Guides & Tutorials

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An example may serve to illustrate the concept of natural and synthetic objects. In a news session on television, a few seconds may be devoted to the weather. The viewers see a weather map of their local geographic region (a computer-generated image) that may zoom in and out and pan. Graphic images of sun, clouds, rain drops, or a rainbow (synthetic scenes) appear, move, and disappear. A person is moving, pointing, and talking (a natural scene), and text (another synthetic scene) may also appear from time to time. All those scenes are mixed by the producers into one audiovisual presentation that’s compressed, transmitted (on television cable, on the air, or into the Internet), received by computers or television sets, decompressed, and displayed (consumed). In general, audiovisual content goes through three stages: production, delivery, and consumption. Each of these stages is summarized below for the traditional approach and for the MPEG-4 approach.

Production. Traditionally, audiovisual data consists of two-dimensional scenes; it is produced with a camera and microphones and contains natural objects. All the mixing of objects (composition of the image) is done during production. The MPEG-4 approach is to allow for both two-dimensional and three-dimensional objects and for natural and synthetic scenes. The composition of objects is explicitly specified by the producers during production by means of a special language. This allows later editing.

Delivery. The traditional approach is to transmit audiovisual data on a few networks, such as local-area networks and satellite transmissions. The MPEG-4 approach is to let practically any data network carry audiovisual data. Protocols exist to transmit audiovisual data over any type of network.

Consumption. Traditionally, a viewer can only watch video and listen to the accompanying audio. Everything is precomposed. The MPEG-4 approach is to allow the user as much freedom of composition as possible. The user should be able to interact with the audiovisual data, watch only parts of it, interactively modify the size, quality, and resolution of the parts being watched, and be as active in the consumption stage as possible. Because of the wide goals and rich variety of tools available as part of MPEG-4, this standard is expected to have many applications. The ones listed here are just a few important examples.

1. Streaming multimedia data over the Internet or over local-area networks. This is important for entertainment and education.
2. Communications, both visual and audio, between vehicles and/or individuals. This has military and law enforcement applications.
3. Broadcasting digital multimedia. This, again, has many entertainment and educational applications.
4. Context-based storage and retrieval. Audiovisual data can be stored in compressed form and retrieved for delivery or consumption.
5. Studio and television postproduction. A movie originally produced in English may be translated to another language by dubbing or subtitling.
6. Surveillance. Low-quality video and audio data can be compressed and transmitted from a surveillance camera to a central monitoring location over an inexpensive, slow communications channel. Control signals may be sent back to the camera through the same channel to rotate or zoom it in order to follow the movements of a suspect.
7. Virtual meetings. This time-saving application is the favorite of busy executives. Our short description of MPEG-4 concludes with a list of the main tools specified by the MPEG-4 standard.

Object descriptor framework. Imagine an individual participating in a video conference. There is an MPEG-4 object representing this individual and there are video and audio streams associated with this object. The object descriptor (OD) provides information on elementary streams available to represent a given MPEG-4 object. The OD also has information on the source location of the streams (perhaps a URL) and on various MPEG-4 decoders available to consume (i.e., display and play sound) the streams. Certain objects place limitations on their consumption, and these are also included in the OD of the object. A common example of a limitation is the need to pay before an object can be consumed. A movie, for example, may be watched only if it has been paid for, and the consumption may be limited to streaming only, so that the consumer cannot copy the original movie.

Systems decoder model. All the basic synchronization and streaming features of the MPEG-4 standard are included in this tool. It specifies how the buffers of the receiver should be initialized and managed during transmission and consumption. It also includes specifications for timing identification and mechanisms for recovery from errors.

Binary format for scenes. An MPEG-4 scene consists of objects, but for the scene to make sense, the objects must be placed at the right locations and moved and manipulated at the right times. This important tool (BIFS for short) is responsible for describing a scene, both spatially and temporally. It contains functions that are used to describe two-dimensional and three-dimensional objects and their movements. It also provides ways to describe and manipulate synthetic scenes, such as text and graphics.

MPEG-J. A user may want to use the Java programming language to implement certain parts of an MPEG-4 content. MPEG-J allows the user to write such MPEGlets and it also includes useful Java APIs that help the user interface with the output device and with the networks used to deliver the content. In addition, MPEG-J also defines a delivery mechanism that allows MPEGlets and other Java classes to be streamed to the output separately.

Extensible MPEG-4 textual format. This tool is a format, abbreviated XMT, that allows authors to exchange MPEG-4 content with other authors. XMT can be described as a framework that uses a textual syntax to represent MPEG-4 scene descriptions.

Transport tools. Two such tools, MP4 and FlexMux, are defined to help users transport multimedia content. The former writes MPEG-4 content on a file, whereas the latter is used to interleave multiple streams into a single stream, including timing information.

Video compression. It has already been mentioned that compression is only one of the many MPEG-4 goals. The video compression tools consist of various algorithms that can compress video data to bitrates between 5 kbits/s (very low bitrate, implying low-resolution and low-quality video) and 1 Gbit/s. Compression methods vary from very lossy to nearly lossless, and some also support progressive and interlaced video. Many MPEG-4 objects consist of polygon meshes, so most of the video compression tools are designed to compress such meshes. Section 8.11 is an example of such a method.

