ACQUIRING AND USING ASYNCHRONOUS TRANSFER MODE IN THE WORKPLACE Asynchronous Transfer Mode (ATM) is a revolutionary new data communications technology that utilizes a high-speed switching protocol to quickly move cells (each containing 53 bytes) between two or more distinct points in a network. ATM uses cell switching for the high-speed transmission of data over a variety of subnetwork technologies; the media used and transmission rates are not a function of ATM. This bulletin assists users in selecting and implementing ATM technology and enables users to properly assimilate ATM products into the workplace. Introduction ATM works by accomplishing two simple functions: cell-switching (in hardware) and path (next hop) determination (virtual connection management done primarily in software). ATM offers the capability of high-speed data transmission with minimum delay, because only these two functions can reside at the network node, eliminating the overhead of additional functionality. The connection which is created, called a virtual circuit (VC), is similar in concept to a telephone connection, which remains in effect until the connection is terminated. Each VC is characterized by a virtual circuit identifier (VCI), which distinguishes one virtual circuit from another. This concept is similar to distinguishing one telephone conversation from another on the same trunk line. The ATM Switch Users most often implement ATM connections via a device called an ATM switch. An ATM switch can handle multiple bit streams, in the same way that a large telephone switch handles multiple conversations from different points. A bit stream is assigned to a port (access point on the ATM switch), either statically or dynamically. Each port has a port identifier, which is known to the ATM switch. Different ports may support different data rates, depending on the physical medium to which the port is attached. There is always a pairing of input port and output port. Each port can handle multiple conversations (VCs); different conversations are identified by their VCI values. Semantically, a conversation may be a particular information type, such as voice or video, or have another meaning of interest to an application. ATM Network There are two kinds of ATM devices: ATM switches and ATM access devices. A network using ATM consists of one or more ATM switches, which connect different ATM access devices, where ATM Adaptation Layer (AAL) functionality (and possibly "higher level" functionality) is implemented. (AAL enhances ATM services for distinct classes of applications as necessary.) ATM access devices send ATM cells that can be processed by the ATM switches. The term "ATM network" is used in this bulletin to denote a network using ATM as the underlying communications technology. Since the ATM network is a subnetwork, the AAL functions are terminated at the ATM network boundaries, and so the AAL functions are not necessarily end-to-end. Thus a transport protocol is necessary to have full end-to-end interoperability. For example, a Transmission Control Protocol/Internet Protocol (TCP/IP) application may use an ATM network (including AAL services) to provide this end-to-end interoperability. Accordingly, the "higher layers" mentioned above denote various services or applications that may use the AAL. Examples of these are a connectionless service, a local area network (LAN) application, a broadband ISDN application, or a user-defined or proprietary application. ATM Interfaces There are two levels of ATM interfaces: a network-to-network interface (NNI) and a user-to-network interface (UNI). AAL functionality, as well as "higher layer" functionality (e.g, routing, transport, and application functionality), may be implemented at the UNI. At the UNI, many terminals can share a given ATM access. Different services are integrated at the UNI, and each service may be associated with one dedicated terminal or a multimedia terminal. The interconnected terminals may form a customer premises network (CPN). When a shared medium is used for ATM access, there is a need to perform contention resolution and bandwidth sharing. There are many methods of implementing the UNI. The UNI is contained in ATM access devices. The NNI governs the interface between different ATM switches; it is more complex than the UNI and is yet to be found in many switches. Different switches may have differing requirements; the NNI must reconcile these differences. Features of ATM ATM data is designed to be transported over high-quality digital lines with minimal cell errors; it is a "fast packet" service. ATM is an ideal transmission mode for bursty traffic prevalent in today's data networks. It provides bandwidth (capacity) on demand and high bandwidth to every host. ATM reduces network response time, allows efficient network usage, and provides for scaling to very large or very fast networks. ATM can accommodate both a wide range of transmission rates (each rate corresponding to a different information type, such as voice, video, etc.) and unpredictable traffic patterns. It can support narrowband as well as broadband uses. ATM connections can be point-to-point or point-to-multipoint. Small cell sizes minimize delay, and cell sequencing means that cells are delivered in order. ATM eliminates the need for separate voice, video, or data networks. It provides circuit emulation (simulating a normal circuit) and allows flexible networks to be built using virtual path (VP) connections. ATM can support a unified addressing capability among these systems. It can carry data, audio, or full-motion video. ATM can be public or private, and can be included in wired or wireless systems. Total throughput is increased by adding more ATM switches and more links at higher rates. The simplicity of both the algorithm for forwarding ATM cells and the constant cell size leads to efficient hardware implementation. ATM is based on a network of switches and dedicated links to each host, so that the aggregate bandwidth of an ATM network increases as hosts are added. Data transfer by a large number of hosts can occur in an ATM network. Since there is a large amount of bandwidth on an ATM network, a user should be able to send information on demand with minimal delay. ATM Products In chronological order, ATM product offerings are likely to be: - ATM local area networks (LANs) - campus ATM (connecting LANs) - ATM wide area networks (WANs) - broadband