Computer Big Lab

ATM is stands for Asynchronous Transfer Mode. This international technology standard is based on transferring multiple service types (such as voice, video, or data) in fixed 53-byte packets.

Some people think that ATM holds the answer to the Internet bandwidth problem, but others are not considering it. ATM creates a fixed channel or route, between two points whenever data transfer begins. ATM differs in working from TCP/IP, in which messages are divided into packets and each packet can take a different route from source to destination. This difference makes it easier to track and bill data usage across an ATM network.

Asynchronous Transfer Mode promises to deliver vast amounts of bandwidth to network users. While ATM was envisioned as technology for public network carriers, its application has been recast, and you can expect to see ATM deployed in private as well as public networks over the next decade.

ATM is the purported solution to the LAN/WAN integration quandary. Companies are looking for an efficient and cost-effective method of integrating their dispersed multiprotocol LANs, and frame relay, SMDS, and T3 are vying as contenders. So far, none has been wholly successful. LAN technologies, with their ability to carry large amounts of data over limited distances, are inherently unsuitable in a geographically large network.

WAN services, although able to efficiently carry voice and to a lesser extent data over long distances, offer limited bandwidth. ATM, however, can effectively integrate the benefits of LAN and WAN technologies while minimizing the side effects of both.

ATM offers a high bandwidth service that is capable of carrying data, voice, and video over great distances. ATM can provide interfaces to transmission speeds ranging from 1Mbit/sec to 10Gbits/sec. It offers low latency, making it suitable for time-sensitive or isochronous services such as video and voice. Plus, it is protocol- and distance-independent.

The following are the salient features of the ATM

Ø Integrates Voice, Video and Data

Ø Uses short fixed length packets called cells

Ø Bandwidth on demand

Ø Best effort delivery system

Ø Connection Oriented technology - Every cell with the same source and destination travels over the same route.

Ø Potential to remove performance bottlenecks in todays LANs and WANs .The ATM service can be categorized into the four following types.

· Constant Bit Rate (CBR): It specifies a fixed bit rate so that data is sent in a steady stream.

· Variable Bit Rate (VBR): It provides a specified throughput capacity but data is not sent evenly. This is a popular for voice and videoconferencing data.

· Available Bit Rate (ABR): It provides a guaranteed minimum capacity but allows data to be bursted at higher capacities when the network is free.

· Unspecified Bit Rate (UBR): It does not guarantee any throughput levels. This is used for applications, such as file transfer, that can tolerate delays.

ATM is the result of a compromise among all of the data-type constituencies to find a single common denominator for all types of data. One alternative to time division multiplexing is to use packet or cell multiplexing.

A stream of bits is broken up into discrete packets or cells, each of which has a header indicating its path and other worthwhile information. If the cell size is made small, and the overall throughput of the circuit is high, delay-sensitive traffic can be carried along with bursty types of data successfully, and everyone gets what they need from the data link. Voice and video work without glitches, and data customers (potentially) get bandwidth-on-demand.

As a universal transport, ATM can plausibly be installed on the desktop, on departmental and campus backbones, on high-capacity wide area services, and even on a global information superhighway system.

During the development of the fundamental ATM definition, the voice interests-particularly the European telephone providers - wanted a 32-byte cell with a 4-byte header, while many North American interests preferred a more efficient 64-byte cell with a 5-byte header. The compromise of a 48-byte cell with a 5-byte header was reached, so, an ATM cell is a 53-byte entity.

ATM AND NETWORKS

Like frame relay and X.25, ATM protocols are connection oriented. ATM sessions take place over virtual circuits (virtual because they need not use particular physical paths, although once the virtual circuit is established, it stays in place for the duration of a session). (For a graphical representation, see Figure 1.) Most, if not all, of today's ATM services offer only permanent virtual circuits (PVCs); setting up and tearing down PVCs is a job for the telephone company unless the ATM network is completely private.

The real promise of bandwidth-on-demand will be fulfilled when switched virtual circuits (SVCs) become available. PVCs are comparable to leased lines, while SVCs are comparable to dial-up voice service. An ATM SVC will typically take only a fraction of a second to be established, however.

With its connection orientation, ATM does not readily compare with shared medium protocols, such as Ethernet and Token Ring, or with connectionless protocols that perform routing, such as IP and IPX.

With the development of LAN emulation standards, ATM services can be made available to Ethernet and Token Ring networks. Products for translating frame relay data to ATM have been announced. IP and Address Resolution Protocol over ATM are described in the Internet RFC1577.

In general, ATM fits into the data link and physical layers, but because connection-oriented protocols don't require routing, it is possible for ATM to provide services to the upper layer protocols directly. This is the sense in which ATM is supposed to sound the death knell for all routers.

ATM PROTOCOLS

The top layer of the ATM protocol stack is the ATM Adaptation Layer (AAL). Different AALs correspond to the different data types ATM supports. Thus AAL1 permits the ATM device to closely resemble a constant bit-rate voice circuit; AAL3/4 and AAL5 are used for variable bit-rate data types, which are those typically found on computer networks.

The AAL is also responsible for integrating the inherently connection-oriented ATM with connectionless data sources, enabling ATM clients to emulate broadcasting and multicasting.