Robustness tools. Data compression is based on removing redundancies from the original data, but this also makes the data more vulnerable to errors. All methods for error detection and correction are based on increasing the redundancy of the data. MPEG-4 includes tools to add robustness, in the form of error-correcting codes, to the compressed content. Such tools are important in applications where data has to be transmitted through unreliable lines. Robustness also has to be added to very low bitrate MPEG-4 streams because these suffer most from errors. Fine-grain scalability. When MPEG-4 content is streamed, it is sometimes desirable to first send a rough image, then improve its visual quality by adding layers of extra information. This is the function of the fine-grain scalability (FGS) tools. Face and body animation. Often, an MPEG-4 file contains human faces and bodies, and they have to be animated. The MPEG-4 standard therefore provides tools for constructing and animating such surfaces.

Speech coding. Speech may often be part of MPEG-4 content and special tools are provided to compress it efficiently at bitrates from 2 kbit/s up to 24 kbit/s. The main algorithm for speech compression is CELP, but there is also a parametric coder.

Audio coding. Several algorithms are available as MPEG-4 tools for audio compression. Examples are (1) advanced audio coding (AAC, based on the filter bank approach), (2) transform-domain weighted interleave vector quantization (Twin VQ, can produce low bitrates such as 6 kbit/s/channel), and (3) harmonic and individual lines plus noise (HILN, a parametric audio coder).

Synthetic audio coding. Algorithms are provided to generate the sound of familiar musical instruments. They can be used to generate synthetic music in compressed format. The MIDI format, popular with computer music users, is also included among these tools. Text-to-speech tools allow authors to write text that will be pronounced when the MPEG-4 content is consumed. This text may include parameters such as pitch contour and phoneme duration that improve the speech quality.

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H.261



In late 1984, the CCITT (currently the ITU-T) organized an expert group to develop a standard for visual telephony for ISDN services. The idea was to send images and sound between special terminals, so that users could talk and see each other. This type of application requires sending large amounts of data, so compression became an important consideration. The group eventually came up with a number of standards, known as the H series (for video) and the G series (for audio) recommendations, all operating at speeds of p×64 Kbit/s for p = 1, 2, . . . , 30.

Members of the p×64 also participated in the development of MPEG, so the two methods have many common elements. There is, however, one important difference between them. In MPEG, the decoder must be fast, since it may have to operate in real time, but the encoder can be slow. This leads to very asymmetric compression, and the encoder can be hundreds of times more complex than the decoder. In H.261, both encoder and decoder operate in real time, so both have to be fast. Still, the H.261 standard defines only the data stream and the decoder. The encoder can use any method as long as it creates a valid compressed stream. The compressed stream is organized in layers, and macroblocks are used as in MPEG. Also, the same 8×8 DCT and the same zigzag order as in MPEG are used. The intra DC coefficient is quantized by always dividing it by 8, and it has no dead zone. The inter DC and all AC coefficients are quantized with a dead zone. Motion compensation is used when pictures are predicted from other pictures, and motion vectors are coded as differences. Blocks that are completely zero can be skipped within a macroblock, and variable-size codes that are very similar to those of MPEG (such as run-level codes), or are even identical (such as motion vector codes) are used. In all these aspects, H.261 and MPEG are very similar. There are, however, important differences between them. H.261 uses a single quantization coefficient instead of an 8×8 table of QCs, and this coefficient can be changed only after 11 macroblocks. AC coefficients that are intra coded have a dead zone. The compressed stream has just four layers, instead of MPEG’s six. The motion vectors are always full-pel and are limited to a range of just ±15 pels. There are no B pictures, and only the immediately preceding picture can be used to predict a P picture.

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H.261



In late 1984, the CCITT (currently the ITU-T) organized an expert group to develop a standard for visual telephony for ISDN services. The idea was to send images and sound between special terminals, so that users could talk and see each other. This type of application requires sending large amounts of data, so compression became an important consideration. The group eventually came up with a number of standards, known as the H series (for video) and the G series (for audio) recommendations, all operating at speeds of p×64 Kbit/s for p = 1, 2, . . . , 30.

Members of the p×64 also participated in the development of MPEG, so the two methods have many common elements. There is, however, one important difference between them. In MPEG, the decoder must be fast, since it may have to operate in real time, but the encoder can be slow. This leads to very asymmetric compression, and the encoder can be hundreds of times more complex than the decoder. In H.261, both encoder and decoder operate in real time, so both have to be fast. Still, the H.261 standard defines only the data stream and the decoder. The encoder can use any method as long as it creates a valid compressed stream. The compressed stream is organized in layers, and macroblocks are used as in MPEG. Also, the same 8×8 DCT and the same zigzag order as in MPEG are used. The intra DC coefficient is quantized by always dividing it by 8, and it has no dead zone. The inter DC and all AC coefficients are quantized with a dead zone. Motion compensation is used when pictures are predicted from other pictures, and motion vectors are coded as differences. Blocks that are completely zero can be skipped within a macroblock, and variable-size codes that are very similar to those of MPEG (such as run-level codes), or are even identical (such as motion vector codes) are used. In all these aspects, H.261 and MPEG are very similar. There are, however, important differences between them. H.261 uses a single quantization coefficient instead of an 8×8 table of QCs, and this coefficient can be changed only after 11 macroblocks. AC coefficients that are intra coded have a dead zone. The compressed stream has just four layers, instead of MPEG’s six. The motion vectors are always full-pel and are limited to a range of just ±15 pels. There are no B pictures, and only the immediately preceding picture can be used to predict a P picture.

