Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that use variable sized packets or frames. ATM provides data link layer services that run over a wide range of OSI physical Layer links. ATM has functional similarity with both circuit switched networking and small packet switched networking. It was designed for a network that must handle both traditional high-throughput data traffic (e.g., file transfers), and real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins. ATM is a core protocol used over the SONET/SDH backbone of the public switched telephone network (PSTN) and Integrated Services Digital Network (ISDN), but its use is declining in favor of All IP.

Asynchronous Transfer Mode (ATM) represents a relatively recently developed communications technology designed to overcome the constraints associated with traditional, and for the most part separate, voice and data networks. ATM has its roots in the work of a CCITT (now known as ITU-T) study group formed to develop broadband ISDN standards during the mid-1980s. In 1988, a cell switching technology was chosen as the foundation for broadband ISDN, and in 1991, the ATM Forum was founded.
The ATM Forum represents an international consortium of public and private equipment vendors, data communications and telecommunications service providers, consultants, and end users established to promote the implementation of ATM. _To accomplish this goal, the ATM Forum develops standards with the ITU and other standards organizations.
The first ATM Forum standard was released in 1992. Various ATM Forum working groups are busy defining additional standards required to enable ATM to provide a communications capability for the wide range of LAN and WAN transmission schemes it is designed to support. This standardization effort will probably remain in effect for a considerable period due to the comprehensive design goal of the technol-ogy, which was developed to support voice, data, and video on both local and wide area networks.

Advantages of the Technology

The use of cell-switching technology in a LAN environment provides some distinct advantages over the shared-medium technology employed by Ethernet, token-ring, and FDDI networks. Two of those advantages are obtaining full bandwidth access to ATM switches for individual workstations and enabling attaching devices to operate at different operating rates. Those advantages are illustrated in Figure 14.2, which shows an ATM switch that could be used to support three distinct operating rates. Workstations could be connected to the switch at 25Mbps, and a local server could be connected at 155Mbps to other switches either to form a larger local LAN or to connect to a communications carrier's network via a different operating rate.

The selection of a 53-byte cell length results in a minimum of latency in comparison to the packet length of traditional LANs, such as Ethernet, which can have a maximum 1526-byte frame length. Because the ATM cell is always 53 bytes in length, cells transporting voice, data, and video can be intermixed without the latency of one cell adversely affecting other cells. Because the length of each cell is fixed and the position of information in each header is known, ATM switching can be accomplished via the use of hardware. In comparison, on traditional LANs, bridging and routing functions are normally performed by software or firmware, which executes more slowly than hardware-based switching.

Two additional features of ATM that warrant discussion are its asynchronous operation and its connection-oriented operation. ATM cells are intermixed via multiplexing, and cells from individual connections are forwarded from switch to switch via a single-cell flow. However, the multiplexing of ATM cells occurs via asynchronous transfer, in which cells are transmitted only when data is present to send. In comparison, in conventional time division multiplexing, keep-alive or synchronization bytes are transmitted when there is no data to be sent. Concerning the connection-oriented technology used by ATM, this means that a connection between the ATM stations must be established before data transfer occurs. The connection process results in the specification of a transmission path between ATM switches and end stations, enabling the header in ATM cells to be used to route the cells on the required path through an ATM network.

Source:

http://technet.microsoft.com/en-us/library/bb726929.aspx