ISDN and cell-relay services offered by subscription from public carriers. ATM LAN products exist now, but from only a few vendors. An ATM LAN comprises one or more ATM switches, dedicated high-speed circuits from each switch to networking elements, and the networking elements (such as workstations) themselves. All of the equipment is owned by a single organization, the distances involved are small, and the ATM switch may "broadcast" a cell to multiple destinations. ATM LANs differ from traditional LANs (e.g., Ethernet) in that they do not use shared media. In addition, ATM switches can be directly connected to LAN servers, LAN switches, or Ethernet hosts. ATM LANs can be used for client-server computing involving workstations and servers. Inhibitors to deployment are incompatibilities between ATM and NICs (network interface cards) for PCs and workstations, and the lack of development of a widespread user market for ATM LANs. These factors may set widespread deployment of ATM LAN systems back several years. Since campus ATM systems interconnect ATM LANs, hubs, bridges, and routers throughout a campus, fewer campus ATM products exist than LAN ATM products. Widespread campus ATM products may also not be available for several years. ATM in the WAN refers to carrier service offerings based on switching ATM cells from a network-entry access location to a remote-destination access location. The address information could be connection-oriented or connectionless. Widespread ATM WAN product offerings are a number of years away. The inhibitors to deployment include economic feasibility and a possible shortage of applications which could benefit from ATM WAN technology. The introduction of widespread broadband ISDN and cell-relay services will not occur until near the end of the decade, and will await the evolution of broadband ISDN and the stabilization of access rates offered by all public carriers. In addition, signalling concerns and concerns over the variety of physical media may delay introduction of ATM on public carriers. Acquiring ATM Products Users must make decisions on a number of important issues when procuring and installing ATM products. The choices that users make will affect the operation of the ATM product for them. Following is a list of concerns and the factors to be considered for each concern. ATM Access The characteristics of ATM access are important to users because user applications have specific transfer requirements and need to know what capabilities to request of an ATM network through the UNI, including type and number of connections. In most cases, users will not procure ATM switches directly, but will have access to ATM switch service provided by others. Users need to determine the types of physical media in their ATM environments, because this affects cost and performance. Users should buy ATM access equipment appropriate to their configuration, application and functional requirements, and appropriate to interfaces with other technologies coexistent with ATM. Users need to look at the state of ATM product offerings to meet the above requirements. For example, users may want to replace private network connections with access to public ATM services; users should ensure that those services are available before considering such a replacement. Users should consider the number and type of ATM access ports (bit streams) supported by the ATM access device. ATM access ports can be directly connected to multimedia workstations and central servers, or can be connected indirectly to hosts through shared-access LANs such as Ethernet. ATM access devices can have other ports (connected to diverse devices) in addition to ATM access ports. ATM access devices could be connected to gateways, routers, or hubs. Synchronous interfaces can be built into an ATM access device. Users need to determine the possible destinations of ATM connections in their ATM access devices. For example, ATM access devices can link to local telephone companies and cable system providers. In an enterprise network, there are a number of different devices connected via an ATM network to an ATM access device, and the ATM access device must recognize these other devices. The more types of devices recognized, the more flexible is the ATM access device. ATM Switches When procuring an ATM switch, users should specify the number of input and output ports supported by the switch, as well as the speeds supported for each port. There are several ATM switch types, and not all of them interwork. For signalling purposes, an ATM switch needs to know to which kind of ATM switch it is connected. Users should determine to which switch types their ATM equipment will be connected, and ensure that the switches can effectively interwork. All ATM switches support permanent virtual circuit (PVC) and UNI capability, but more and more switches are supporting switched virtual circuit (SVC) and NNI capability; users should specify if these additional capabilities are needed. ATM switches must (if the application demands) be able to handle SVC calls, and must be able to handle PVC calls if necessary. Applications where SVC capability is important include videoconferencing and transmitting trading information, as well as transmitting 3-D images over multiple routes to multiple destinations. Ports on ATM switches should be flexibly assigned, and must keep traffic counts on different service classes so that usage-based billing can be supported. ATM switches can have non-ATM interfaces (e.g., Ethernet) directly connected to them. ATM switches must also segregate traffic with differing delay- tolerance characteristics (for example, file transfer vs. interactive video). The time delays supported by ATM switches vary widely from nanoseconds (jitter) to microseconds, and could pose problems for continuous bit-rate traffic, such as voice and videoconferencing. Users need to specify acceptable delay- tolerance characteristics for ATM switches. Security When procuring and installing ATM equipment, security is important because users want to ensure that data that passes through an ATM switch has its integrity preserved, is subject to access control restrictions, and is subject to authentication and identification restrictions. The scope of security for ATM includes aspects of network security, as well as administrative and operational security. ATM products are still in their infancy, so a number of features have