1

Chapter 5Chapter 5

Asynchronous Asynchronous Transfer ModeTransfer Mode(ATM)(ATM)

Chapter 5 ATM2

Introduction Introduction

ATM Protocol ArchitectureATM Protocol ArchitectureLogical connectionsLogical connectionsATM cell structureATM cell structureService levels/categoriesService levels/categoriesATM Adaptation Layer (AAL)ATM Adaptation Layer (AAL)

Chapter 5 ATM3

Introduction Introduction ATM evolved from B-ISDN development ATM evolved from B-ISDN development

effortsefforts– Frame Relay: high-speed WAN (1.5+ Mbps)Frame Relay: high-speed WAN (1.5+ Mbps)– ATM: very high speed WAN (155 Mbps and ATM: very high speed WAN (155 Mbps and

622Mbps)622Mbps) ATM, like Frame Relay, was built on the ATM, like Frame Relay, was built on the

assumption that the underlying physical assumption that the underlying physical media was reliable and flexiblemedia was reliable and flexible– minimal error and flow control capabilitiesminimal error and flow control capabilities– even more streamlined, therefore faster, even more streamlined, therefore faster,

than Frame Relaythan Frame Relay Specifications developed by ITU-T and Specifications developed by ITU-T and

ATM ForumATM Forum

Chapter 5 ATM4

ATM Protocol ArchitectureATM Protocol Architecture

Fixed-size packets called cellsFixed-size packets called cells– ““cell switchingcell switching” like packet switching” like packet switching

2 primary protocol layers relate to 2 primary protocol layers relate to ATM functions:ATM functions:– Common layer providing packet Common layer providing packet

transfers, logical connections (transfers, logical connections (ATMATM))– Service dependent ATM adaptation Service dependent ATM adaptation

layer (layer (AALAAL))AAL maps other protocols to ATMAAL maps other protocols to ATM

– like IP (AAL5)like IP (AAL5)

Chapter 5 ATM5

Protocol Model has 3 planesProtocol Model has 3 planes

UserUser – provides for user information – provides for user information transfer and associated controls transfer and associated controls (flow control, congestion control)(flow control, congestion control)

ControlControl – performs call control and – performs call control and connection control functions connection control functions (signaling)(signaling)

ManagementManagement – provides plane – provides plane management and layer management and layer management and coordination management and coordination functionsfunctions

Chapter 5 ATM6

ATM Protocol Reference ATM Protocol Reference ModelModel

Various data rates (155.52 Mbps, 622.08 Mbps) over various physical media types (Fiber Optic, SONET, UTP, etc.)

Framing, cell structure& Logical Connections

Map data to the ATM cellstructure

Chapter 5 ATM7

User Plane LayersUser Plane Layers

AALAAL

ATMATM

User information

User information

AALAAL

ATMATM

PHYPHYPHYPHY

ATMATM

PHYPHY

ATMATM

PHYPHY

……

End systemEnd system End systemEnd systemNetworkNetwork

Chapter 5 ATM8

User Plane LayersUser Plane LayersUser

informationUser

information

Chapter 5 ATM9

Logical ConnectionsLogical ConnectionsVCC (VCC (Virtual ChannelVirtual Channel Connection): Connection):

a logical connection analogous to a logical connection analogous to a virtual circuit in X.25, or Frame a virtual circuit in X.25, or Frame Relay data link connectionRelay data link connection– full-duplex flow between end usersfull-duplex flow between end users– user-network control signalinguser-network control signaling– network-network network-network

management/routingmanagement/routing

VPC (VPC (Virtual PathVirtual Path Connection): a Connection): a bundle of VCCs with the bundle of VCCs with the same end same end pointspoints (not necessarily same end- (not necessarily same end-users)users)– and switched along the same pathand switched along the same path

Chapter 5 ATM10

ATM Connection ATM Connection RelationshipsRelationships

Virtual ChannelVirtual Channel: basic logical communications channel: basic logical communications channelVirtual PathVirtual Path: groups of “common” virtual channels: groups of “common” virtual channelsPhysical Transmission PathPhysical Transmission Path: physical communications link: physical communications link

