier) were numbers in the cell header that told each switch where to send the cell next. They worked like a combination lock: the switch looked up the VPI and VCI in a table and forwarded the cell to the correct outgoing port.