yet to be defined. High-speed ATM traffic poses difficulties with network security. Current methods of data encryption are unable to keep up, because in real-time processing, encryption times are too slow for the data rates. With ATM, (possibly replicated) cells can be sent on different paths to a destination, thus "scattering" the data; this is a possible network security mechanism. Users should ensure that their network (and other) security concerns are satisfied in ATM products. Management Users want to access resource management, performance, accounting, and configuration information for ATM networks. They will want to know, for a call placed through an ATM access device, did the call complete? The payload ID field of an ATM cell header distinguishes management information from user and other signalling information in an ATM cell. Users should ensure that management concerns (such as resource management) are satisfied by placement of appropriate information in this field. Resource management involves the proper allocation of memory, processors, and devices to support the requirements of an ATM network. Performance is important in ATM networks. ATM networks must be able to ensure consistent performance in the presence of widely varying traffic characteristics, because many high-speed applications require guaranteed throughput. ATM access devices are responsible for guaranteeing good performance for a mix of traffic types in an ATM network. ATM access devices can perform traffic management. Traffic prioritization and load balancing are examples of performance metrics for ATM access devices. Other performance metrics include device data rate (throughput) and full (peak) ATM cell rate, both in number of cells/second, number of PVC and SVC connections supported per module, and the time it takes to set up an ATM connection. ATM access devices can also offer different connection qualities of service. Users should specify a desired quality of service. ATM access devices should be efficient, in that all existing paths should be completely filled with cells before new connection requests occur. ATM connections are refused if insufficient bandwidth is available at connection request time, so users should specify the bandwidth reservation capability. ATM switches may be implemented with inadequate congestion management capabilities. The buffers may be too small and they may discard data when the network becomes too congested. There is also concern about traffic delays. An alternative to a congestion management algorithm is over-provisioning of existing lines. Configuration may involve setting up buffer management parameters. Buffer management ensures that different types of traffic will be handled according to the quality of service each requires. Users should specify appropriate buffer management capabilities. The Future of ATM ATM technology is a unifying communications paradigm; it insulates protocols from underlying changes in the physical medium. Use of the ATM technology promises: - cost savings - speed improvements - flexibility gains in the design and operation of networks - consolidation of voice, data, video and other information types Whether the user's need is for wide area connectivity or LAN links, ATM promises to bring the swift and sure transport of information. Any application or service can be supported by cells on an ATM network. ATM makes the interface between LANs, WANs, and MANs transparent to users. ATM provides better application integration and speeds up application development. By far the most important advantage that ATM offers over competing technologies is its open-ended growth path. As a LAN architecture, ATM is not locked into a single physical medium or speed. Instead, it can utilize any combination of speeds and a variety of media, including metallic and optical. There are many opportunities for cost reduction in designing ATM networks. ATM can be implemented without affecting the local user environment. The ATM technology separates the basic transfer mechanism (that is, cell relaying) from policies governing routing, quality of service, and security, facilitating the efficient interconnection of administrative domains. ATM networks provide access to centrally stored video information through the network, two-way or multipoint videoconferencing, and remote classroom instruction using video. For distributed computing, ATM is being used as the interconnection network for a distributed memory multiprocessor. ATM networks can also be used in: - LAN interconnectivity - publishing (newspapers, journals) - healthcare (such as telemedicine) - multimedia libraries - financial applications - government - entertainment Cable companies are experimenting with ATM for delivering real- time interactive TV and video on demand. Some ATM LAN applications are: - transport of real-time video - development of new transport protocols - parallel computation on ATM-connected workstation clusters - development of distributed multimedia applications For multimedia, the real-time traffic can be sent directly over an ATM connection, with or without AAL processing, as appropriate. Corporate applications using the ATM technology include: - desk-to-desk videoconferencing - multimedia conferencing - multimedia messaging - distance learning - imaging tasks (including CAD/CAM) - animation - cooperative work (for example, joint document editing) - desktop applications for enterprise networks ATM is most likely to succeed if there is a LAN-based ATM presence in the workplace and an established need for very high bandwidth over the wide area; the wide area cost is reasonable; wide area ATM service is a ubiquitous and uniform offering (a lesson learned from narrowband ISDN); and there is a graceful migration from other technologies and services to ATM. For more information This bulletin summarizes NISTIR 5561, Asynchronous Transfer Mode Procurement and Usage Guide, by Tim Boland, computer scientist, Computer Systems Laboratory, NIST. For a copy of this document, contact the National Technical Information Service (NTIS) at (703) 487-4650; the order number is PB95-174967 and the price is $17.50. The bulletin is also available online through NIST's WWW server. To access the CSL Home Page, use the URL http://www.ncsl.nist.gov From the CSL Home Page, click on Publications, then click on Online NISTIRs. Select NISTIR 5561 which is provided in WordPerfect and ASCII formats.