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Asynchronous Transfer Mode (ATM) Networks

Demand for fast and dependable access to Web-based applications and real-time delivery of multimedia transmissions via an integrated network infrastructure drives implementation of ATM (Asynchronous Transfer Mode) broadband solutions. ATM is a high-speed, high-performance multiplexing and switching technology that provides bandwidth on-demand for seamless transport of full-motion video, audio, data, animations, and still images in local and wider area environments.

A flexible and extendible telecommunications solution, ATM interlinks distributed networks and heterogeneous technologies into integrated configurations, thereby eliminating the need for multiple network overlays. ATM interworks with diverse narrowband and broadband wireline architectures, protocols, and technical solutions such as SONET/SDH (Synchronous Optical Network and Synchronous Digital Hierarchy), WDM (Wavelength Division Multiplexing), and DWDM (Dense WDM). ATM also interoperates with FDDI (Fiber Data Distributed Interface), Ethernet, Fast Ethernet, Frame Relay, ISDN, and IP (Internet Protocol) and wireline and wireless residential broadband access networks employing cable modem, DSL (Digital Subscriber Line), and VSAT (Very Small Aperture Terminal) solutions. In addition, ATM works in conjunction with second-generation GSM (Global System for Mobile Communications) and third-generation UMTS (Universal Mobile Telecommunications Systems) cellular communications technologies.

The ATM platform enables fast access to basic and sophisticated tele-education, telemedicine, electronic commerce (E-commerce), and electronic government (E-government) services. ATM networks are reliable, dependable, and scalable, and flexibly accommodate an array of topologies, applications, and services.


PURPOSE

ATM is a complex cell multiplexing and switching technology. This chapter provides a high-level introduction to ATM technical attributes, features, and functions. Representative ATM implementations that support a diverse and powerful mix of applications are examined. Wireless ATM (WATM) configurations are described, and the capabilities of next-generation ATM networks are explored. Research initiatives in the ATM arena are highlighted.


ATM DEVELOPMENT

The ATM platform enables multimedia transmission via fixed-sized 53-byte packets called cells in network environments ranging from desk area networks (DANs) to global implementations. The term “Asynchronous” refers to ATM support of intermittent bit rates and traffic patterns in accordance with actual demand. The phrase “Transfer Mode” denotes ATM multiplexing capabilities in transmitting and switching multiple types of network traffic.

Bell Labs initiated work on ATM research projects in the 1960s and subsequently developed cell relay technology and cell switching architecture for handling bursty transmissions. Originally, ATM was called Asynchronous Time-Division Multiplexing (ATDM) and regarded as a successor to TDM (Time-Division Multiplexing). As with TDM, ATDM supports transmission of delay-sensitive and delay-insensitive traffic. TDM and ATDM assign each fixed-sized cell or information packet to a fixed timeslot. By contrast, ATM supports dynamic allocation of timeslots to cells ondemand. In comparison to ATM, TDM and ATDM protocols are limited in optimizing utilization of available bandwidth for effectively handling volume-intensive multimedia applications.

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ATM TECHNICAL FUNDAMENTALS

An ATM broadband network enables dependable delivery of multimedia traffic in wireline and/or wireless environments. Prior to ATM implementation, individual networks carried data, voice, and video traffic separately on individual channels or circuits.


ATM CELL

ATM networks employ a standard, fixed-size 53-byte cell comprised of a 5-byte header and a 48-byte payload or information field as the basic unit of transmission. The 5-byte header includes an error detection field and a Virtual Channel Identifier (VCI) or Virtual Path Indicator (VPI) for transporting a cell payload to a destination address.

Through utilization of a common cell format, ATM enables real-time services, public and private network interconnectivity, and global interoperability. ISDN employs STDM (Statistical Time-Division Multiplexing) for enabling transmission of frames via designated timeslots at specified intervals. In contrast to ISDN installations, the ATM protocol supports dynamic allocation of timeslots to cells on-demand for optimizing traffic throughput in high-performance network configurations.

ATM technology employs a priority switching technique for enabling ATM cells carrying delay-sensitive signals to access the first available timeslot. Because the ATM cell size is fixed and the buffer memory size is constant for each cell, switch queuing delays are predictable and jitter or the variation in signal delay is minimized. By contrast, signal delays degrade performance of real-time applications such as videoconferencing and interactive video-on-demand (IVOD) in networks such as Frame Relay (FR) and Ethernet that transport variable length packets.

