Asynchronous Transfer Mode Introductory Presentation

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ASYNCHRONOUS TRANSFER MODE

1.0 INTRODUCTION
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Asynchronous Transfer Mode (ATM) is a technology designed for the high-speed transfer of voice, video and data
through public and private networks using cell relay technology
ATM provides functionality that is both similar to Circuit switching and packet switching networks.
ATM protocol was designed to support the transfer of data with guaranteed Q.o.S in mind. It divides the user
data into small fixed-length packets called cells and it transport these cells over virtual connections.
ATM is connection-oriented in a sense that before the two end devices communicates, resources have to be
reserved in the intermediate switches similarly to telephone systems.

INTRODUCTION….
11/10/2018 GROUP 2 3
BASIC POINTS…
The term transfer in this context means both transmission and switching
And also the term asynchronous here means that cells belonging to same conversation can be multiplexed and
appear in irregular pattern.
ATM is the transfer mode for implementing Broadband Integrated services digital network BISDN
ATM was driven by the integration of service and performance required of both telephony and data networking.
Was therefore envisioned as the technology of providing B-ISDN

2.1HOW ATM WORKS
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This cell-based transmission is in contrast to typical local area network (LAN) variable-length packet mechanisms,
which means that ATM connections are predictable and can be managed so that no single data type or
connection can monopolize the transmission path.
ATM technology originated in broadband ISDN (B-ISDN) technology and works primarily at layer 2 of the Open
Systems Interconnection (OSI) reference model.
ATM connects devices over a WAN using virtual channels (VCs) and virtual paths (VPs). Virtual channels consist of
one or more physical ATM links connected in a series for transmitting data between remote stations

2.0 HOW ATM WORKS….
11/10/2018 GROUP 2 4
ATM uses fixed-size packets called “cells.” Each 53-byte ATM cell contains 48 bytes of data payload and 5 bytes
of control and routing information in the header. The header provides addressing information for switching the
packet to its destination. The payload section carries the actual information, which can be data, voice, or video.
The payload is properly called the user information field.
An ATM network is made up of ATM switch and ATM endpoints. ATM switch is responsible for cell transit through
an ATM network. ATM switch accepts the incoming cell from ATM endpoints or another ATM switch reads & update
the cell header info and quickly switches the cell to an output interface toward its destination.

3.0 BROADBAND ISDN
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is a broadband communication network developed by International Telegraph And Telephone Consultative
Committee(CCITT) that enables the transmission of design simulations and other multimedia transmission that
include text, voice, video and graphics in one network
This B-ISDN would be able to provide end users with increased transmission rate, up to 155.54Mbits/s on a
switching basis.
This is a great improvement as compared to the earlier rate of 64kbits/s employed in the ISDN which is not
suitable for high definition moving pictures.

4.0 ATM REFERENCE MODEL
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The ATM protocol model is based on standards developed by the ITU.
The model is divided into three layers:
1. ATM adaptation layer
2. ATM layer
3. Physical layer
Just like all/most layer based protocols, the top layer acts as an interface to the below layers and the bottom
layer which is the physical layer transmits the data.
ATM ADAPTATION
LAYER (AAL)
ATM LAYER
PHYSICAL LAYER

4.1 PHYSICAL LAYER
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The physical layer is divided into two sublayers:
1- The physical medium sub layer
2- The transmission convergence sub layer
Physical Medium (PM) sub layer
The physical medium sublayer performs medium-dependent functions. For example
 It provides bit transmission capabilities including bit alignment, line coding and electrical/optical conversion.
 Also responsible for bit timing, i.e., the insertion and extraction of bit timing information.
 The PM sublayer currently supports two types of interface: optical and electrical.
Transmission Convergence (TC) sublayer
Above the physical medium sublayer
 primarily responsible for framing of data transported over the physical medium.

