Asynchronous Transfer Mode (ATM)

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Asynchronous Transfer Mode (ATM)

is the cell relay protocol designed by the ATM
Forum and adopted by the ITU
T. The combination of ATM and B
ISDN will allow high
interconnection of all the world’s networks. In fact, ATM can be though of as the “highway” of
the information superhigh

The Asynchronous Transfer Mode (ATM) protocols and architecture have managed to gather an
impressive amount of market and media attention over the last several years. Intended as a
echnique to achieve a working compromise between the rigidity of the
synchronous architecture and packet network's unpredictable load behavior, ATM products are
appearing for everything from high
speed switching to local area networking. ATM has caught
the interest of both the telecommunications community
as a broadband carrier for Integrated
Services Digital Network (ISDN) networks as well as the computer industry, who view ATM as
a strong candidate for high
speed Local Area Networking.

ATM Architecture

As in the case of many large systems, there are a r
ange of components and connections involv
in the ATM networks. Figure


network architecture. All connections in the ATM
network are point
point, with traffic being switched through the network by the switching

The user access devices, called the end points are connected through
User Network
Interface (UNI)

to the switches inside the network
The switches are connected through


ATM Sample Network Architecture

Virtual Paths and
Virtual Channels

ATM uses virtual paths and channels to logically divide the bandwidth of the transmission
path.The following figure, “Channels within a Path inside the Transmission Medium,” shows
how virtual paths and channels are divided within the trans
mission medium

Channels Within a Path Inside the Transmission Medium

Transmission Path:
The transmission path consists of the physical cable connected to a
particular port of an ATM switch. The cable has a defined bandwidth, such as 155 megabits per
second for an Optical Carrier
3 (OC
3) optical fiber link.

Virtual Path:
The bandwidth of the transmission path is logically divided into separate virtual
paths and identified using the VPI in the ATM header. Each virtual path is allocated a fixed
amount o
f bandwidth. Virtual paths do not dynamically vary their bandwidths beyond what has
been allocated.

Virtual Channel :
The bandwidth of a virtual path is logically divided into separate virtual
channels using a virtual channel identifier in the ATM header.
Unlike virtual paths, virtual
channels share the bandwidth dynamically within a virtual path.


At the core of the ATM architecture is a fixed length "cell." An ATM cell is a short, fixed length
block of data that contains a short header with addressi
ng information, followed by the upper
layer traffic, or "payload." The cell structure, shown in Figur
, is 53 octets long, with a 5 octet
header, followed by 48 bytes of payload. By fixing the length of each cell, the timing
characteristics of the links an
d the corresponding network are regular and relatively easy to
predict; predicting the dynamics of variable length packet switched networks isn't always easy.

ATM Cell Structure (UNI Format)

ATM Layers




The ATM Adaptation layer facilitates the use of packets larger than a cell.
Packets are segmented by the ATM interface, transmitted individually, and
then reassembled on the receiving end. The ATM Adaptation Layer includes
the Segmentation and Reasse
mbly and Convergence sublayers.


The ATM layer regulates cells and cell transport and establishes and releases
Virtual Circuits. The ATM layer has no sublayers


The Physical layer represents the physical medium and regulates Physical layer
functions such as voltages and bit timing. The Physical layer consists of the
Transmission Convergence and the Physical Medium Dependent sublayers

ATM Adaptation Layer

or Ap
plication Adaptation Layer

The ATM Adaptation Layer (AAL) creates and receives

byte payloads through the lower
layers of ATM on behalf of different types of applications. Although there are five different
types of AALs, each providing a distinct class
of service, Windows


2003 supports only
AAL5. ATM Adaptation is necessary to link the cell
based technology at the ATM Layer to the
stream technology of digital devices (such as telephones and video cameras) and the packet
stream technology of mo
dern data networks (such as frame relay, X.25 or LAN protocols such as
TCP/IP or Ethernet).

The ATM Adaptation layer adapts user traffic into cell
format. AALs break
data of various types, having different characteristics into 48 byte
cells. At the receivi
ng end the
cells are reassembled into the original form by the AAL. Different AALs exist for different types
of traffic.

AALs are divided into 2 sub
layers namely Convergence Sub
layer (CS) and the Segmentation
and Reassembly (SAR) sub
layers. The functio
n of the CS layer is to describe the method in
which non
ATM traffic is converted into ATM traffic. The function of SAR layer is to insert
data into ATM cells and adds header information. There are different types of AALs, each
supporting a different type
of traffic or service.

Type1: Constant Bit Rate (CBR) service: AAL1 supports a connection
oriented service in
which the bit rate is constant. Examples of this service include 64 Kbit/sec voice, fixed
rate uncompressed video and leased lines for private dat
a networks.

