ATM Layers

Fig. 11.32 shows an ATM end point operational model and an ATM switch operational model.

encode bit stream

Fig. 11.32. Headers of ATM cell. ATM end point operational model:

The ATM end point operational model consists of three layers

1. ATM adaptation layer (AAL)

2. ATM layer and

3. Physical layer

The ATM adaptation layer (AAL) is described in the section 11.7.5. Remaining two layers are explained below.

Physical layer. The physical layers are divided into two sublayers. They are Physical Medium Dependent (PMD) layer and the transmission convergence (TC) layer.

The DMP sublayer provides bit transmission, coding, electrical and optical conversion and bit timing. The functions of the TC sublayer are as follows:

1. Extracting the cell from physical layer

2. Scrambling the cell before transmission and descrambling the cell after transmission.

encode bit stream

Fig. 11.32. Headers of ATM cell. ATM end point operational model:

The ATM end point operational model consists of three layers

1. ATM adaptation layer (AAL)

2. ATM layer and

3. Physical layer

The ATM adaptation layer (AAL) is described in the section 11.7.5. Remaining two layers are explained below.

Physical layer. The physical layers are divided into two sublayers. They are Physical Medium Dependent (PMD) layer and the transmission convergence (TC) layer.

The DMP sublayer provides bit transmission, coding, electrical and optical conversion and bit timing. The functions of the TC sublayer are as follows:

1. Extracting the cell from physical layer

2. Scrambling the cell before transmission and descrambling the cell after transmission.

3. After receiving a cell from physical layer, TC calculate the HEC header and compares with the cell's HEC header. The results of the comparison are used for error correction in the cell header. If the error can not be corrected, the cell will be discarded.

ATM layer. There is no sublayers. It performs the following function.

1. It adds cell header and remove cell header.

2. The VPI/VCI values of incoming cell is translated to a new VPI/VCI value for the outgoing cell.

3. Generic flow control—to determine the destination of the receiving cell.

4. Multiplexes and demultiplexes cell.

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