Asynchronous Transfer Mode (ATM) is a very high-speed transmission technology for voice, data, video, and television that combines the best of circuit switching and packet switching. Background One of the major challenges faced by both telephone and cable companies is the delivery of broadband communications to specific receivers or customers in a cost-efficient manner over existing networks. Currently, circuit switching is used in telephone networks to transmit voice and data signals. Circuit switching establishes a dedicated connection between the sender and receiver. This connection consumes network capacity whether or not there is an active transmission taking place; for example when a caller is put on hold. Packet switching. on the other hand. ensures that the network is filled at all times. It does this by sending signals in the small unused segments of transmission 1 for example, between the words of a conversation or when a caller is put on hold.
However in packet switching, there can be variations in the timing when the digital bits are received. For normal voice and data communications this is not a problem [ for broadband signals, such as television, it is a huge problem that causes the picture to jerk and the audio to be out of sync with the picture. Asynchronous transfer mode is by definition not synchronous. In synchronous transmission, a synchronized connection must be made between the sender and receiver because there must be a constant time interval between each successive bit. character. or event. with a start bit preceding and a stop bit following each one. Universal Compatibility Asynchronous transmissions do not require a constant time interval because each fixed-length cell contains 53 bytes. five of which are a header that defines the address. routing, forward error correction, and a plus bit for priority handling and network management.
The remaining 48 bytes are the user data. Cells in the network can be filled from various sources; voice, video. data. or television and. since all cells contain data, the efficiency of the network is greater. The position of each cell is random, and data can wait in queue until a cell is available. ATM protocols enable cells containing time-dependent data (such as television signal) to receive priority over cells carrying normal data or voice signals. ATM can be used in existing twisted pair. fiber-optic, coaxial. and hybrid fiber/coax (HFC) networks for local area network (LAN) and wide area network (WAN) communications. Because ATM was developed to have such a wrde range of compatibility with existing networks, its implementation does not require replacement or overbuilding of telephone, data, or cable networks, ATM is also compatible with Wireless and satellite communications.
The layered architecture of ATM includes: the adaptation layer that divides all types of user data into 48»byte cells, the ATM layer that adds the fiverbyte header information to direct the user data to its destination, and the physical layer that defines the electrical characteristics and network interfaces (based on the transmission medium [ copper, fiber optic. coaxial, HFC, wireless). As a highrbandwidth medium with low delay and the capability to be switched or routed to a specific destination. ATM provides a uniformity that meets the needs of the telephone, cable teleVISIon, video, and data industries. This universal compatibility makes it possible to interconnect the networks [1 something that IS not currently possible because of the various transmission standards used by each industry. One of the key advantages of ATM is its ability to transmit video without creating a jittery picture or losing the synchronization of the sound and picture.
In addition to this, ATM is also extremely fast. AT&T has developed ATM switches capable of transmitting 20 gigabits of data per second (Gbps) and a shared swnch that can transmit up to 662 Gbps. At 20 Gbps the ATM switch can handle the equivalent of 1,600 copies of MelvilIeDs novel “Moby Dick” each second. Telephone networks connect every telephone to every other telephone using a dedicated path. but carry narrow bandwidth signals. Cable networks carry broadband signals. but only connect subscribers to centralized locations, To build a network that would provide a dedicated connection between sender and receiver for broadband communications would be prohibitively expensive. For this reason, ATM seems to be the best hope since it can use existing networks to deliver simple voice and data as well as complex and time-sensitive television signals. ATM can also handle bi-directional communications easily, something CATV distribution systems were not built to do.
Levels of Service Proponents of ATM (primarily the ATM Forum, which is comprised of 800 member companies worldwide), have banded together to develop technical standards and forward them to the International Telecommunications Union (ITU) so the deployment of ATM will be speeded up. The ATM Forum proposes four escalating levels ofATM sewice I] Constant Bit Rate (CBR). Variable Bit Rate (VBR), Available Bit Rate (ABR), and Unspecified Bit Rate (UBR). At the outset of a transmission, users would specrfy which level of service they require. with an associated increase in cost. Communications requiring a fully reserved channel. such as television, video conferencing or video e-mail, distance learning, and video on demand (VOD), would select CBR to achieve full-motion images and fully synchronized sound. At the lower end, a user could select UBR for data applications background file transfers, and messaging and telecommuting from home to office. Worldwide Tests and Trials Tests and trials of ATM technology are in progress to prove ATMUs viability for distribution of information. learning, medical images, interactive games, and multimedia to the desktop.
Time Warner has staged a home trial in Orlando, Florida to test the ability to offer VOD, home shopping, and information on demand. A Similar test has been done in Cambridge, England and other tests have been done in Canada, Finland, France, Germany, Japan. and Australia. An 18rmonth long transrAtlantic test of ATM that ended in 1993 found that transmission time for a L75 gigabyte file was reduced from 14 hours to 2 hours. The test, conducted by Cable and Wireless magazine, connected Tandem Computer customer support applications in Europe to its headquarters in Austin, Texas. Private networks in the United States have begun embracmg ATM for use on LANs, Chrysler Corporation has a 3,000 node network in Detroit that uses ATM to interconnect PCs, workstations, servers. and supercomputers.
The network carries broadcast video, Video conferencing, computer aided design (CAD) program updating, and real-time remote use of supercomputers. McDonaldl’ls Corporation uses ATM to sen/e 150 subnets that support existing applications and expand network capacity. The United States Geological Survey (USGS) uses a 13-switch ATM network for the transmission of Interactive graphics and Video conferencing. In addition, tests have been done to see if ATM could function in a wireless system. In 1994, AT&T. Telestra of Australia, and KDD of Japan tested satellite transmission using ATM. Using satellite communications and ATM, high-speed transmission would be possible in countries that donIJt have sophisticated communication networks. ATM in the Home While business communications have created the most interest to date for the implementation of ATM, Internationsal Data Corporation (IDC) conducted a study of 2,500 residences to determine how households fit into the ATM picture.
Its findings indicated that 40% of the households wanted entertainment on demand, 20% wanted Information on demand or home»office capability and 10% wanted home shopping. From its study. IDC estimates 58.2 million households (almost 60% of U.S. households) are likely to adopt ATM applications over the next 5 to 15 years. IDCDs sun/ey also identified the features that will create demand for these ATM applications T graphical user interface (GUI) controllers clear consistent audio and video quality voice activation integrity of file transler, full-motion video, multitasking, and high-speed connectivity to data sources and intelligent agents. Because of its compatibility for all types of digital communications and its speed of transmission, ATM may be the technology that levels the playing field for the telephone, cable, and utility industries which are vying for the attention and dollars of consumers. As the ATM Forum prepares standards and the ITU and other regulatory organizations adopt them for use, more equipment will be installed and the user base wrll drive expanded services.