Chapter 5 ATM12

Advantages of Virtual PathsAdvantages of Virtual PathsSimplified network architecture – Simplified network architecture – allows allows

separation of functionality into into individual logical separation of functionality into into individual logical connections and related groups of logical connections and related groups of logical connectionsconnections

Increased network performance and Increased network performance and reliability – reliability – network consists of fewer aggregated network consists of fewer aggregated entitiesentities

Reduced processing and short Reduced processing and short connection setup time – connection setup time – complex setup complex setup tasks are in virtual paths, simplifies setup of new tasks are in virtual paths, simplifies setup of new virtual channels over existing virtual pathvirtual channels over existing virtual path

Enhanced network services – Enhanced network services – supports supports user-specified closed groups/networks of VC bundlesuser-specified closed groups/networks of VC bundles

Chapter 5 ATM13

Virtual Path/Virtual Channel Virtual Path/Virtual Channel TerminologyTerminologyVirtual ChannelVirtual Channel (VC) A generic term used to (VC) A generic term used to

describe unidirectional transport describe unidirectional transport of cells associated by a common of cells associated by a common unique identifierunique identifier

Virtual Channel Identifier Virtual Channel Identifier (VCI) A unique numerical tag for (VCI) A unique numerical tag for a particular VC linka particular VC link

Virtual Channel LinkVirtual Channel Link A means of unidirectional A means of unidirectional transport of cells between the transport of cells between the point where a VCI is assigned point where a VCI is assigned and where it is translated or and where it is translated or terminatedterminated

Virtual Channel Connection Virtual Channel Connection (VCC) (VCC) A concatenation of VC links that A concatenation of VC links that extends between two connected extends between two connected ATM end-pointsATM end-points

Chapter 5 ATM14

Virtual Path/Virtual Channel Virtual Path/Virtual Channel TerminologyTerminologyVirtual Path Virtual Path (VP) A generic term which (VP) A generic term which

describes unidirectional transfer of describes unidirectional transfer of cells that are associated with a cells that are associated with a common unique identifiercommon unique identifier

Virtual Path IdentifierVirtual Path Identifier (VPI) Identifies a particular VP(VPI) Identifies a particular VPVirtual Path LinkVirtual Path Link A group of VC links identified by a A group of VC links identified by a

common identifier between the common identifier between the point where the identifier (VPI) is point where the identifier (VPI) is assigned and where it is translated assigned and where it is translated or terminatedor terminated

Virtual Path Connection Virtual Path Connection (VPC) A concatenation of VP links (VPC) A concatenation of VP links that extends between ATM end-that extends between ATM end-points where the VCIs are assigned points where the VCIs are assigned and where they are translated or and where they are translated or terminatedterminated

Chapter 5 ATM15

ATM VPC/VCCATM VPC/VCC

c ATMSw1

ATMSw4

ATMSw2

ATMSw3

ab

de

VP3 VP5

VP2

VP1

a

bc

de

Sw = switch

DCC = Cross-connect switch

ATMDCC

VP6

Chapter 5 ATM16

ATM Connection ATM Connection RelationshipsRelationships

Chapter 5 ATM17

VPC/VCC CharacteristicsVPC/VCC CharacteristicsQuality of Service (Quality of Service (QoSQoS))Switched and semi-permanentSwitched and semi-permanent

virtual channel connectionsvirtual channel connectionsCell sequenceCell sequence integrity integrityTraffic parameterTraffic parameter negotiation and negotiation and

usage monitoringusage monitoring– average rate, peak rate, burstiness, average rate, peak rate, burstiness,

peak duration, etc.peak duration, etc. ((VPC onlyVPC only) virtual channel ) virtual channel

identifier restriction within a VPCidentifier restriction within a VPC

Chapter 5 ATM20

Control SignalingControl SignalingA mechanism to A mechanism to establish and release establish and release

VPCs and VCCsVPCs and VCCs (per ITU-T Rec. I.150) (per ITU-T Rec. I.150)4 methods for VCCs:4 methods for VCCs:

