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Asynchronous Transfer Mode

(ATM)

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Introduction Introduction A high-speed, connection oriented

switching and multiplexing technology.A transfer mode in which the information

(voice, video and data) is organized into cells.

It is asynchronous in the sense that the information streams can be sent independently without a common clock.

Asynchronous transfer mode (= Statistical multiplexing)

– Sources packetize data. Packets are sent only if there is data

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FeaturesFeaturesEnables high-bandwidth Distributed

Applications (155Mbps, 622 Mbps)Provides high transmission speeds for both

LAN and WAN.Supports both circuit switching and packet

switching (cell switching).Virtual Networking (share available

bandwidth among multiple applications).Support for multicasting and broadcasting.Enables supply of bandwidth on demand

and billing as per cell basis.Scalable.

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ATM’s Key ConceptsATM’s Key Concepts

Hybrid form of switching technology.ATM requires connections to be

established before any communication takes place.

A connection is called a virtual circuit which could be permanent or switched.

Streamlined: minimal error and flow control.

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Architecture of an ATM NetworkArchitecture of an ATM Network

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Logical ConnectionsLogical Connections

VCC (Virtual Channel Connection): a logical connection analogous to virtual circuit in X.25

VPC (Virtual Path Connection): a bundle of VCCs with same endpoints

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Figure 5.2Figure 5.2

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TP, VPs, and VCsTP, VPs, and VCs

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Example of VPs and VCsExample of VPs and VCs

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Connection IdentifiersConnection Identifiers

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Virtual Connection Identifiers Virtual Connection Identifiers in UNIs and NNIsin UNIs and NNIs

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Advantages of Virtual PathsAdvantages of Virtual Paths

Simplified network architectureIncreased network performance and

reliabilityReduced processing and short connection

setup timeEnhanced network services

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ATM LayersATM Layers

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ATM Layers in End-Point Devices and ATM Layers in End-Point Devices and SwitchesSwitches

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Functions of the LayersFunctions of the Layers

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ATM Layers and PlanesATM Layers and Planes ATM is a three dimensional model consisting of layers and

planes. Physical layer deals with physical media issues, further

divided into -PMD and TC sub layers ATM layer defines cell format and connection setup. ATM Adaptation Layer segments and reassembles upper layer

into and from cells, divided into -SAR and CS sub layers.

User planes deals with data transport, flow and error control. Control plane deals with connection management. The Layer and Plane management deals with resource

management and interlayer coordination.

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ATM LayerATM Layer

Defines the cell format and connection setup.

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Cell SwitchingCell Switching ATM packets are small and have a fixed size

– Packets in ATM are called cells– Small packets are good for voice and video transmissions

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ATM CellsATM Cells

Why 53 Bytes? A 48 byte payload was the result of a

compromise between a 32 byte payload and a 64 byte payload

• Advantages– Low packetization delay for continuous bit rate applications (video, audio)– Processing at switches is easier

• Disadvantages– High overhead (5 Bytes per 48)

2020

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ATM Adaptation Layer (AAL)ATM Adaptation Layer (AAL)

Support non-ATM protocols– e.g., PCM voice, LAPF

AAL Services– Handle transmission errors– Segmentation/reassembly (SAR)– Handle lost and misinserted cell conditions– Flow control and timing control

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AAL ProtocolsAAL Protocols

AAL layer has 2 sublayers:– Convergence Sublayer (CS)

Supports specific applications using AALservice dependant

– Segmentation and Reassembly Layer (SAR)Packages data from CS into cells and unpacks at

other end

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AAL TypesAAL Types

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ATM Network ArchitectureATM Network ArchitectureATM Adaptation Layer (AAL) -

Convergence sublayerDifferent service classes are accessed via

AAL-SAPAssociated with Convergence sublayer is

SAR protocol– AAL types

4 different AAL protocols or typesOriginally correspond with 4 classes

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ATM Network ArchitectureATM Network ArchitectureATM Adaptation Layer (AAL)

Segmentation and Reassembly (SAR) sublayer

AAL - Type 1Carries Class A Services

– e.g. CBR packetised voice (traditional circuit switched)

– 48 octet payload47 speech samples (samples acquired every 125

micro secs) 1 octet holds sequence # and seq # check

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ATM Network ArchitectureATM Network Architecture

ATM Adaptation Layer (AAL) Segmentation and Reassembly (SAR) sublayer

AAL - Type 2Carries Class B Services

– Not fully developed yet, but some characteristics are:-

Transfer of Service Data Units with variable source bit rate

Transfer of timing information

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ATM Network ArchitectureATM Network Architecture

ATM Adaptation Layer (AAL) Segmentation and Reassembly (SAR) sublayer

AAL - Type 3/4Carries Class C&D Services

– Can operate in message or stream mode– Message mode entire message carried in one

CS-PDU– Stream mode involves many CS-PDUs.

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ATM Network ArchitectureATM Network ArchitectureATM Adaptation Layer (AAL)

Segmentation and Reassembly (SAR) sublayer

AAL - Type 5 (Simple and Efficient Adaptation Layer).

Carries Class C & D Services– Designed for same class of traffic as AAL3/4– Simpler, less overhead– SAR-PDU for instance has no header / trailer– Essentially CS-PDU is segmented and segments

mapped to ATM payload

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Problems and ChallengesProblems and Challenges

Interoperability– LAN Emulation over ATM– IP over ATM

Bandwidth management– Peak rate allocation method– Minimum throughput allocation method– Bursty traffic specification method

Latency/Bandwidth Tradeoff