The ATM Forum defines procedures for monitoring the effectiveness of network transmission based on cellular throughput. Cell Loss Ratio (CLR) describes the percentage of cells that are not transported to their destination addresses as a consequence of buffer overloads and network congestion. Cell Transfer Delay (CTD) refers to propagation and queuing delays experienced by cells transiting the network.
Cell Delay Variation (CDV) measures variations in transmission delay between adjacent cells. Minimum Cell Rate (MCR) refers to the lowest cell rate supported by ABR (Available Bit Rate) service. In addition, metrics for Cell Delay Variation Tolerance (CDVT) and parameters for Maximum Cell Transfer Delay (MCTD) are also defined. The effectiveness of QoS delivery in ATM networks depends on such variables as Cell Transfer Delay (CDT) and Cell Delay Variation (CDV).


ATM APPLICATIONS

ATM is a connection-oriented virtual network transmission and switching technology that combines the low-delay of circuit-switched networks with the bandwidth flexibility and high-speed of packet-switched networks. ATM is an enabler of basic and advanced applications such as remote sensing, 3-D (three-dimensional) interactive simulations, tele-instruction, biological teleresearch, and medical teleconsultations. Edge devices at the boundary of an ATM network convert non-ATM traffic streams into standard ATM cells.
ATM technology is implemented in backbone, enterprise, and edge switches as well as hubs, routers, bridges, multiplexers, servers, server farms, and NICs (Network Interface Cards) in high-end Internet appliances. The ATM Data Exchange Interface (DXI) enables fast access to public network services. A flexible and extendible networking solution, ATM technology supports network configurations that include DANs (Desk Area Networks), LANs, MANs (Metropolitan Area Networks), WANs (Wide Area Networks), and GANs (Global Area Networks).


ATM TRANSMISSION RATES

ATM technology enables wireline transmissions via optical fiber, twisted copper pair, and hybrid optical fiber and coaxial cable media. To support bandwidth-intensive operations, the ATM platform multiplexes and relays a diverse mix of network traffic via optical fiber at rates that include 155.52 Mbps (OC-3 or Optical Carrier-Level 3) and 622.08 Mbps (OC-12). ATM also sustains speeds at 2.488 Gbps (OC-48), 10 Gbps (OC-192), and 13.21 Gbps (OC-255). Optical Carrier (OC) levels are sets of signal rates that describe digital transmission speeds over a fiber optic plant. In the United States, these levels are based on multiples of the base rate of 51.84 Mbps (OC-1).

In addition to optical fiber, ATM supports landline information transport via hybrid optical fiber and coaxial cable (HFC) connections, and ordinary twisted copper pair found in public networks such as the PSTN (Public Switched Telephone Network) at lower speeds. For example, ATM enables transmission at 1.544 Mbps or T-1 and DS- 1 (Digital Signal-1) in terms of the North American digital hierarchy. In the European Union, ATM supports information transport at 2.048 Mbps or E-1 (European-1) and DS-1 (Digital Signal-1) in accordance with European Union digital specifications. ATM also supports rates at 44.746 Mbps or T-3 and DS-3 in North America, and 34.368 Mbps or E-3 and DS-3 in the European Union. By enabling transport of concurrent voice, still images, video, and data traffic in local, municipal, and wider area configurations, ATM technology promotes development of an integrated and scalable multiservice network infrastructure that optimizes resource sharing and user productivity.


ATM PROTOCOL STACK

ATM services are based on protocol layer operations.The Physical Layer in the ATM protocol stack consists of the Physical Medium Sublayer and the Transmission Convergence Sublayer. These sublayers enable the use of diverse physical media, interfaces, and transmission speeds. In addition, these sublayers transform signals into electronic or optical formats, map and encapsulate IP packets into cells, and provision multiplexing services for transmitting cells over the same physical link. In addition, the Physical Layer defines the process for routing and switching cells in accordance with Virtual Path Identifiers (VPIs) or Virtual Channel Identifiers (VCIs). The ATM Physical Layer corresponds to Layer 1 or the Physical Layer of the Open Systems Interconnection (OSI) Reference Model.

Situated directly above the Physical Layer, the ATM Layer employs the 53-byte cell as the basic transmission unit. The ATM Layer operates independently of the ATM Physical Layer. At the ATM Layer, ATM switches route cellular streams received from the ATM Adaptation Layer (AAL) to destination addresses in accordance with the Virtual Channel Identifier (VCI) or Virtual Path Indicator (VPI) contained in each cell header.