– Semi-permanent VCC:Semi-permanent VCC: no control no control signaling requiredsignaling required

– Meta-signaling channel:Meta-signaling channel: permanent, low permanent, low data rate channel for setting up data rate channel for setting up signaling channelssignaling channels

– User-to-network signaling virtual User-to-network signaling virtual channel:channel: set up between user and set up between user and network network

– User-to-user signaling virtual channel:User-to-user signaling virtual channel: set up between users within a VPC, set up between users within a VPC, allowing users to set up and tear down allowing users to set up and tear down VCCs, without network interventionVCCs, without network intervention

Chapter 5 ATM22

ATM CellsATM Cells

Fixed sizeFixed size– 5-octet header5-octet header– 48-octet information field48-octet information field

Small cells may reduce queuing Small cells may reduce queuing delay for high-priority cells delay for high-priority cells ((essential for low delayessential for low delay))

Fixed size facilitates more efficient Fixed size facilitates more efficient switching in hardware (switching in hardware (essential essential for very high data ratesfor very high data rates))

Chapter 5 ATM23

ATM Cell Format ATM Cell Format (p. 98)(p. 98)

Chapter 5 ATM24

Header FormatHeader Format

Generic flow control (Generic flow control (more ->more ->))Virtual path identifier (VPI)Virtual path identifier (VPI)Virtual channel identifier (VCI)Virtual channel identifier (VCI)Payload type (3 bits: identifies cell Payload type (3 bits: identifies cell

as user data or network as user data or network management cell, presence of management cell, presence of congestion, SDU type)congestion, SDU type)

Cell loss priority (0: high; 1: low)Cell loss priority (0: high; 1: low)Header error control (Header error control (more ->more ->))

Chapter 5 ATM25

Generic Flow ControlGeneric Flow ControlUsed to Used to control traffic flowcontrol traffic flow at user- at user-

network interface (UNI) to alleviate network interface (UNI) to alleviate short-term overload conditionsshort-term overload conditions– Note: Note: not employed in network corenot employed in network core

When GFC is enabled at the UNI, When GFC is enabled at the UNI, two procedures are used:two procedures are used:– Uncontrolled transmission: not subject Uncontrolled transmission: not subject

to flow controlto flow control– Controlled transmission: flow control Controlled transmission: flow control

constraints (using GFC mechanism) constraints (using GFC mechanism) are in forceare in force

Chapter 5 ATM27

Header Error ControlHeader Error Control8-bit field - calculated based on 8-bit field - calculated based on

the other 32 bits in the headerthe other 32 bits in the header– CRC based on xCRC based on x88 + x + x22 + x + 1 + x + 1

-> generator is 100000111 -> generator is 100000111error detectionerror detection in some cases, error correction of in some cases, error correction of

single-bit errors in headersingle-bit errors in header2 modes: 2 modes:

– Error detectionError detection– Error correctionError correction

Chapter 5 ATM28

HEC Operation at ReceiverHEC Operation at Receiver

Based on recognition of fact Based on recognition of fact that bit errors occur in bursts. that bit errors occur in bursts.

Chapter 5 ATM31

ATM Service CategoriesATM Service Categories

Real-time serviceReal-time service– Constant bit rate (Constant bit rate (CBRCBR))– Real-time variable bit rate (Real-time variable bit rate (rt-VBRrt-VBR))

Non-real-time serviceNon-real-time service– Non-real-time variable bit rate (Non-real-time variable bit rate (nrt-nrt-

VBRVBR))– Available bit rate (Available bit rate (ABRABR))– Unspecified bit rate (Unspecified bit rate (UBRUBR))– Guaranteed frame rate (Guaranteed frame rate (GFRGFR))

Chapter 5 ATM32

ATM Bit Rate Service LevelsATM Bit Rate Service Levels

Chapter 5 ATM33

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

Support higher-level protocols Support higher-level protocols and/or native applicationsand/or native applications– e.g., PCM voice, LAPF, e.g., PCM voice, LAPF, IPIP

