Asynchronous Transfer Mode (ATM)

Before reading this you may find the following tutorials useful:
Protocol Basics, Formatting for Transmission, Internet Protocol (IP) Life is full of compromises. The telecommunications industry is quite life-like in this respect – ideal solutions are a rarity and so compromises often need to be made. Out of the range of protocols available, it is perhaps the role of Asynchronous Transfer Mode (ATM) to keep everyone happy.

ATM is a data-link layer protocol like Ethernet, aimed at wide area networks (WANs) as well as local area networks (LANs). Whereas Ethernet is really geared towards carrying only Internet Protocol (IP) traffic, ATM is designed to integrate both data and voice needs in one network.

If you can imagine your government outlawing all legal tender apart from one single coin, you may have an idea of how ATM works. Picture life with just one coin, perhaps worth enough to buy, say, a pair of shoes. You just have a supply of this one coin, and you must use that to cover all of your money transactions.

ATM has fixed-length “cells” of 53 bytes in length in contrast to Ethernet’s variable-length “frames.” It is this size of cell that is a compromise between the large frame requirements of data transmission and the relatively small needs of voice calls – just as your one denomination of coin is a compromise between the small amount of money needed to buy a cup of coffee and the relatively large amount required to buy a television.

By catering to both forms of network traffic, ATM can be used to handle a company’s entire networking needs, removing the need for separate data and voice networks. The performance, however, can also be compromised, and the network may not be as efficient as dedicated networks for each service. Try to buy a house with a bagful of coins and you should realize the problem.

ATM’s 53-byte cells each contain a 5-byte header along with 48 bytes of data, and overall transmission speeds from a few tens of Mbit/s up to several Gbit/s are possible. ATM is a connection-oriented protocol, meaning that it must establish a connection (or “circuit”) between two devices before it can begin to send information between them. This is different from the working of Ethernet where frames are sent out with addressing information to find their way around (so-called “connectionless” networking).

An ATM Cell The benefit of the fixed-length cells is that the traffic in the network is very predictable, and very fast switching of cells is possible due to the inbuilt clocking of cells travelling through. Due to this predictability, it now becomes possible to guarantee specific levels of service to traffic that may be very time sensitive (such as voice calls and video conferencing). IP networks using Ethernet are what is known as “best effort,” meaning that no guarantees of quality of service (QOS) can be given, and the network will just do its best to get information from A to B as quickly as it can. In this respect, ATM really has the edge.

When ATM sets up a connection between two devices, it uses what are known as virtual channels (VCs) and virtual paths (VPs). A VP is a route between two points, and within it can be many (up to several thousand) VCs that are initiated for each specific information exchange. The advantage of having the paths in place is that all channels within a path can benefit from the same management functions. An additional plus is that adding new channels is simpler, as the foundations of the link are already in place. Once such a VC is in place, cells can be switched relatively quickly through intermediate points and direct to their destination.

Virtual Path containing many Virtual Channels The 5-byte header in the ATM cell contains a virtual path identifier (VPI) and a virtual channel identifier (VCI) that serve to identify the exact path and channel each cell belongs to – forming a “virtual circuit.” It should be noted that there are two types of connection that can be made. Permanent virtual circuits (PVCs) are connections that are constantly in place, whereas switched virtual circuits (SVCs) are those that can be dynamically created and destroyed as required by the network.

The final piece of ATM-speak we shall be covering for now is that relating to ATM Adaptation Layers (AALs). AALs convert information into ATM cells, and are used to determine the QOS that specific traffic will receive. The four main types of AAL you may hear about are CBR, VBR, ABR, and UBR. Constant Bit Rate (CBR) gives a fixed and guaranteed amount of network capacity to a circuit, and is therefore useful for time-sensitive information such as voice. Variable Bit Rate (VBR) circuits have the ability to grow and shrink in size with their data needs, but some loss of cells may occur if requirements cannot be met due to higher priority traffic from other sources. Available Bit Rate (ABR) guarantees a minimum level of data transfer with the possibility of higher rates if available. And finally, Unspecified Bit Rate (UBR) circuits just provide whatever spare capacity the network has at any one time.

As a final note, in a real-life application it is likely that IP traffic may be travelling through an ATM network that may then go out onto optical fiber through Synchronous Digital Hierarchy (SDH). Such a scheme is referred to as IP over ATM over SDH.

Key Points

  • A data-link layer protocol designed to support voice and data
  • Fixed-length "cells" of 53 bytes (including 5-byte header)
  • Connection-oriented protocol with predictable network traffic due to fixed-length cells
  • Quality of service is possible, in contrast to Ethernet’s "best-effort" service
  • Virtual paths (VPs) containing several virtual channels (VCs); identifiers for each (VPI and VCI) in the headers define a virtual circuit
  • Permanent virtual circuits (PVCs) and switched virtual circuits (SVCs) are possible
  • Adaptation layers provide different levels of service from Constant Bit Rate (CBR) through to Unspecified Bit Rate (UBR)

Further Reading

Sonet (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy)
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mnicholas 12/5/2012 | 1:21:34 AM
re: Asynchronous Transfer Mode (ATM) Nortel have a 70 Gbps ( Passport 20K) ATM switch.
When you count the processing speed you must
consider the architecture buss or switched and also what it's carry on. Ethernet it's on other
end to ATM

csae 12/4/2012 | 11:30:11 PM
re: Asynchronous Transfer Mode (ATM) Dear Author of this article -

I just wanted to point out that there is a fairly large error in your discussion on AAL and QoS.

