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The Beauty of

Asynchronous Transfer Mode

Thao Mong Le

Note: This Asynchronous Transfer Mode (ATM) series column discusses the aspects of an ATM network and may be used as a guide to build an ATM data and computer communications network, suitable for both technical and non-technical people who have little technical background in the ATM communications field. The readers will benefit from reading this column by better understanding the concepts and fundamentals of ATM communications network architectures, protocols, and devices. This column was not intended to cover all ATM network architectures and its communications equipment, or to delve into the design of ATM network elements itself.

Everyone can agree that more work needs to be done in completing the ATM technology. But there is uncertainty about what we should do over the next few years?, whether to look on ATM for interim and target dates. Is there a sufficient set of defined standards and products for your organization to proceed with full scale operational use of ATM or is it prudent to wait for further standardization and more complete products before changing to ATM?



A number of changes are occurring in the computing, data communications, and telecommunications fields. As computing systems become increasingly powerful and versatile, data communications and telecommunications networks must evolve to support the performance of computing systems. Probably the best cell-base switching and multiplexing technology used to support the development of these types of high-performance communications networks is the Asynchronous Transfer Mode (ATM). ATM combines a user�s data, voice, and video into fixed length cells, and multiplexes it into a single bit stream transmitted across a physical medium. ATM supports a wide range of high-performance data communications and telecommunications services for local, national and international communities. In addition to providing a technical overview of ATM in terms that are understandable to the readers, this paper will discuss the driving forces behind ATM, and various ATM applications that make the "future" telecommunications come to the world.


The Asynchronous Transfer Mode (ATM) is one of today�s most attractive communications technologies. It is an emerging technology that uses a very flexible method to provide a high bandwidth, as well as a very high transmission rate for a communications or telecommunications network. ATM is a method of communication which can be used as the basis for both high-speed local area network (LAN) and wide area network (WAN) technologies to support the performance of advanced computing systems. Technically, ATM is a packet-like switching and multiplexing technique that relays traffic via an address contained within the packet for broadband signals, and performs two simple functions: path determination which will be done mainly in software, and cell-switching done in hardware.

ATM provides a tool that allows network designers to solve many pressing network problems - from internetworking to electronic publishing, electronic fund transfer, and financial industry reports, from videoconferencing to medical image, more affordable health care, to expanding telecommuting, work at home, and many, many more.

The flexibility bandwidth of ATM allows various types of services (e.g., data, voice , video, and image ) with differing requirements to map onto a single network. With ATM, separate networks for data, voice, video and image will not be required.

ATM is promised to be fast, reliable, and a cost effective bandwidth for meeting both the current and future communications and telecommunications needs.


Today, telecommunications companies around the world are building digital transmission facilities implemented various modern switches to meet the current and future needs of their business clients. Business users are concerned with maximizing their investment in computing and data communications equipment. Users want a network where any one user can connect with any type of interfaces and protocols. This evolution in technology is the primary motivating factor to develop ATM. There are several forces driving the move to this ATM technology. Among them are:

· ATM is chosen as an underlying router-based network since ATM can work in a hierarchical network that provides a wide range of telecommunications services for local, national and international communities.

· ATM is chosen since it is the only international standard designed to accommodate the simultaneous transmission of data, voice, video, and image at a very high speed over a single network. This international standard allowing interoperability of all types of information, regardless of the "end-system". Telecommunications companies may minimize the loss when investing in the ATM technology.

· ATM meets the growing telecommunications demand, as business becomes more geographically diperse, for domestic and international digital communications service for data, voice, video and image. ATM provides a network that can integrate all these types of communication into one universal network.

· ATM provides network a "unlimited" bandwidth with scalability. Scalability is the ability to increase the amount of bandwidth without changing the information structure. Today�s telecommunications networks are not constructed to provide such features. Packet data services, for example, are typically limited to transmission rates of about 56 kbps, and circuit switched services are typically limited to speed of about 45 Mbps; as the demand for the rapid transfer of information increases dramatically, these speeds are no longer adequate and other transmission technology must be found.

· ATM provides an effective communication protocol for LAN-LAN, LAN-WAN and WAN-WAN interconnection. It has added to users� needs for connectivity expand from the local to national, and finally international connectivity. As ATM continues to be deployed, the line between LAN and WAN will blur to form an ATM- based global network.

· The ATM protocol allows implementation of many advanced network applications that would be difficult to use with current network technologies. This protocol allows network designers to increase network simplicity, flexibility, and control at an economical price.