Type2: Variable Bit Rate (VBR) service: AAL2 supports a connection
oriented service in
which the bit rate is variable but requires a bounded delay for delivery. Examples of this
service include compressed packetized voice or video. The require
ment on bounded delay
for delivery is necessary for the receiver to reconstruct the original uncompressed voice
or video. AAL2 has not been fully developed yet.

Type3: Connection
oriented data service: Examples of this service include connection
file transfer and in general, data network applications where a connection is set
up before data is transferred. This service has variable bit rate and does not require
bounded delay for delivery. The ITU originally recommended two types of AAL
protocols t
o support this service class, but these two types have been merged into

a single
type, called AAL3/4.

Type4: Connectionless data service: Examples of this service include datagram traffic
and in general, data network applications where no connection is se
t up before data is
transferred. Either AAL3/4 or AAL5 can be used to support this class of service


AAL5 provides a way for non
isochronous (time
dependent), variable bit rate, connectionless
applications to send and receive data. AAL5 was developed
as a way to provide a more efficient
transfer of network traffic than AAL3/4. AAL5 merely adds a trailer to the payload to indicate
size and provide error detection. AAL5 is the preferred AAL when sending connection
or connectionless LAN protocol
traffic over an ATM network. Windows


2003 supports

AAL5 provides a straightforward framing at the Common Part Convergence Sublayer (CPCS)
that behaves more like LAN technologies, such as Ethernet. The following figure, “Breakdown
of an AAL5 Ce
ll Header and Payload,” shows a detailed breakdown of an AAL5 Cell Header
and Payload, followed by a detailed description of each of the components.

Breakdown of an AAL5 Cell Header and Payload

With AAL5, there is no longer a dual encapsulation. The service class frames cells at the CPCS,
but not at the Segmentation and Reassembly sublayer to minimize overhead. It also uses a bit in
the Payload Type (PT) field of the ATM header rather than a sepa
rate SAR framing.

AAL5 is the AAL of choice when sending connection
oriented (X.25 or Frame Relay) or
connectionless (IP or IPX) LAN protocol traffic over an ATM network.

ATM Layer

The ATM layer provides cell multiplexing, demultiplexing, and VPI/VCI routi
ng functions. The
ATM layer also supervises the cell flow to ensure that all connections remain within their
negotiated cell throughput limits. If connections operate outside their negotiated parameters, the
ATM layer can take corrective action so the misb
ehaving connections do not affect connections
that are obeying their negotiated connection contract. The ATM layer also maintains the cell
sequence from any source.

The ATM layer multiplexes and demultiplexes and routes ATM cells, and ensures their sequenc
from end to end. However, if a cell is dropped by a switch due to congestion or corruption, it is
not the responsibility of the ATM layer to correct the dropped cell by means of retransmission or
to notify other layers of the dropped cell. Layers above t
he ATM layer must detect the lost cell
and decide whether to correct it or disregard it.

In the case of interactive voice or video, a lost cell is typically disregarded because it takes too
long to resend the cell and place it in the proper sequence to rec
onstruct the audio or video
signal. A significant number of dropped cells in time
dependent services, such as voice or video,
results in a choppy audio or video playback, but the ATM layer cannot correct the problem
unless a higher Quality of Service is sp
ecified for the connection.

In the case of data (such as a file transfer), the upper layer application must sense the absence of
the cell and retransmit it. A file with randomly missing 48
bytes chunks is a corrupted file that is
unacceptable to the receiv
er. Because operations, such as file transfers, are not time dependent,
the contents of the cell can be recovered by incurring a delay in the transmission of the file
corresponding to the recovery of the lost cell.

Physical Layer

The physical layer provide
s for the transmission and reception of ATM cells across a physical
medium between two ATM devices. This can be a transmission between an ATM endpoint and
an ATM switch, or it can be between two ATM switches.
It also defines the encoding and
electrical to
optical transformation. It provides

convergence with physical transport protocols,
such as SONET and T
3, as well as mechanisms for transforming the flow of cells into a flow of

ATM Applications

ATM is used in both LANs and WANs.

ATM is
basically a WAN technology that delivers cells over a long distance. In
this type of application, ATM is mainly used to connect LANs or other WANs together. A router
between the ATM network and the other network serves as an end point. The router has two
acks of protocols: one belonging to the ATM and the other belonging to the other protocol.

ATM was originally designed as a WAN technology. However, the high data rate
of the technology (
155 and 622 Mbps

has attracted the attention of designers
who are looking
for more and more speed in LANs. At the surface level, the use of ATM technology in LANs
seems very natural.

International Institute of Professional Studies

Devi Ahilya Vishwavidyalaya, Indore, M.P.

Session Jan




Data and Computer Communication

Master of Technology (Information Technology)

Semester VI

Submitted By:

Ritu Rajput