Asynchronous Transfer Mode (ATM) Pros and Cons

ATM Origins and Development

During development of the standards for the Asynchronous Transfer Mode (ATM), in the mid 1980s, the goals were to create a unified networking strategy that could act as an all-round transport system for real-time video and audio as well as for image, text and email. ATM is pretty much a “Jack-of-all-trades” transport system. The two groups primarily responsible for the development of the ATM standards were the International Telecommunications Union [ITU 2004] and the ATM Forum [ATM 2004]

Implementations - Over time we have seen that the majority of implementations and uses that ATM has fulfilled have been primary concerned with telephony and IP networks. Ethernet and the Internet Protocol (IP) are packet-switched network technologies that use packets of variable size referred to as frames.

Circuit-Switched - In marked contrast; ATM is a connection-oriented, Data Link Layer (OSI Reference Model Layer 2), circuit-switched, cell relay protocol that runs over Synchronous Optical Network (SONET) Physical Layer (OSI Reference Model Layer 1) links using cells of identical and never varying size. Consistent predictability is the underlying ethos here.

Connection-Oriented - Being a connection-oriented channel-based technology means that ATM must always establish a “logical” connection between the two endpoints prior to commencement of data exchange.

Uniform Cell Size - Significantly, ATM encodes data traffic into small uniform fixed-sized cells. ATM cells are always 53 bytes in size and are comprised of 48 bytes of data and 5 bytes of header information.

Benefits of Using Small Fixed Size Cells

The major benefits derived from using small data cells were a reduction in queue delay and jitter; particularly in multiplexing data streams. By using small, fixed-sized cells ATM is able to transport large data files all the while maintaining minimal queuing delays. Minimal queuing delays are essential to the delivery of voice/video communications.

Queue Delay – Queue delay related issues include problems associated with end-to-end-round-trip delays and delay variance particularly when carrying voice traffic. High traffic volumes and/or congested networks along with the arrival variance associated with variable route routing are among the main causes of queue delay issues.

Jitter – Although jitter results from queuing delay issues deviations or displacement of various aspects of high frequency pulses such as amplitude, phase timing and signal pulse width as a direct result of electromagnetic interference (EMI) and crosstalk (noise) also cause jitter. Think of jitter as being the production of “jerky” results or in video applications flicker. By using small fixed-size cells ATM is able to overcome the effects of queue delay as well as other types/sources of jitter.

Multi Purpose Transport Protocol - Asynchronous Transfer Mode (ATM) carries many different data types and formats (text, audio, video, graphics, photos etc.) from a multitude of sources and of variable sizes. When combined with standard queuing strategies, maximum queuing delays were common. This is totally unacceptable where voice and real-time video traffic is concerned.

Compression/Decompression Algorithms (Codecs)

Time - The nature of time as we humans perceive it is an analogue continuum (that is to say time is a linear progression). Once past, there is no way as yet to recover the loss.

Jitter and Queue Delay - Jitter and queue delay are of great importance because of the nature and manner of operation of the compression/decompression (codec) algorithms used in the conversion of a digitalized data stream back into an analogue audio signal. This conversion process (digital-to-analogue) is very much a “real-time, on-the-fly” process and is more attuned to” just-in-time” transport protocols.

Real-Time Streaming - In order to produce reliable, consistently “acceptable” output the codec needs the data items (the digitized voice data) to be presented to it in a predictable, regulated and evenly spaced in time data stream, hence the term “real-time streaming”.

Late Arrivals - If the data arrives after its allotted position/reception window in the time sequence (relating to that part of the data-stream) the codec will simply drop it. Not surprisingly this is unacceptable for IP telephony. Remember to keep in mind that time is analogue in nature and once a “time window” elapses, the “lost” time becomes unrecoverable.

Codec Packet Handling Options - If the transport protocol is unable to present the data as and when the codec expects it, the codec, has no choice but to assume either silence, make a “best guess” or simply drop the packet. Any way is unacceptable where voice is concerned as the conversation rapidly becomes untenable and the message does not get through.

Asynchronous Transfer Mode (ATM) Deployment Indicators and Scenarios

ATM WAN Core Implementation - ATM production environment implementations have over time proved to be very successful in the Wide Area Network (WAN) scenarios. Numerous telecommunication providers and Internet Service Providers (ISPs) have implemented ATM in their Wide Area Network (WAN) cores.

Slow Links - For slow links less than 2M-bit/s, ATM still makes sense, which is why many ADSL systems use ATM as an intermediate layer between the physical link layer and a Layer 2 protocol like PPP or Ethernet.

Linear Audio and Video Streams - Interest in using native ATM for carrying live video and audio has increased recently. It is in these environments, where ATM can deliver the low latency and very high Quality of Service (QoS) required for handling linear audio and video streams.

Gigabit Ethernet – Today we are finding that for both new WAN implementations and for existing WAN implementation upgrades, high speed, high performance Ethernet (Gigabit Ethernet, 10Gbit Ethernet, and Metro Ethernet etc.) are rapidly replacing ATM as the technology of choice.

Relative Performance - At the time ATM was designed, 155Mbit/s (135Mbit/s payload) over fiber-optic cable was very fast in comparison to the other carrier/transport technologies available at the time. Since then however; these other technologies have evolved and are now considerably faster than they once were.

Jitter – Today; a 1,500 byte (12,000 bit) full-size Ethernet packet takes only 1.2 µs to transmit across a 10Gbit/s optical network. With this sort of speed, jitter is no longer the issue it once was. By overcoming the potential adverse effects of jitter through this ramping up of network transfer speeds we have at the same time removed the need for using small uniform cells to overcome jitter.

Complexity - Unfortunately, due to ATM’s complexity it proved to be unsuitable for deployment in many of the scenarios that its creators had originally intended.

Converged Networks - The speed and traffic shaping requirements of many converged networks are also proving to be very challenging for ATM.