· ATM is chosen since it provides primarily a connection-oriented service, but it can support a connectionless service. Moreover, ATM uses virtual calls to transport user information in the closest approximation to true bandwidth-on-demand service at both a constant bit rate (CBR) and variable bit rate (VBR).


* Fast Packet Switching

Unlike most current networks, which use circuit or packet switching, ATM uses fast packet switching. By definition, fast packet switching is a statistical switching mechanism, also know as asynchronous time division, allowing its network to provide a large amount of bandwidth, high throughput with a very small delaying time.

Fast packet switching consists of two techniques, frame relay and cell relay, that allow its network to simultaneously transfer a variety of types of traffic with high quality and reliability. Frame relay operates with a variable length frame, normally in the range of 64 octets to 1024 octets. Cell relay uses a fixed length frame consisting of 53 octets. Frame relay and cell relay enable their networks to provide end-to-end error-checking and minimize flow control overhead. These allow a reduction of delaying time during the transfer of information. ATM employs cell relay.

* Standards

The ATM standards are in development within the American National Standards Institute (ANSI) and the International Telecommunications Union - Telecommunications (ITU-T) as transport protocols for use in an ATM-based network, such as the Broadband Integrated Service Digital Network (B-ISDN). Although much of the initial interest and early research on ATM was done by the public telecommunication providers and ITU, the private network providers driven by the benefits of ATM became interested in the development and standardization of this new technology. This interest from the private network providers led to the establishment of the ATM Forum, whose initial objective was to ensure interoperability between private and public ATM implementations. This forum includes almost all major players in the telecommunications area, and it is essentially supplanting the ITU as the major driver of private and public ATM networking standards.

Today, the specification of the ATM cell structure is in place and implementable. The ITU I.361 standard defines the physical make-up of ATM cells, but the development of the higher-layer specifications is still underway. The ITU I.121 standard provides the initial references to ATM as the target solution for providing B-ISDN support. Further partial standards described more details of ATM are as follows:

I.150 - ATM protocol, I.15X - General aspects of B-ISDN, I.2XX - Service principles, and I.413 - Details of the User-Network Interface.

Remaining open ATM issues are being addressed rapidly within an industry consortium as the ATM Forum. The ATM Forum developed the user-network interface specification, which assists vendors in developing interoperable ATM products. The initial payload data rate envisioned for ATM is 44.736 Mbps, transported via the carrier existing T-3 (44.736 Mbps) networks. The ATM Forum also worked on the development of interface transmission specifications of 155 Mbps and other interfaces for ATM networks.

* ATM Transmission Facility

ATM use SONET (Synchronous Optical Network) as a digital transmission facility. SONET is a standardized hierarchical transport scheme designed to address the need for communications and telecommunications networks that desire transmission rates higher than 44.736 Mbps (i.e., T3 or DS-3). It was originally developed for the public telephone network to improve overall network performance and management and provide a needed bandwidth to the customer.

ATM use SONET over the public telephone network to convert synchronous transport signals (STS) (i.e., electrical signals) to optical carrier (OC) signals over a fiber optic network. SONET's basic building block is a 125 micro-second frame. The frame can be broken down into overhead and user information or payload. The STS-n electrical signal generated by SONET transport equipment is converted directly into an optical signal with no further processing or added overhead, therefore the OC-n are identical with the STS-n rates. The hierarchy is made up of a basic transport tributary called Optical Carrier level 1 or OC-1 running at a rate of 51. 84 Mbps. Higher level OC can be generated by multiplexing of OC-1. In this hierarchical structure, standard rates of each OC transports are OC-1 (51.84 Mbps), OC-3 (155.52 Mbps), OC-9 (466.56 Mbps), OC-12 (622.08 Mbps), OC-24 (1,244.16 Mbps), OC-36 (1,866.24 Mbps), and OC 48 (2.488.32 Mbps). Currently, the rates below OC-3 have been regionally optimized, and the rates between OC-3 and OC-12 have been adopted for international use.

The ATM Forum originally was recommending SONET as the primary transmission vehicle for ATM, but with the premature SONET, many existing ATM products use T1 (1.544 Mbps) or T3 as its physical layer.

* Interfaces

An ATM network is modeled as a set of end-systems (e.g., workstations) and a set of intermediate nodes (e.g., ATM switches), all linked by a set of point-to-point ATM links. Two major types of interfaces in ATM networks are the User-to-Network Interface (UNI) and the Network-to-Network Interface (NNI). The use of these terms has been broadened to include both ATM private and ATM public networks, as illustrated in Figure 2. While the ITU defines standards for NNI, the ATM Forum developed standards for public and private UNI. The major differences between these types of interfaces are administrative and signaling functions.