Your article states: "The four main types of AAL you may hear about are CBR, VBR, ABR, and UBR." I think that you meant to say that the 4 main types of QoS are the above"

I assume that this was a typo since the Adaptation Layers (AAL1, AAL2, AAL3/4, and AAL5) which define the way the cells are formatted are something quite different than the Quality of Service (CBR, VBR-rt, VBR-nrt, UBR) traffic contracts that are established between the transport layer and the network layer and which are based on the timing and bit rate needs of the type of data being transmitted.

I just thought that I would point this out in case you felt that this article needed to be editied to correct this.
debasish71 12/4/2012 | 10:49:41 PM
re: Asynchronous Transfer Mode (ATM) I have a very good pdf doc on your topic. I think you are looking for xDSL over ATM which is what companies are providing as a compromise between truely broadband solutions on one hand and noisy 56kbps modems on the other hand. Cost and economics and to avoid new instrument installation, xDSL has become something called personal broadband systems at speed of 2-8 mbps, which is fast enough. After all, nobody wants thousands of movies with video dial tone at every residence if that translates into connecting a STM-1 fibre into every home. The economics just dont justify it. Do you remember how ISDN(i still dont know !) died inspite of all its myths?

If you are still looking for something in depth, contact me at [email protected]
debasish71 12/4/2012 | 10:49:40 PM
re: Asynchronous Transfer Mode (ATM) what is the typical backplane processing speed of contemporary ATM switch? Is 2.4 Gbps processing power( 16 STM-1 ports=16 x 155 mbps = 2.4 gbps) truely broadband? i have heard that companies have deployed terabit switches.Any ideas?
debasish71 12/4/2012 | 10:46:32 PM
re: Asynchronous Transfer Mode (ATM) There is a lot of fuss regarding which AAL to use for which application. For voice transmission, there are many possibilities, like PCM(uncompressed, G.723) voice over ATM, and VOIP(compressed, G.721).There may be other possibilities as well, such as VoIPoFRoATM !!

which is optimal for voice and video?
Does AAL5 has any utility other than non-real time message transfers?

gbennett 12/4/2012 | 8:09:10 PM
re: Asynchronous Transfer Mode (ATM) Dear LR,

Nice summary, but I wanted to throw a couple of comments at you.

- ATM isn't just a Data Link technology. ATM standards include definitions for Physical Layer, Data Link and Network Layer protocols. I cringe when I hear people, even within my own company describing ATM as a "layer 2" to an IP network (in the context of ATM as a Traffic Engineering layer).

In fact, ATM is a connection-oriented technology. In order to establish connections, it makes use of full Layer 3 addressing (the ATM Private Address, or a public version of this). Once the connections are established, ATM uses a label swapping mechanism similar to MPLS (which is often, rather painfully described as a "Layer 2.5" technology).

- At the end of the article you pull out a one-off example of IP-over-ATM-over-SDH. It's true that ATM plays an important role in this application as the Traffic Engineering layer for IP networks. However, this may be one of the first areas in which MPLS displaces ATM. To really give some idea of the future of ATM, why not mention that it's the de facto aggregation technology for a range of access techniques, including xDSL, FSAN PONs and 3G Mobile?

petteri 12/4/2012 | 8:07:20 PM
re: Asynchronous Transfer Mode (ATM) Let us say that ATM is a great tech but it is also a religion and I fear to mess with it. But anyway in this part of world ATM is no more success story, and as far as I know it is not selling-point in America either. So why this article now; may it be that ATM is getting second lease of life ?

flanker 12/4/2012 | 8:07:18 PM
re: Asynchronous Transfer Mode (ATM) ...So why this article now; may it be that ATM is getting second lease of life ?...

Yes, Didn't you know Silicon Valley venture capitalists have been dumping BILLIONS into ATM transport? Don't let Tachion fool you, ATM is poised for great things, by 2003 it will be carrying 91.7% of the nation's data and voice traffic. Optical IP and GigE don't have a chance against ATM.

JACK00 12/4/2012 | 8:03:37 PM
re: Asynchronous Transfer Mode (ATM) May I ask why ATM will win Against optical IP and GigE? thx
petteri 12/4/2012 | 8:03:27 PM
re: Asynchronous Transfer Mode (ATM) > JACK00 - May I ask why ATM will win Against optical IP and GigE?

Even half a year ago I was of the opinion that ATM is a goner. But think:

1. The direction of telecom was something, say a year ago. If same direction had continued someone would have lost big money.
2. So direction was restored by force; too bad that a slump also took place GÇÖaccidentallyGÇÖ. The amount of losses rose greatly but what happened also was that ATM&Sonet won round one in fight with IP&Ethernet.
3. Slogan GÇÖbandwith glutGÇÖ was coined. Was it invented because IP&Ethernet beats ATM&Sonet hands down if there are fat pipes ?

So the question may not be what is better, but who wins. Graph about sales of ATM&Sonet-equipment versus years would clarify this story but I cannot find any. Can you or someone help, please ?

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