Situated directly above the ATM Layer, the ATM Adaptation Layer (AAL) facilitates cell segmentation and reassembly by dividing media streams received from upper protocol layers into 53-byte ATM cells for transmission over the physical link. For reassembly, the AAL reconstitutes original media streams from cells that were received via the Physical Layer. The reconstructed media streams are then ready for transport by upper protocol layers to destination addresses. The ATM Adaptation Layer (AAL) consists of five sublayers, ranging from AAL1 to AAL5. Each sublayer has specific and overlapping responsibilities for enabling dependable and robust transmissions. AAL1 supports cell segmentation and cell reassembly and the transmission of data, video, and high-quality audio signals for enabling real-time applications. AAL2 enables transport of connectionoriented VBR (Variable Bit Rate) packetized video-over-ATM and voice-over-ATM. Defined in the ITU-T I.363 Recommendation, AAL Sublayers 3/4 (3 and 4) handle connectionless and connection-oriented VBR transmissions. As with AAL1, AAL5 supports cell segmentation and cell reassembly operations.

The ATM Forum describes the features and functions of the ATM protocol stack and clarifies procedures for interworking ATM with technologies that include DSL (Digital Subscriber Line), SONET/SDH (Synchronous Optical Network and Synchronous Digital Hierarchy), WDM (Wavelength Division Multiplexing), and DWDM (Dense WDM). In addition, approaches for enabling ATM to interwork with SMDS, Frame Relay, cable modem, MMDS (Multichannel Multipoint Distribution System), and LMDS (Local Multipoint Distribution System) implementations are also clarified. The ATM Forum works with other standards organizations such as the Digital Video Broadcasting/Digital AudioVisual Council (DVB and DAVIC) and the Full Service Access Network (FSAN) Consortium in developing specifications for ATM television broadcasts and APONs (ATM Passive Optical Networks). APONs interwork with FTTC (Fiber-to-the-Curb) and FTTH (Fiber-to-the-Home) broadband residential access networks.

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ATM OPERATIONS

SVCs (SWITCHED VIRTUAL CIRCUITS) AND PVCS (PERMANENT VIRTUAL CIRCUITS)

ATM networks sustain point-to-point links for direct connectivity, point-to-multipoint connections for broadcast and multicast services, and multipoint-to-multipoint connections for applications such as interactive videoconferencing and telecollaborative teleresearch. Path specifications for moving traffic across ATM networks are termed Switched Virtual Circuits (SVCs) and Permanent Virtual Circuits (PVCs).

SVCs are created virtually on a semi-permanent basis for enabling multimedia transmission. SVCs establish connections on a call-by-call basis for accommodating bursty transmissions and bandwidth on-demand. UBR (Unspecified Bit Rate) service for SVC (Switched Virtual Circuit) connections supports information delivery on a best-effort basis. SVC connections do not guarantee the availability of bandwidth for enabling QoS (Quality of Service) transmissions.

In comparison to SVCs, PVCs are static virtual connections between network endpoints that support always-available and assured bandwidth allocations for current and emergent network applications and services. As a consequence, PVCs enable stable, dependable, and reliable transmission of voice, video, and data traffic with QoS guarantees.


ATM SWITCHES

ATM networks consist of routers, servers, switches, and endpoint devices such as network nodes and stations. The ATM switch family includes workgroup, campus, enterprisewide, and next-generation switches that provide services in a variety of LAN, MAN, and WAN environments. For example, ATM switches enable LATMs (Local Area ATM Networks) to provision services to legacy workstations and support sophisticated network backbone operations for advanced academic and research networks.

ATM multiservice switches provide the underlying physical infrastructure for the network configuration and control network processing speed. These devices uniformly facilitate cell relay operations, sustain throughput and end-to-end network performance, interlink nodes on ATM networks, and route multiple cells concurrently to destination addresses. It is important to note that ATM switches also support diverse applications, services, and operations, and vary in structure, capacity, value-added capabilities, interoperability support, and traffic management functions in order to accommodate a wide range of E-government (electronic government), E-business (electronic businesses), telemedicine, teleresearch, and/or tele-education requirements.


User-to-Network Interfaces (UNIs), Network-to-Node and Network-to-Network Interfaces (NNIs), and Private Network-to-Node or Network-to-Network Interfaces (PNNIs)

ATM installations consist of a set of ATM switches or internetworking devices that are interconnected by point-to-point ATM interfaces. ATM interfaces or virtual connections include User-to-Network Interfaces (UNIs) and NNIs (Network-to- Node Interfaces or Network-to-Network Interfaces). UNIs are ATM protocols that define standard interfaces between customer premise equipment (CPE) and the network switch. For example, FUNI (Frame UNI) clarifies parameters for integrating legacy devices with ATM switching equipment in mixed-mode Frame Relay and ATM network configurations.

PNNIs (Private Network-to-Node or Private Network-to-Network Interfaces) are NNI protocols that define ATM interfaces within and between private networks. PNNIs determine approaches for routing ATM connection-oriented requests across an ATM network or between ATM networks.

Moreover, PNNIs employ signaling technologies to support SVCs and PVCs in multivendor environments, provision QoS guarantees, and foster distribution of reserved bandwidth. PNNIs also establish the format for the Broadband-Intercarrier Interface (B-ICI) between public networks for enabling seamless multicarrier multivendor multiservice ATM implementations.