By definition, the UNI represents a regulatory boundary between an end-device and an ATM switch, public or private. The NNI refers to an interface between two public ATM switches, which exchange information such as routing tables that are unrelated to a particular circuit set-up. For links between private ATM switches and between different public switches, switch-to-switch links, also known as Inter-Switching-System- Interfaces (SSSI), are used. The ATM Forum proposed the use of the same signaling protocols across all types of interfaces, which will greatly facilitate implementations. * ATM Related Functional Layers

In terms of the Open Systems Interconnection (OSI) 7-layer Reference Mode, ATM operates roughly between Layer 1 (Physical layer) and Layer 3 (Network layer). Figure 1 shows the ATM related functional layers. The lowest layer is the physical layer, which is capable for transmitting ATM cells over various media, such as fiber optic cable, electrical wire, and satellite. The ATM Physical Layer, which consists of two sub-layers: the Physical medium Dependent (PMD) sublayer, and the Transmission Convergence (TC) sublayer. The PMD sublayer deals with electrical interfaces, such as bit timing and the physical medium (e.g., cables, connectors). The TC sublayer performs a convergence function that receives a bit stream from the PMD, and extracts cells to pass to the ATM Layer. The ATM Physical Layer uses SONET as a transport medium facility to scale to the very high speeds required for ATM.

Higher Layers

ATM Adaptation Layer (AAL)

ATM Layer

Physical Layer (optical, electrical, etc)

Figure 3: The Functional ATM Layers Model

Next, the layer above the Physical Layer is the ATM Layer , which performs the basic operations of ATM, such as multiplexing, demultiplexing, cell relaying, delay handling, and cell loss priority processing.

On the top of the ATM Layer is the ATM Adaptation Layer (AAL). This layer categorizes five types of protocol layers: 1,2,3,4, and 5. The AAL allows the ATM network to provide bandwidth savings by taking advantage of the bursty nature of packet services. The main functions of the AAL include cell assembly and disassembly, handling of loss cell conditions, cells recovery, mapping of control signal into ATM cell stream, dividing information blocks up into ATM cells, and rate adaptation. The AAL provides a service to the higher layers that correspond to the four classes of traffic as defined below.

Class A: CBR, connection oriented, synchronous traffic (e.g., uncompressed voice or video)

Class B: VBR, connection oriented, synchronous traffic (e.g., compressed voice and video) each of which was designed to optimally carry one of the four above classes of traffic.

Class C: VBR, connection oriented, asynchronous traffic (e.g., X.25, Frame Relay services, etc.)

Class D: Connectionless packet data (e.g., LAN traffic, SMDS, etc.)

Finally, the Higher Layers, whose protocols are defined across the ATM layers to support signaling and control. The main functions of the Higher Layers are to perform transfer of connectionless network service (CLNS) and connection-oriented network service (CONS) data and video/voice communications at high speeds over the ATM network.

* ATM Cells

ATM is an emerging technology that involves a high speed protocol to move cells. Each ATM cell is 53 octets (bytes) long, which is divided into two fields. The header is 5 octets consisting of four octets of routing/switching and status information and one byte of header error correcting code. The payload is the remaining 48 octets used to carry the user�s information (e.g., data, voice, video, and image). The use of a small cell offers ATM two main advantages. First, the length of queuing time used to process in an ATM-based network is very short and manageable since cells from different kind of traffic sources (e.g., data, voice, video, and image) can be interleaved, Second, the use of small fixed size cells allows the ATM-based network to be switchable efficiently.

The ATM cell header field is used by the UNI, NNI, and switches to route the cells. This header field holds the information on dividual cell routing and connection type. The organization of the header will vary slightly depending on whether it contains information related to NNI or UNI. Three principal functions of the header field are virtual channel identification, error detection on the header, and unassigned cell indication. The other main functions performed by the header field are error correction on the header, cell loss detection, cell sequence numbering, terminal identifier, virtual path identification, and line equipment identification. In operation, header values are assigned to each section of a connection when required, and release when no longer required. The connection identified by the headers remains unchanged during the lifetime of a call.

* ATM Switch

ATM connections are most often implemented via an ATM switch, which performs the statistical multiplexing and virtual connection management functions. An ATM switch is capable of handling both types of virtual circuits: switched and permanent. For a switched virtual circuit (SCV), when an ATM switch receives a connection request signal, it examines the destination request, and verifies the sufficiency of memory, port, and bandwidth of the network before allowing a connection. An ATM permanent virtual circuit (PVC) switch has dedicated path through the switches for information transfer, therefore it is capable for handling a larger volume of information and allowing faster transferring than the use of an ATM SCV switch.


