CHAPTER #6CHAPTER #6

IntroductiIntroduction on to ATMto ATM

ContentsContents IntroductionIntroduction ATM CellsATM Cells ATM Architecture ATM Architecture ATM ConnectionsATM Connections Addressing and SignalingAddressing and Signaling IP over ATMIP over ATM

6.1. Introduction6.1. Introduction ATM-Asynchronous Transfer ModeATM-Asynchronous Transfer Mode A transfer mode in which the A transfer mode in which the

information is organized into cellsinformation is organized into cells It isIt is asynchronous asynchronous in the sense in the sense

that the recurrence of cells that the recurrence of cells containing information from a containing information from a particular user is not necessarily particular user is not necessarily periodicperiodic

Asynchronous transfer mode (= Asynchronous transfer mode (= Statistical multiplexing)Statistical multiplexing)

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

ATM requires connections to ATM requires connections to be established before any be established before any communication takes placecommunication takes place

A connection is called a A connection is called a virtual circuit (VC) which virtual circuit (VC) which could be permanent or could be permanent or switchedswitched

Information to be transmitted is Information to be transmitted is divided into cells consisting of: 5 divided into cells consisting of: 5 bytes header and 48 bytes user bytes header and 48 bytes user data. data.

Fixed sized cells enable HW Fixed sized cells enable HW switchingswitching

Typical speeds of ATM are 155 Typical speeds of ATM are 155 and 622 Mbps and 622 Mbps

6.2.ATM’s Key Concepts6.2.ATM’s Key Concepts ATM uses Virtual-Circuit Packet ATM uses Virtual-Circuit Packet

Switching(VCPS)Switching(VCPS) ATM can reserve capacity for a VC ATM can reserve capacity for a VC This is useful for voice and video, This is useful for voice and video,

which require a minimum level of which require a minimum level of serviceservice

Overhead for setting up a connection is Overhead for setting up a connection is expensive if data expensive if data

transmission is short transmission is short

(e.g., web browsing)(e.g., web browsing)

6.2. (Follow)6.2. (Follow) ATM packets are small and have a fixed ATM packets are small and have a fixed

sizesize

– – Packets in ATM are called Packets in ATM are called cellscells

– – Small packets are good for voice and Small packets are good for voice and video video transmissions transmissions

6.3.ATM Cells6.3.ATM Cells

Why 53 Bytes?Why 53 Bytes?

AdvantagesAdvantages

DisadvantagesDisadvantages

6.3.1.Why 53 Bytes?6.3.1.Why 53 Bytes?

A 48 byte payload was the A 48 byte payload was the result of a compromise result of a compromise between a 32 byte payload between a 32 byte payload and a 64 byte payloadand a 64 byte payload

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6.3.2.Advantages6.3.2.Advantages

– – Low packetization delay Low packetization delay for continuous bit rate for continuous bit rate applic- ations (video, audio)applic- ations (video, audio)

– – Processing at switches is Processing at switches is easiereasier

6.3.3.Disadvantages6.3.3.Disadvantages

– – High overhead (5 Bytes High overhead (5 Bytes per 48)per 48)

– – Poor utilization at lower Poor utilization at lower line rates linksline rates links

6.3.4. UNI Cells6.3.4. UNI Cells

6.3.5. NNI Cells6.3.5. NNI Cells

6.4.ATM Architecture6.4.ATM Architecture

The ATM Reference Model:The ATM Reference Model:

ATM technology has its ATM technology has its own protocol architectureown protocol architecture

6.5.ATM Reference Model6.5.ATM Reference Model

6.6.ATM Layers and 6.6.ATM Layers and PlanesPlanes

ATM is a three dimensional ATM is a three dimensional model consisting of layers and model consisting of layers and planesplanes

Physical layer deals with physical Physical layer deals with physical media issues,further divided into: media issues,further divided into:

(1) PMD and (1) PMD and (2) TC sub layers(2) TC sub layers

ATM layer defines cell format ATM layer defines cell format and connection setupand connection setup

6.6.(Follow)6.6.(Follow)

ATM Adaptation Layer ATM Adaptation Layer (AAL) segments and (AAL) segments and reassembles upper layer reassembles upper layer into and from into and from cells,divided into : cells,divided into :

1.1. SAR and SAR and

2.2. CS sub layersCS sub layers

User planes deals with data User planes deals with data transport,flow and error transport,flow and error controlcontrol

