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Successes and failures of ATM technology

ATM has proven very successful in the WAN scenario and numerous telecommunication providers have implemented ATM in their wide-area network cores. Many ADSL implementations also use ATM. However, ATM has failed to gain wide use as a LAN technology, and its complexity has held back its full deployment as the single integrating network technology in the way that its inventors originally intended. Since there will always be both brand-new and obsolescent link-layer technologies, particularly in the LAN area, not all of them will fit neatly into the synchronous optical networking model for which ATM was designed. Therefore, a protocol is needed to provide a unifying layer over both ATM and non-ATM link layers, as ATM itself cannot fill that role. IP already does that; therefore, there is often no point in implementing ATM at the network layer.

In addition, the need for cells to reduce jitter has declined as transport speeds increased (see below), and improvements in Voice over IP (VoIP) have made the integration of speech and data possible at the IP layer, again removing the incentive for ubiquitous deployment of ATM. Most Telcos are now planning to integrate their voice network activities into their IP networks, rather than their IP networks into the voice infrastructure.

Many technically sound ideas from ATM were adopted by MPLS, a generic Layer 2 packet switching protocol. ATM remains widely deployed, and is used as a multiplexing service in DSL networks, where its compromises fit DSL's low-data-rate needs well. In turn, DSL networks support IP (and IP services such as VoIP) via PPP over ATM and Ethernet over ATM (RFC 2684).

ATM will remain deployed for some time in higher-speed interconnects where carriers have already committed themselves to existing ATM deployments; ATM is used here as a way of unifying PDH/SDH traffic and packet-switched traffic under a single infrastructure. However, ATM is increasingly challenged by speed and traffic shaping requirements of converged networks. In particular, the complexity of SAR imposes a performance bottleneck, as the fastest SARs known run at 10 Gbit/s and have limited traffic shaping capabilities. Currently it seems likely that gigabit Ethernet implementations (10Gbit-Ethernet, Metro Ethernet) will replace ATM as a technology of choice in new WAN implementions.

this source taken from http://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode

ATM Addressing

A Virtual Channel (VC) denotes the transport of ATM cells which have the same unique identifier, called the Virtual Channel Identifier (VCI). This identifier is encoded in the cell header. A virtual channel represents the basic means of communication between two end-points, and is analogous to an X.25 virtual circuit.^[1]

A Virtual Path (VP) denotes the transport of ATM cells belonging to virtual channels which share a common identifier, called the Virtual Path Identifier (VPI), which is also encoded in the cell header. A virtual path, in other words, is a grouping of virtual channels which connect the same end-points, and which share a traffic allocation. This two layer approach can be used to separate the management of routes and bandwidth from the setup of individual connections.

Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a cell relay, packet switching network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells. ATM provides data link layer services that run over Layer 1 links. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used. ATM is a connection-oriented technology, in which a logical connection is established between the two endpoints before the actual data exchange begins.

The standards for ATM were first developed in the mid 1980s. The goal was to design a single networking strategy that could transport real-time video and audio as well as image files, text and email. Two groups, the International Telecommunications Union [ITU 2004] and the ATM Forum [ATM 2004] were involved in the creation of the standards. ATM has been used primarily with telephone and IP networks.