ATM CLASS OF SERVICE (COS) AND QUALITY OF SERVICE (QOS)

ATM networks employ Classes of Service (CoS) for optimizing network performance and supporting applications with specified bandwidth or throughput requirements. ATM service classes resolve congestion problems and traffic management issues in order to ensure seamless transmission in multivendor environments. A Class of Service (CoS) refers to a category of ATM connections that features identical traffic patterns and resource requirements. Each class provisions a distinct level of service and associated QoS guarantees. Depending upon the format of the QoS service requested, the ATM network defines a series of CoS categories. The Variable Bit Rate (VBR) Class of Service consists of applications with specific requirements for delays and throughputs such as packetized voice and data applications. The real-time Variable Bit Rate (VBR-rt) Class of Service requires real-time support for provisioning applications such as video-on-demand (VOD) and voice-over-IP (VoIP). VBR-rt bandwidth requirements vary over time. However, delay and delay variance limits are clearly established.

The non-real-time variable bit rate (VBR-nrt) Class of Service eliminates the need for guaranteed delivery of applications such as multimedia e-mail, bulk file transmissions, and business and educational database transactions with minimal service requirements. Bandwidth for VBR-nrt applications varies within a specified range. However, delay and delay variance requirements are not fully defined. The Available Bit Rate (ABR) Class of Service requires the use of flow control mechanisms for ensuring allocation of bandwidth on-demand for non-real-time, mission-critical applications. With ABR applications, guaranteed minimum transmission rates are specified for the duration of the connection. In addition, ABR also establishes peak transmission rates for data bursts when bandwidth is available. As a consequence, the ABR service class tolerates delay variations. Applications grouped into this category allow priority traffic to consume bandwidth first. ABR applications include LAN emulation (LANE), file and data distribution, and LAN interconnections.

The Unspecified Bit Rate (UBR) Class of Service is equivalent to best-effort delivery in IP networks. Delay-tolerant UBR applications include Web browsing and IP transmissions. Because UBR applications require minimal network support, QoS guarantees and pre-established throughput levels are not defined. The Constant Bit Rate (CBR) Class of Service (CoS) requires utilization of a virtual channel with constant bandwidth for seamlessly transporting applications in accordance with pre-defined response time requirements. CBR applications include videoconferencing, telephony services, and television broadcasts.

In conjunction with establishing a CoS, ATM networks define cell rates and burst size to facilitate seamless network performance. For example, Peak Cell Rate (PCR) indicates the maximum rate at which cells transit the network for brief time periods. Sustainable Cell Rate (SCR) refers to the cell rate that is sustained for a specified period of time. Maximum Burst Size (MBS) defines the maximum number of back-to-back cells that transit the network.

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IP-OVER-ATM

The popularity of IP (Internet Protocol) applications contributes to implementation of IP overlays on top of multiservice ATM networks. IP-over-ATM solutions employ protocols such as MPOA (MultiProtocol-over-ATM) and MPLS (MultiProtocol Label Switching) for leveraging IP enhancements. Moreover, IP packets can be mapped to ATM service classes to transport, for instance, IP-based voice and video traffic via CRC and VBR-rt. Additionally, IP-over-ATM implementations support VPNs (Virtual Private Networks) and ATM emulated LAN (ELAN) protocols that work in concert with the Network Layer or Layer 3 and the Transport Layer or Layer 4 of the OSI (Open Systems Interconnection) Reference Model. Sponsored by the ATM Forum, the ATM-IP Collaborative Working Group (AIC) develops specifications for coordinating the provision of IP services with ATM technology. Approaches for mapping ATM QoS to the IP DiffServ (Differentiated Services) protocol are also in development.

In order to interoperate with IP packet-switched services, ATM defines a framing structure for carrying IP packets as sets of ATM cells. ATM PVCs (Permanent Virtual Circuits) support virtual connections within an IP network. In provisioning IP integrated services over an ATM infrastructure, a portion of the available bandwidth is reserved for specified CoS transmissions. By employing fixed capacity virtual connections for designated CoS transmissions, the ATM infrastructure guarantees the availability of reserved bandwidth on-demand.


CIP-OVER-ATM (CLASSICAL IP-OVER-ATM)

Protocols for preserving in-place infrastructure investments in ATM environments include Classical IP-over-ATM (CIP-over-ATM). A CIP-over-ATM solution employs Permanent Virtual Circuits (PVCs) or dynamic Switched Virtual Circuits (SVCs) for transporting IP packets to ATM addresses. Moreover, CIP-over-ATM deployments enable access to ATM services and connectivity to legacy IP applications. CIP-over- ATM implementations require modification of the IP Address Resolution Protocol (ARP) in order to establish ATM connections that correspond to IP addresses.


MULTIPROTOCOL-OVER-ATM (MPOA) PROTOCOL

Endorsed by the ATM Forum, the MPOA (MultiProtocol over ATM) protocol defines Network Layer or Layer 3 services for enabling ATM implementations. IMPOA employs Next Hop Resolution Protocol (NHRP) for mapping IP packets to ATM cells at AAL5 of the ATM protocol stack. In addition, MPOA routes ATM traffic directly between ELANs (Emulated LANs) and employs SVCs (Switched Virtual Circuits) to ensure reliable and dependable voice, video, and/or data delivery to destination addresses. Robust transmissions are achieved by reducing the number of nodes participating in the internetwork transmission process.

