ATM
Asynchronous Transfer Mode (ATM) is a technology that has the potential of revolutionizing data communications and telecommunications. Based on the emerging standards for Broadband Integrated Services Digital Networks (B-ISDN), ATM offers the economically sound "bandwidth on demand" features of packet-switching technology at the high speeds required for today's LAN and WAN networks -- and tomorrow's.
ATM is a cell-relay technology that divides upper-level data units into 53-byte cells for transmission over the physical medium. It operates independently of the type of transmission being generated at the upper layers AND of the type and speed of the physical-layer medium below it.
This allows the ATM technology to transport all kinds of transmissions (e.g, data, voice, video, etc.) in a single integrated data stream over any medium, ranging from existing T1/E1 lines, to SONET OC-3 at speeds of 155 Mbps, and beyond.
Evolution
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.
ATM Standards
The following are some of the basic ATM standards documents available from the International Telecommunications Union (ITU).
- ITU-T I.361 - Defines the ATM Layer functions.
- ITU-T I.363 - Defines the ATM Adaptation Layer protocols.
- ITU-T I.610 - Defines the ATM Operation and Maintenance (OAM) functions.
Architecture
ATM is based on the switching of 53-byte cells, in which each cell consists of a 5-byte header and a payload of 48 bytes of information. The figure illustrates the format of the ATM cell, including the explosion of its 5-byte header to indicate the fields carried in the header.
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The ATM Protocol Reference Model
Three layers in the ATM architecture form the basis for the ATM Protocol Reference model, illustrated in Figure below. Those layers are the Physical layer, the ATM layer, and the ATM Adaptation layer.
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Using Cells and Virtual Circuits for Traffic Engineering
Another key ATM concept involves the traffic contract. When an ATM circuit is set up each switch on the circuit is informed of the traffic class of the connection. The performance of ATM is often expressed in the form of OC (Optical Carrier) levels, written as "OC-xxx." Performance levels as high as 10 Gbps (OC-192) are technically feasible with ATM. More common performance levels for ATM are 155 Mbps (OC-3) and 622 Mbps (OC-12).
ATM traffic contracts form part of the mechanism by which "quality of service" (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.
1. CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.
2. VBR - Variable bit rate: an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic.
3. ABR - Available bit rate: a minimum guaranteed rate is specified.
4. UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity.
VBR has real-time and non-real-time variants, and serves for "bursty" traffic. Non-real-time is sometimes abbreviated to vbr-nrt. Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT), which defines the "clumping" of cells in time. To maintain traffic contracts, networks usually use "shaping", a combination of queuing and marking of cells. "Traffic policing" generally enforces traffic contracts.
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