ATM Transmission

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ATM Transmission

Asynchronous Transfer Mode (ATM) networking is an outgrowth of efforts during the 1970s and 1980s to develop a broadband Integrated Service Digital Network (ISDN) capability. ATM provides a transport mechanism that allows digital data to be efficiently transmitted over high-speed links. Currently, most of the high-speed backbone networks throughout the world use ATM technology. It is also used to some extent as a local area networking technology, although the availability of low-cost 100 megabyte and gigabyte Ethernet equipment reduces its appeal for this application.

ATM technology was developed to support a blending of circuit-switching and packet-switching technologies. It is intended to support traffic that requires a fairly constant rate of data delivery, such as voice and video, as well as variable data rate traffic, such as most computer data. ATM is a connection-oriented technology. This means that a fixed path through the network must be established before data can be transmitted. In this respect, ATM is similar to earlier telephone technologies in which physical wires between switching centers were allocated for the duration of each telephone call. Establishing an ATM connection causes a virtual channel connection (VCC) or virtual circuit (VC) to be created through the ATM network between the end users of the connection.

Virtual channels can be bundled into virtual paths in much the same way that physical wires were bundled into trunk lines. Virtual channels can be either permanent virtual channels (PVCs), established manually and persisting for long periods of time, or switched virtual channels (SVCs), set up dynamically as needed and torn down when the need no longer exists.

Data traveling over a VC are divided into fixed-length packets called cells. Each cell contains forty-eight bytes of user data and five bytes of header. Three of the header bytes are used to identify the virtual path (eight bits ) and virtual channel (sixteen bits). One byte is used for header error checking, and the remaining eight bits are used for flow control (four bits), payload type (three bits), and priority (one bit). The small payload size benefits services such as voice and video, where timely and regular delivery are required.

ATM supports five different classes of service:

Constant bit rate (CBR) allows the desired bit rate to be set when the virtual circuit is established; it is used for services such as uncompressed voice and video;
Variable bit rate–non-real time (VBR–NRT) allows statistical techniques to be used to optimize network throughput when the rate at which data is available varies;
Variable bit rate–real time (VBR-RT) is intended for applications such as compressed speech and video, where data delivery must occur at regular intervals;
Available bit rate (ABR) is used for non-time-critical operations such as bulk file transfers that can adjust their rate of input to use available network capacity; minimum acceptable rates can be specified to ensure some service at all times.
Unspecified bit rate (UBR) is the residual class with no guaranteed properties; it is used primarily for TCP/IP data traffic.

When an ATM connection is established, a number of parameters may be specified to ensure desired service properties such as acceptable cell loss percentage, maximum delivery time, variation in delivery time, and the variability of variable rate sources, which specify peak and average data rates and the maximum duration of a burst of peak-rate traffic. Not all parameters apply to all classes of service. Variability parameters make no sense for constant-rate connections, for example. The ability to specify both the type of service needed and parameters controlling the quality of service make ATM well suited to deliver data for multimedia applications.

The ATM Forum is a non-profit international organization dedicated to speeding the development and mass-market deployment of ATM broadband communications technologies. The forum is focused on development of interoperability specifications, promotion of industry-wide cooperation, and educational awareness of the technology's capabilities. Among its other activities, the forum defines standards for connecting other networking technologies to ATM systems. This is necessary because few if any applications use the forty-eight byte data cell payloads of ATM as their native format. A number of ATM adaptation layer (AAL) standards exist that specify the methods to be used.

AAL-1 provides for the conversion of voice and video circuits to CBR ATM virtual channels. The use of PVCs emulates fixed physical circuits and is generally wasteful of bandwidth, as few point-to-point circuits carry fixed traffic levels for long periods of time. The use of SVCs for this traffic represents an improvement, but is still far from optimum, because voice traffic is characterized by lengthy periods of silence, such as when one party is listening to the other. AAL-2 provides a VBR–RT trunking mechanism that uses statistical multiplexing techniques to eliminate the cells that would contain silence.

Compressed video in the MPEG-2 format is accommodated by either AAL-1 or CBR AAL-5. The use of AAL-1 provides for forward error correction at the expense of increased bandwidth and delivery delay. It also allows compensation for Cell Delay Variation (CDV) and the replacement of lost cells. AAL-5 does not compensate for CDV or for bit errors, and lost cells cannot be replaced. For these reasons, AAL-1 is recommended when high video quality is needed.

Internet data traffic also travels over ATM circuits. These data typically take the form of Internet Protocol (IP) datagrams that range in length from a few bytes to thousands of bytes. At the lowest levels of the protocol stack, each datagram is treated independently, and delivery is on a best-effort basis where some loss is deemed acceptable. Higher-level protocols add additional information to the datagram payloads to ensure that they are delivered reliably and in the proper order, retransmitting lost datagrams as necessary. These functions are provided at the end points and are not part of the network routing structure. It would be possible to set up a VCC for a single datagram and tear it down once the packet had been delivered, but the overheads would be excessive. Instead, ATM connections are established between Internet routers. These connections are treated as equivalent to direct physical links between the routers, with the virtual circuit carrying traffic for multiple users. IP over ATM typically uses UBR AAL-5 connections. A potential problem occurs in mapping IP datagrams into ATM cell payloads, because loss of a single cell necessitates retransmission of the entire datagram.

see also Asynchronous and Synchronous Transmission; Network Design; Telecommunications.

John McHugh