| 3.3 | Layer 2 Protocol Overview - WAN Protocols | ||
| 3.3.11 | Asynchronous transfer mode |
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Asynchronous Transfer Mode (ATM) is an
evolving technology designed for the high-speed transfer of voice,
video, and data through public and private networks in a
cost-effective manner. ATM is based on the efforts of Study Group
XVIII of the International Telecommunication Union Telecommunication
Standardization Sector and the American National Standards Institute
(ANSI) to apply very-large-scale integration (VLSI) technology to the
transfer of data within public networks. Officially, the ATM layer of
the Broadband ISDN (BISDN) model is defined by CCITT I.361.
Current efforts to bring ATM technology to private networks and to guarantee interoperability between private and public networks is being carried out by the ATM Forum, which was jointly founded in 1991 by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint. Today, 90 percent of computing power resides on desktops, and that power is growing exponentially. Distributed applications are increasingly bandwidth hungry, and the emergence of the Internet is driving most LAN architectures to the limit. Voice communications have increased significantly with increasing reliance on centralized voice-mail systems for verbal communications. The internetwork is the critical tool for information flow. Internetworks are being pressured to cost less yet support emerging applications and higher number of users with increased performance. To date, local and wide-area communications have remained logically separate. In the LAN, bandwidth is free and connectivity is limited only by hardware and implementation cost. The LAN has carried data only. In the WAN, bandwidth has been the overriding cost and delay-sensitive traffic, such as voice, has remained separate from data. New applications and the economics of supporting them, however, are forcing these conventions to change. The Internet is the first source of multimedia to the desktop, and it immediately breaks the rules. Such Internet applications as voice and real-time video require better, more predictable LAN and WAN performance. In addition, the Internet also requires that the WAN recognize the traffic in the LAN stream, thereby driving LAN/WAN integration. ATM has emerged as one of the technologies for integrating LANs and WANs. ATM can support any traffic type in separate or mixed streams, delay-sensitive traffic, and non-delay-sensitive traffic, as shown in the figure to the left. ATM can also scale from low to high speeds. It has been adopted by all the industry's equipment vendors for incorporation into LANs and PBXs. With ATM, network designers can integrate LANs and WANs, support emerging applications in the enterprise, and support legacy protocols with added efficiency. On the other hand, to some extent ATM has been eclipsed by other technologies as a core switching technology, mostly as a result of the relative cost and inherent complexity of ATM. This has affected a concomitant lost of market share for ATM in the recent past.
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