Business users are concerned with maximizing their investment in computing and data communications equipment. Users want a network where any one user can connect with any type of interfaces and protocols. This is the expectation that an ATM network is setting today. ATM provides a better application integration and speeds up application development. ATM offers a number of advanced data, voice, video and image applications with different performance, quality, and business requirements. For example, ATM can support television, and video telephony services ranging from 2 Mbps to 600 Mbps. ATM video conferencing will extend to applications in different locations to be able to communicate with each other. The ATM network will also be used for TV distribution services to be provided on the same transmission equipment for all voice, data, and video traffic. Some consumer and commercial service applications requiring the use of ATM are listed below.

· Multipoint videoconferencing service, video-on-demand, and on-line video libraries

· Telecommuting (work at home)

· Home entertainment/shopping service · E-mail and data/audio/voice Internet service

· Interactive multimedia

· Medical imaging

· Distributed data access

· Distance learning

· Videoconferencing

· LAN, LAN-WAN, and WAN-WAN interconnection.

Today, industry, government, and community leaders began to recognize innovation and excellence in use of ATM. Some ATM applications areas, whose successful stories can be found at "" are listed below.

· Business (e.g., GTE Telephone Operations National Network Monitoring and Support Network, The Virtual Corporation, Agile Cable Production Service for The Information Highway);

· Department Store (e.g. JC Penney);

· Education (e.g., Oregon ATM Network, Technical University of Delft, Gigaswitch/ ATM at La Trobe University, Shared Remote Teleclasses on the North Carolina Information Highway, St. Louis Public Schools District Facility LAN Infrastructure Model, NYNet)

· Energy (e.g., Westinghouse, Amoco);

· Entertainment (e.g., World Cup USA 1994 Tournament, Cinema of the Future);

· Financial Services (e.g., Lufkin, Donaldson, Jenrette);

· Government (e.g., Commonwealth Telecommunications Network NAVSEA, Sandia National Laboratories Supercomputer Consolidation Network)

· Health & Science (e.g., USC-ABC's Advanced Telemedicine Network, Penn Allegheny Health, ATM Testbed Network for Health Care in Neuro-Imaging, Congressional Healthcare Demonstration; Department of Surgery Grand Rounds, I-SNET Pilot Telemedicine Service, NIIT's National Telemedicine Demonstration, Teleradiology/Telemedicine Demonstration at the Radiological, Society of North American Conference);

· Research and Development (e.g., Monterey Bay Aquarium Research Institute, NASA, Cablevision Systems Corporation, Concurrent Technologies Corp.)


ATM is a technology that is being applied to support a variety of advanced applications. An ATM-based network, such as B-ISDN, can support very high speed interfaces for various classes of traffic - data, voice, video and image - in the one transmission and switching fabric. An ATM network can provide flexible bandwidth allocation and simple routing using connection oriented technology. ATM allows for the integration of networks improving efficiency and manageability.

ATM brings the digital network to the world. ATM offers dramatic increases in the speeds of data transfer, real-time videoconferencing, video-on-demand, and remote broacasting and audio transfer. ATM is well suited for emerging multimedia applications, such as interactive data, voice, video and imaging, suitably applicable for entertainment, telecommuting, home shopping, video on demand, global virtual collaboration, international product design, worldwide banking, video conferencing, distance learning, high speed image visualization, and tele-medicine. ATM will allow its network to bring increased capabilities, reduces costs, and improves productivity to organizations both large and small. Moreover, ATM using wireless capability and satellite links is under very serious consideration. ATM has a potential to be a center for interconnecting different types of networks to build the National Information Infrastructure.


[1] Andrew S. Tannenbaum, "Computer Networks", Prentice -Hall, Inc., 1981

[2] William Stalling, "ISDN and Broadband ISDN", Macmillian, 1992

[3] David E. McDysan, et .... "ATM", McGraw-Hill, Inc., 1995

[4] Craig Partridge, "Gigabit Networking", Addison-Wiseley, 1994

[5] Thao M. Le, "A Guide to ISDN and B-ISDN", GSA, 1994

[6] Thao M. Le, "ISDN and B-ISDN: Communications network Needs & Technologies", VACETS Technical Magazine, Volume 1, February 1996

Thao Mong Le
[email protected]

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Copyright © 1994 - 1998 by VACETS and Thao M. Le


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