With MPOA, network stations or nodes on different subnetworks establish Permanent Virtual Connections (PVCs) or shortcuts, thereby eliminating the need for intermediate cell segmentation and cell reassembly. In contrast to MPOA, LANE (LAN Emulation) and CIP protocols use intermediate routers for enabling intercommunications between subnetwork nodes. This process limits ATM transmission rates and the amount of voice, video, and data throughput transported via the network by requiring intermediate cell segmentation and reassembly.


MULTIPROTOCOL LABEL SWITCHING (MPLS)

Developed by the IETF (Internet Engineering Task Force), the MPLS (MultiProtocol Label Switching) protocol enables the provision of merged IP and ATM services within the same networking environment. To accomplish this objective, the MPLS protocol interlinks the IP Layer and the ATM Layer and interconnects IP routers and ATM switches, thereby enabling IP transmissions to take advantage of ATM traffic management capabilities in provisioning CoS assurances. IP also benefits from ATM broadband transmission rates for enabling high-speed and dependable multimedia delivery.

MPLS technology enables operations at the Data-Link Layer or Layer 2 of the OSI Reference Model, supports connection-oriented switching based on IP routing and control protocols, and employs fixed-length labels for rapidly routing transmissions to destination addresses. The MPLS protocol works in concert with its own LDP (Label Distribution Protocol) in establishing links and shortcuts in accordance with IP addresses, ATM CoS requirements, and ATM QoS guarantees.

MPLS implementation requires development of a Label-Switching Path (LSP) for handling volume-intensive traffic that takes a specific destination route over the network and supporting identification of a communications channel with high capacity and minimal congestion to accommodate application bandwidth requirements. MPLS solutions optimize network performance, control network operating costs, minimize congestion, decrease the number of information packets dropped as a consequence of network instability, and provision preferential service for delivery of priority transmissions. The MPLS protocol works in concert with IPv4 (Internet Protocol version 4) and supports migration to IPv6 (Internet Protocol version 6) operations. In addition to ATM, the MPLS protocol optimizes performance of network configurations based on POS (Packet over SONET/SDH), Frame Relay, Ethernet, Fast Ethernet, and Gigabit Ethernet technologies.

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ATM LAN EMULATION (LANE)

ATM LANE FUNDAMENTALS

ATM LAN emulation (LANE) enables virtual LAN (VLAN) implementations across ATM backbone networks that reflect the logical associations of workgroups regardless of the physical location of workgroup participants. Modifications in virtual ATM LANE topologies are accomplished by redefining workgroups in the network management system and reconfiguring software in ATM switches. MPOA (MultiProtocol-over-ATM) enables direct transmission of virtual ATM LANE traffic over the ATM Physical Layer or Layer 1 of the OSI Reference Model. The Cells-in-Frames (CIF) Alliance supports implementation of ATM desk area networks (DANs) that operate in concert with the virtual ATM LANE infrastructure. ATM LANES are also called ATM ELANs (Emulated LANs).


ATM EMULATED LANS (LANES) IN ACTION

ATM LANEs are scalable and flexible, feature sophisticated network management and control capabilities, and perform functions equivalent to those supported by conventional Ethernet and Token Ring VLANs (Virtual LANs). ATM LANES enable each participant in a logical workgroup to take part in collaborative networking activities. Internetworking devices such as bridges and routers support voice, video, and data exchange between participants in enterprisewide ATM LANEs.

In an ATM LANE, local networking applications access an ATM network con- figuration via IP protocols. IP packets are transported in ATM cells. ATM LANEs use LAN Emulation User-to-Network Interfaces (LUNIs) and LAN Emulation Network- to-Node or Network-to-Network Interfaces (LNNIs) to provision QoS guarantees, Internet telephony, and connectionless unicast and multicast delivery. ATM LANEs support MAC (Medium Access Control) operations at the Data-Link Layer or Layer 2 of the OSI Reference Model.

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ATM IMPLEMENTATION CONSIDERATIONS

In the academic arena, ATM technology facilitates fast, reliable, and dependable access to an expanding array of Web initiatives and institutional resources. ATM enables tele-education, telementoring, and real-time interactions with subject experts in remote locations; multimedia applications; and curricular enhancement and enrichment. ATM also promotes deployment of virtual schools, virtual universities, virtual museums, and virtual communities.

ATM pilot trials and initiatives support the design and implementation of extendible, reliable, and scalable ATM configurations to accommodate current and anticipated network requirements. In addition, the ATM platform delivers high-capacity, high-speed multimedia services and applications. However, it is also important to note that major regulatory, technical, logistical, and economic issues associated with ATM deployment remain unresolved. As a consequence, the ATM acronym also stands for “All That Money.”

ATM is an evolving technology. As a consequence, standards and testing methods are still in development. Congestion on ATM networks can lead to cell loss before traditional network tools detect problems. Problems associated with providing effective traffic management, seamless network performance, and network-level security for information integrity and high-speed interactive data, video, and voice delivery must be resolved through further research. ATM functions are also constrained by the lack of cross-vendor support.