AAL ServicesAAL Services– Handle transmission errorsHandle transmission errors– Segmentation/reassembly (Segmentation/reassembly (SARSAR))– Handle lost and misinserted cell Handle lost and misinserted cell

conditionsconditions– Flow control and timing controlFlow control and timing control

Chapter 5 ATM34

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

Chapter 5 ATM35

Applications of AAL and Applications of AAL and ATMATMCircuit emulation (e.g., T-1 Circuit emulation (e.g., T-1

synchronous TDM circuits)synchronous TDM circuits)VBR voice and videoVBR voice and videoGeneral data servicesGeneral data services IP over ATMIP over ATMMultiprotocol encapsulation over Multiprotocol encapsulation over

ATM (MPOA)ATM (MPOA)LAN emulation (LANE)LAN emulation (LANE)

Chapter 5 ATM36

AAL Protocol and ServicesAAL Protocol and Services

Basis for classification:Basis for classification:•requirement for a timing relationship requirement for a timing relationship between source and destinationbetween source and destination

•requirement for a constant bit rate data requirement for a constant bit rate data flowflow

•connection or connectionless transferconnection or connectionless transfer

Chapter 5 ATM37

AAL ProtocolsAAL ProtocolsAAL layer has 2 sublayers:AAL layer has 2 sublayers:

– Convergence SublayerConvergence Sublayer (CS) (CS) Supports specific applications/protocols Supports specific applications/protocols

using AALusing AAL Users attach via the Service Access Users attach via the Service Access

Point (like a port number)Point (like a port number) Common part (CPCS) and application Common part (CPCS) and application

service-specific part (SSCS)service-specific part (SSCS)

– Segmentation and Reassembly Segmentation and Reassembly SublayerSublayer (SAR) (SAR)

Packages data from CS into ATM cells Packages data from CS into ATM cells and unpacks at other endand unpacks at other end

Chapter 5 ATM38

AAL Protocols and PDUsAAL Protocols and PDUs

Chapter 5 ATM39

AAL Protocol DescriptionsAAL Protocol Descriptions

Chapter 5 ATM40

Segmentation and Segmentation and Reassembly PDUsReassembly PDUs

Chapter 5 ATM41

AAL Type 1AAL Type 1

Constant-bit-rate sourceConstant-bit-rate sourceSAR simply packs bits into cells SAR simply packs bits into cells

and unpacks them at destinationand unpacks them at destinationOne-octet header contains 3-bit One-octet header contains 3-bit

SC field to provide an 8-cell frame SC field to provide an 8-cell frame structurestructure

No CS PDU structure is defined No CS PDU structure is defined since CS sublayer primarily for since CS sublayer primarily for clocking and synchronizationclocking and synchronization

Chapter 5 ATM42

AAL Type 1AAL Type 1

Chapter 5 ATM43

AAL Type 2AAL Type 2

Intended for variable bit rate Intended for variable bit rate applications that generate bursty data applications that generate bursty data and demand low lossand demand low loss

Originally, connectionless (AAL4) or Originally, connectionless (AAL4) or connection (AAL3) oriented, now connection (AAL3) oriented, now combined into single format (AAL 3/4)combined into single format (AAL 3/4)

Provides comprehensive sequencing Provides comprehensive sequencing and error control mechanismsand error control mechanisms

AAL Type 3/4AAL Type 3/4

Intended for use with applications with Intended for use with applications with variable bit-rate service on multiple variable bit-rate service on multiple channels (multiplexing), or low bit rate, channels (multiplexing), or low bit rate, short-frame trafficshort-frame traffic

Chapter 5 ATM45

AAL 3/4AAL 3/4

Chapter 5 ATM46

AAL Type 5AAL Type 5

Streamlined transport for Streamlined transport for connection oriented protocolsconnection oriented protocols– Reduce protocol processing Reduce protocol processing

overheadoverhead– Reduce transmission overheadReduce transmission overhead– Ensure adaptability to existing Ensure adaptability to existing

transport protocolstransport protocols– primary function is segmentation primary function is segmentation

and reassembly of higher-level PDUsand reassembly of higher-level PDUs

Chapter 5 ATM48

AAL5AAL5