ATM BASICS - Chapter 2 pot

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Chapter 2

How Does ATM Work

This chapter explains fundamental concepts that lay the basis for ATM

technology. The reader is given the in-depth understanding of the terms

such as ATM cells, statistical multiplexing, ATM switching and ATM layer

processing. The layers of ATM reference model are discussed and explained.

This in-depth view includes the Physical Layer, the ATM and the ATM

Adaptation Layer. The basic understanding of these terms is recommended

prior to reading through the following chapters.

2.1 ATM Protocol Reference Model

As it was mentioned before, ATM can be viewed as a part of the B-ISDN con￾cept. The development of B-ISDN protocols was facilitated by the definition

of the B-ISDN Protocol Reference Model (PRM). The model was developed

using the layered communication architecture based on the distinction

between layer functions developed by the ISO (International Standards

Organization). ATM plays a significant role in the B-ISDN PRM. The formal

ATM PRM is a three-dimensional model but the relations between layers

can be better viewed using one-dimensional layer model.

It is important to understand that the layers in the ATM PRM (presented in

the Fig.2-1) don’t have one-to-one mapping relationship with the seven lay￾ers OSI protocol reference model. Some of the layers of ATM PRM provide

the functionality of more than one OSI layer. For instance, the AAL (ATM

Adaptation Layer) represent some of the features of OSI layer 4 (transport

control), layer 5 (session control) and layer 7 (application control). Most of

the ATM PRM layers can be further subdivided into a number of sublayers.

2.2 Physical Layer

The physical layer (PHY) constitutes the lowest level of the ATM PRM. Its

major task is to transmit ATM cells between ATM devices over the physical

medium. ATM is designed to operate over potentially error free media.

Therefore, successful transmission of ATM cells between ATM devices

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Fig. 2-1, Simplified ATM Protocol Reference Model

requires very low values of BER (BER = 10-12 or better). There exist today a

large variety of standards defining ATM physical interfaces. This situation

is mainly caused by a number of underlying technologies that can be used

by ATM.

2.2.1 Sub-layers

A physical layer takes complete cells from the mid-layer and transmits them

over the physical medium. The physical layer itself is subdivided into two

sub-layers:

•the Transmission Convergence (TC) sub-layer,

•the Physical Medium Dependent (PMD) sub-layer.

These two sub-layers work together to ensure that the physical interfaces

receive and transmit cells efficiently, with the appropriate timing structure

in place.

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Fig. 2-2, Sublayers of the Physical Layer

The Physical Medium Dependent is concerned with getting the bits on and

off the wire. The PMD bit transmission includes bit transfer and bit align￾ment. Technically, it covers bit timing, line coding, opto-electric conversion,

modulation and demodulation functions necessary to transfer bits over a

given medium. The physical connectors and signal characteristics differ

from medium to medium.

The Transmission Convergence sublayer is separated from details and char￾acteristics of the physical medium being used. Due to the presence of the

PMD sublayer the TC is specified independently of the underlying physical

medium and operates over different media. In general, the purpose of the

TC sublayer is to provide a uniform interface to the ATM layer in both direc￾tions. The cells received from the ATM layer are encoded and pushed into

the medium as a bit or a byte stream. The work of the TC sublayer can be

characterized by the following functions:

•Cell rate decoupling. This mechanism is used to insert idle cells in

the transmit direction in order to compensate for the variable rate of the

generation of ATM cells. At the receiving side all idle cells are identified and

suppressed.

•Header checksum generation and extraction. The TC sublayer can

detect and if necessary correct errors affecting the contents of the ATM cell

header. At the transmitting side the Header Error Check (HEC) field is gen￾erated in hardware and inserted into the cell header. At the receiving side

the HEC is recalculated and compared to the value that is extracted from

the header of the received cell. The capabilities of the algorithm used to cal￾culate the HEC allow for the detection and correction of single errors as well

as detection of double errors.

•Unpacking cells from the enclosing envelope. This function is also

referred to as cell delineation. The receiver must be able to recover the cell

boundaries. The TC sublayer must delineate the individual cells in the

received bit stream, either directly from the TDM frame or with the help of

the HEC field in ATM cells. This function can be complemented by the

scrambling/descrambling operation.

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