Migration to an ATM solution typically requires acquisition of ATM products and services from a single vendor. The majority of ATM switches in use by early adopters of ATM technology are expected to be incompatible with next-generation ATM switches. As a result, replacement of expensive in-place ATM switches with costly next-generation ATM switches appears to be necessary for enabling ATM services.

Successful ATM deployment requires the use of carefully executed measures to manage traffic flows and accommodate application requirements. Inasmuch as ATM support of multiple QoS parameters contributes to difficulties in managing ATM configurations, development and implementation of network management policies are indispensable in facilitating realization of the full potential of ATM technology.

An understanding of ATM technical capabilities is essential in order to effectively address pedagogical challenges associated with ATM implementation. Although ATM supports multifaceted options for information delivery to the desktop, SOHO venues, and local and wider area environments, deployment of ATM technology does not automatically guarantee its effective utilization in the educational domain. In implementing ATM applications and services in school and university environments, the capabilities of the proposed infrastructure must be determined. Requirements for a high-performance ATM infrastructure that is modular, reliable, secure, expandable, and available to accommodate bandwidth demands over time must be clarified. Effective ATM implementation in the tele-education milieu also involves developing ATM telelearning paradigms for supporting problem-solving skills and accomplishment of learning goals and objectives. Effective ATM deployment in the telelearning environment ultimately depends on its ability to foster knowledge-building competencies and exploratory learning, quality education, and focused research and facilitate instructional innovation and creativity. Future research involving ATM deployment in school and university settings must also focus on the practical design and deployment of pedagogical strategies and collaborative instructional activities for optimizing student skills in broadband tele-education environments.

In the broadband networking arena, ATM’s major competitor is Gigabit Ethernet technology. Gigabit Ethernet technology is compatible with the installed base of Ethernet and Fast Ethernet solutions in local area and wider area network environments. In comparison to ATM, Gigabit Ethernet does not provision information transport with QoS guarantees. However, Gigabit Ethernet leverages capabilities of newer technologies and protocols such as the Resource Reservation Protocol (RSVP) and the MultiProtocol Link Aggregation (MPLA) protocol to support scalable bandwidth, fault tolerance, network resiliency, and streamlined packet transmission for provisioning higher-level networking services. In addition, Gigabit Ethernet implementations are more affordable and easier to implement than complex ATM solutions.


SUMMARY

There is a growing consensus that ATM reliably and dependably accommodates requirements for high-speed, high-performance networking operations while also enabling a seamless migration path to the network of the future. Increasing numbers of ATM field trials and full-scale implementations demonstrate ATM capabilities in providing access to worldwide learning resources and supporting innovative telelearning activities and applications.
Distinctive attributes of major national and international ATM initiatives and research efforts that contribute to establishing a global ATM infrastructure are examined. ATM systems featuring a mix of wireline and wireless technologies for enabling transborder interdisciplinary research and global connectivity to innovative instructional programs are explored.

ATM technology is uniquely suited for supporting error-free multimedia transport in high-speed network configurations. Moreover, ATM is an enabler of network traffic consolidation, thereby streamlining network management operations and optimizing utilization of high-speed network connections. In addition, ATM provisions networking services via twisted copper pair, optical fiber, and hybrid optical fiber and coaxial cable (HFC) wireline media and wireless technical solutions. National and international standards organizations such as the ITU-T, the Institute of Electrical and Electronic Engineers (IEEE), the American National Standards Institute (ANSI), and the European Telecommunications Standards Institute (ETSI) endorse ATM specifications.

ATM solutions are designed to function in multiservice, multivendor environments. However, debate persists about the suitability of ATM technology in accommodating mission, goals, and requirements economically and effectively in the academic arena. Potential barriers to ATM deployment include high costs, lack of universally accepted standards, restricted geographical availability, equipment incompatibilities, and insufficient research data on the capabilities of ATM in provisioning Quality of Service (QoS) guarantees.

Despite these constraints, ATM is regarded as a key enabler for tele-education, telebusiness, E-government, and telemedicine applications. ATM provisions dependable Internet, intranet, and extranet connectivity; facilitates implementation of Virtual Reality (VR) applications; and supports reliable access to broadband multimedia services.

ATM networks resolve problems associated with internetwork congestion and enable seamless voice, video, and data transmission over wireless, wireline, and hybrid wireline and wireless network configurations. In the distance education domain, ATM enables access to new student populations in remote locations, promotes transborder research and telecollaboration, and facilitates curricular enrichment. Globally, ATM technology supports development and deployment of major research and education networks such as Abilene, vBNS+, Internet2, ESnet, CA*net II, and SuperJANET4. Moreover, ATM promotes incorporation of emergent network architectures, protocols, and transmission technologies into an integrated infrastructure. Continued research on the design and implementation of pedagogical approaches and methods for supporting student learning and achievement in ATM instructional settings is essential for achieving effective ATM implementation in school and university environments.

[Addyklub]

thank you very much