Explain the ATM Reference Model.

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25.Explain the ATM Reference Model.

ATM Reference Model: 

ATM has its own reference model, different from the OSI model and also different from the TCP/IP model. This model is shown in Fig. 12.1. It consists of three layers, the physical, ATM, and ATM adaptation layers, plus whatever users want to put on top of that.

ATM Reference Model
ATM Reference Model

The physical layer deals with the physical medium: voltages, bit timing, and various other issues. ATM does not prescribe a particular set of rules but instead says that ATM cells can be sent on a wire or fiber by themselves, but they can also be packaged inside the payload of other carrier systems. In other words, ATM has been designed to be independent of the transmission medium.

The ATM layer deals with cells and cell transport. It defines the layout of a cell and tells what the header fields mean. It also deals with establishment and release of virtual circuits. Congestion control is also located here.

Because most applications do not want to work directly with cells (although some may), a layer above the ATM layer has been defined to allow users to send packets larger than a cell. The ATM interface segments these packets, transmits the cells individually, and reassembles them at the other end. This layer is the AAL (ATM Adaptation Layer).

Unlike the earlier two-dimensional reference models, the ATM model is defined as being three-dimensional, as shown in Fig. 12.1. The user plane deals with data transport, flow control, error correction, and other user functions. In contrast, the control plane is concerned with connection management. The layer and plane management functions relate to resource management and inter layer coordination.

The physical and AAL layers are each divided into two sub layers, one at the bottom that does the work and a convergence sub layer on top that provides the proper interface to the layer above it. The functions of the layers and sub layers are given in Fig. 12.2.

The ATM layers and sub layers, and their functions


The PMD (Physical Medium Dependent) sub layer interfaces to the actual cable. It moves the bits on and off and handles the bit timing. For different carriers and cables, this layer will be different.

The other sub layer of the physical layer is the TC (Transmission Convergence) sub layer. When cells are transmitted, the TC layer sends them as a string of bits to the PMD layer. Doing this is easy. At the other end, the TC sub layer gets a pure incoming bit stream from the PMD sub layer. Its job is to convert this bit stream into a cell stream for the ATM layer. It handles all the issues related to telling where cells begin and end in the bit stream. In the ATM model, this functionality is in the physical layer. In the OSI model and in pretty much all other networks, the job of framing, that is, turning a raw bit stream into a sequence of frames or cells, is the data link layer's task.

The ATM layer manages cells, including their generation and transport. Most of the interesting aspects of ATM are located here. It is a mixture of the OSI data link and network layers; it is not split into sub layers.

The AAL layer is split into a SAR (Segmentation And Reassembly) sub layer and a CS (Convergence Sub layer). The lower sub layer breaks up packets into cells on the transmission side and puts them back together again at the destination. The upper sub layer makes it possible to have ATM systems offer different kinds of services to different applications (e.g., file transfer and video on demand have different requirements concerning error handling, timing, etc.).

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