Control plane deals with Control plane deals with connection managementconnection management

The Layer and Plane The Layer and Plane management deals with management deals with resource management and resource management and interlayer coordinationinterlayer coordination

6.7.ATM Adaptation Layer (AAL)6.7.ATM Adaptation Layer (AAL)

AAL encapsulates user-AAL encapsulates user-level datalevel data

Performs segmentation Performs segmentation and reassembly of user-and reassembly of user-level messageslevel messages

6.8.Layers of 6.8.Layers of ATMATM

6.8.1..Functions of Layers6.8.1..Functions of Layers

6.9.ATM Layer6.9.ATM Layer

The ATM Layer is The ATM Layer is responsible for the responsible for the transport of 53 byte cells transport of 53 byte cells across an ATM networkacross an ATM network

Multiplex logical channels Multiplex logical channels within a physical channelwithin a physical channel

The ATM Layer can provide a variety of The ATM Layer can provide a variety of services for cells from an ATM virtual services for cells from an ATM virtual connection:connection:-Constant Bit Rate (CBR)-Constant Bit Rate (CBR)-Variable Bit Rate(VBR)-Variable Bit Rate(VBR)- Available Bit Rate(ABR)- Available Bit Rate(ABR)-Unspecified Bit Rate(UBR)-Unspecified Bit Rate(UBR)-Guarantees Frame Rate(GFR)-Guarantees Frame Rate(GFR)

6.10. Correspondence 6.10. Correspondence between ATM and between ATM and OSI Reference Models OSI Reference Models

6.11.ATM 6.11.ATM ConnectionsConnections

A Packet SwitchA Packet Switch

6.12.Forwarding with VCs6.12.Forwarding with VCs

6.13.Addressing and 6.13.Addressing and SignalingSignaling

ATM End system Addresses ATM End system Addresses (AESA)(AESA)

ALL ATM addresses are 20 ALL ATM addresses are 20 bytes longbytes long

Source and destination Source and destination address are supplied when address are supplied when setting up a connectionsetting up a connection

ATM endpoints use the ATM endpoints use the NSAP (Network Service NSAP (Network Service Access Point) format from Access Point) format from ISO OSIISO OSI

Three different types of Three different types of addresses : addresses :

1.1. NSAP encodingNSAP encoding for E.164: for E.164: ISDN telephone ISDN telephone numbers numbers (e.g., 001-434-9822200)(e.g., 001-434-9822200)

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2.DCC format2.DCC format: for public : for public networks, networks,

3.ICD format3.ICD format: for private : for private networksnetworks

6.14.ATM End system Addresses 6.14.ATM End system Addresses (AESA)(AESA)

6.15.Formats of an ATM 6.15.Formats of an ATM addressaddress

6.16.ATM UNI Signaling6.16.ATM UNI Signaling

Significant Signaling Significant Signaling ProtocolsProtocols

ATM Forum:ATM Forum:

1.1. UNI 3.0. UNI signaling UNI 3.0. UNI signaling protocol for point-to- protocol for point-to- point connections.point connections.

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2.UNI 3.1. Supports point-to-2.UNI 3.1. Supports point-to-multipoint connections.multipoint connections.

3.UNI 4.0. Supports Leaf 3.UNI 4.0. Supports Leaf initiated join multipoint initiated join multipoint connectionsconnections

4. -PNNI. for network node 4. -PNNI. for network node signalingsignaling

6.17.Which Address Format To Use?6.17.Which Address Format To Use?

Currently each service Currently each service provider makes its own provider makes its own choicechoice

– – This introduces This introduces problems (SVC problems (SVC compatibility)compatibility)

Most ATM switches Most ATM switches support multiple formatssupport multiple formats

ATM Forum prepares ATM Forum prepares standards to translate standards to translate addresses at network addresses at network boundaries (NNI interfaces)boundaries (NNI interfaces)

– –

Interworking of ATM Interworking of ATM Networks (IAN)Networks (IAN)

6.18.IP over ATM6.18.IP over ATM

Issues with sending IP traffic Issues with sending IP traffic over ATMover ATM

Address resolution:Address resolution:

– – IP address IP address VPI/VCIVPI/VCI

– – IP address IP address ATM ATM addressaddress

Emulation of broadcast Emulation of broadcast operation on IP sub operation on IP sub networksnetworks

RoutingRouting