4.1 PHYSICAL LAYER..
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 The TC sub layer is responsible for the following four functions:
I. Cell Rate Decoupling : Cell rate decoupling is the insertion of idle cells at the sending side to
adapt the ATM cell stream’s rate to the rate of the transmission path
II. Header Error Control : Header error control is the insertion of an 8-bit CRC value in the ATM cell header
to protocol the contents of the ATM cell header
III. Cell Delineation : Cell delineation is the detection of cell boundaries
IV. Transmission Frame Adaptation : Transmission frame adaptation is the encapsulation of departing
cells into an appropriate framing structure (either cell-based or SDH based).

4.2 ATM ADAPTATION LAYER (AAL)
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The basic function of the ATM adaptation layer is to convert the user data supplied by
higher layers into 48-byte blocks of data. The higher layer protocols accepts user data arrange it into packets
and hand it over to the AAL
The ATM adaptation layer is divided into two sub-layers
1. Convergence sub-layer
2. Segmentation and Re-assembly sub-layer.
The convergence sub-layer : provides services to higher layers through a set of Protocols
The segmentation and re-assembly : separates the messages from the convergence sub-layer into ATM cells.
the two sub-layers above adds some protocol information, which is transported as the payload of 53 bytes ATM
cells
The below figure depicts this.

4.2 ATM ADAPTATION LAYER (AAL)..
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4.3 ATM LAYER
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We can say the primary function of ATM layer is associated with routing and switching of ATM cells
But then the ATM layer changes functionality depending on current interface in use.
There are two interfaces in ATM network
1-User-Network interface [between the ATM end point and ATM switch]
2-Network-Node interface [between ATM switches]
The interfaces only differs in the cell header format.
ATM FUNCTIONALITY
 VPI/VCI translation
 Multiplexes and de-multiplexes cells
 Cell Header creation and extraction before or after the cell is delivered to the AAL
 Maintains flow control using the GFC bits of the header

5.0 ATM PLANE
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In addition to the layers, the ATM reference model also has planes which are used to group functionality of the
layers and for management purpose .
The ATM reference model has three planes
1- User Plane : This plane is responsible for managing the transfer of data
2- Control Plane : This plane is responsible for generating and managing signaling requests
3- Management Plane : This plane contains two components:
o Layer management : manages layer-specific functions, such as the detection of
failures and protocol problems.
o Plane management : manages and coordinates functions related to the complete
system

5.1 ATM LAYER
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5.2 ATM PLANE
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NOTE:
 Within the user and control planes is a hierarchical set of layers.
 The user plane defines a set of functions for the transfer of user information between communication end-
points.
 The control plane defines control functions such as call establishment, call maintenance, and call release.
 The management plane defines the operations necessary to control information flow between planes and
layers, and maintain accurate and fault tolerant network operation.
 Also notice that the ATM functionality corresponds to the physical layer and part of the data link layer of the
OSI reference model.
 As the ATM physical layer is analogous to physical layer of OSI model
 The ATM Layer combined with ATM Adaptation Layer is analogous to the data link layer of the Osi
reference model.

6.0 ATM CELL STRUCTURE
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As previously said, the ATM network has two interfaces the UNI and NNI interfaces.
Both these interfaces uses a 5byte cell header but they differ slightly.
Below is the header format for both.

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The VCI and VPI fields are identifier values for VC and VP respectively.
[ A virtual channel connects two ATM communication end-points.
A virtual path connects two ATM devices, which can be switches or end-points, and several
virtual channels may be multiplexed onto the same virtual path.]
The 2-bit PT field
Identifiers whether the cell payload contains data or control information.
The CLP bit
Used by the user for explicit indication of cell loss priority. If the value
of the CLP is 1, then the cell is subject to discarding in case of congestion.
The HEC
Field is an 8-bit CRC value that protects the contents of the cell
header.
The GFC field
which appears only at the UNI, is used to assist the
customer premises network in controlling the traffic flow for different qualities of
service.

11/10/2018 GROUP 2 17
A transmission path may comprise several virtual paths and each virtual path may carry several virtual channels

Asynchronous Transfer Mode Introductory Presentation

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