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ATM Physical layer

Categorized into two parts:

Transmission convergence Functions-include

header error control, Cell rate decoupling, celldelineation, scrambling.

Physical medium Dependent-timing and

synchronization, encoding and bit

transmission.

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ATM physical layer

Transmission Convergence (TC) Sub-Layer

The functions of this layer are:--

Header error control Cell rate decoupling

Cell delineation

Cell Scrambling

Transmission Frame Adaptation

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Header Error Control

The sender computer the checksum over thefour bytes and places it in fifth byte.

The receiver checks for the error difference.

HEC has two modes of operations: Correctionmode and detection mode.

The error control procedures on the receiver

side depends on the mode. In correction mode, The receiver provides for

single bit correction

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HEC

In detection mode, only error detection is

done and all cells with error are discarded.

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Cell rate decoupling

If cells are to be transferred over a

synchronous physical medium , fixed number

of cells have to be transmitted periodically

When user doesnt have anything to transfer

the dummy cells are used to fill time slots.

Inserting idle cells allows rate of outgoing cells

to match the link rate.-CRD.

Idle cells contain specific bit pattern.

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Cell Delineation

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Cell Delineation

To identify start and end of cell, TC layerdelineates the boundaries of the cells.

End of one cell identifies the start of the next.

The position of the subsequent cells can beidentified.

When it is searching for cell boundaries-hunt

mode When it finds correct HEC with syndrome

matching with fifth byte PRESYNC mode.

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Cell Delineation

If HEC of all cells found correct then it moves

to SYNCH state.

If receiver receives less than ALPHA no. of cellswith incorrect HEC it remains in SYNCH state-

ensures rare bit error does not effect cell

delineation

And after ALPHA it again changes to HUNT

state.

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Cell Scrambling

Mechanism used for security and robustness.

Undesired patterns in data is exchanged at

source and changes are revert back at thedestination.

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Transmission Frame Adaptation

The mapping of cells into a framed structure

and carried over a network is known as

transmission frame adaptation.

At destination the cells can be extracted from

the structure and delivered to other layers.

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Physical Medium dependent(PMD)

1.Timing and Synchronization-In order to correctlyreceive and interpret bit patterns the timingrelationship is to be maintained by receiver andsender.

Explicit timing-Both maintain a clock with samefrequency.

Implicit Timing-alternate mask inversion

2.Encoding and Transmission-Two different voltagesor frequencies are used.

In fiber optics-Absence and presence of Photons

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Physical Layer Standards for ATM

Two main classes of standards:-

1.PDH-plesiochronous digital hierarchy-two

signals occur at nearly same rate-two networkelements can make use of two reference

clocks-clock time separated by small

difference called plesiochronous difference.

2.SDH-digital transitions occur at same rate-

following a same reference clock known as

primary reference clock.

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Physical layer standards for ATM

In PDH , there are two ways to carry cells.

First is to directly map ATM cells in to payload

part , without any regard to frame format Second is to define another framed structure

which is then carried in the payload part.