Asynchronous Asynchronous Transfer Mode Transfer Mode (ATM)(ATM)

Asynchronous Transfer ModeAsynchronous Transfer Mode

One man had a vision of railways that would link all the One man had a vision of railways that would link all the mainline railroad termini. His name was Charles Pearson mainline railroad termini. His name was Charles Pearson and, though born the son of an upholsterer, he became and, though born the son of an upholsterer, he became Solicitor to the city of London. There had previously been a Solicitor to the city of London. There had previously been a plan for gaslit subway streets through which horse-drawn plan for gaslit subway streets through which horse-drawn traffic could pass. This was rejected on the grounds that such traffic could pass. This was rejected on the grounds that such sinister tunnels would become lurking places for thieves. sinister tunnels would become lurking places for thieves. Twenty years before his system was built, Pearson envisaged a Twenty years before his system was built, Pearson envisaged a line running through "a spacious archway," well-lit and well-line running through "a spacious archway," well-lit and well-ventilated.ventilated.

His was a scheme for trains in a drain.His was a scheme for trains in a drain.

—King Solomon's Carpet, Barbara Vine (Ruth Rendell)

ATMATM

a streamlined packet transfer interfacea streamlined packet transfer interface similarities to packet switchingsimilarities to packet switching

transfers data in discrete chunkstransfers data in discrete chunks supports multiple logical connections over a supports multiple logical connections over a

single physical interfacesingle physical interface ATM uses fixed sized packets called cellsATM uses fixed sized packets called cells with minimal error and flow controlwith minimal error and flow control data rates of 25.6Mbps to 622.08Mbpsdata rates of 25.6Mbps to 622.08Mbps

Protocol ArchitectureProtocol Architecture

Reference Model PlanesReference Model Planes

user planeuser plane provides for user information transferprovides for user information transfer

control planecontrol plane call and connection controlcall and connection control

management planemanagement plane plane managementplane management

• whole system functionswhole system functions layer managementlayer management

• Resources and parameters in protocol entitiesResources and parameters in protocol entities

ATM Logical ConnectionsATM Logical Connections

virtual channel connections (VCC)virtual channel connections (VCC) analogous to virtual circuit in X.25analogous to virtual circuit in X.25

basic unit of switching between two end usersbasic unit of switching between two end users full duplexfull duplex fixed size cellsfixed size cells

also for also for user-network exchange (control) user-network exchange (control) network-network exchange (network mgmt & routing)network-network exchange (network mgmt & routing)

ATM Virtual Path ConnectionATM Virtual Path Connection

virtual path connection (VPC)virtual path connection (VPC) bundle of VCC with same end pointsbundle of VCC with same end points

Advantages of Virtual PathsAdvantages of Virtual Paths

simplified network architecturesimplified network architecture increased network performance and increased network performance and

reliabilityreliability reduced processingreduced processing short connection setup timeshort connection setup time enhanced network servicesenhanced network services

Call Call EstablishEstablish

ment ment Using VPsUsing VPs

Virtual Channel Connection Virtual Channel Connection UsesUses

between end usersbetween end users end to end user dataend to end user data Control signalsControl signals VPC provides overall capacityVPC provides overall capacity

• VCC organization done by usersVCC organization done by users

between end user and networkbetween end user and network control signalingcontrol signaling

between network entitiesbetween network entities network traffic managementnetwork traffic management routingrouting

VP/VC CharacteristicsVP/VC Characteristics

quality of servicequality of service switched and semi-permanent channel switched and semi-permanent channel

connectionsconnections call sequence integritycall sequence integrity traffic parameter negotiation and usage traffic parameter negotiation and usage

monitoringmonitoring

VPC onlyVPC only virtual channel identifier restriction within VPCvirtual channel identifier restriction within VPC

Control Signaling - VCCControl Signaling - VCC

to establish or release VCCs & VPCs to establish or release VCCs & VPCs uses a separate connectionuses a separate connection methods are:methods are:

1.1. semi-permanent VCCsemi-permanent VCC

2.2. meta-signaling channelmeta-signaling channel

3.3. user to network signaling virtual channeluser to network signaling virtual channel

4.4. user to user signaling virtual channeluser to user signaling virtual channel

Control Signaling - VPCControl Signaling - VPC

methods for control signalling for VPCs:methods for control signalling for VPCs:1.1. Semi-permanentSemi-permanent

2.2. Customer controlledCustomer controlled

3.3. Network controlledNetwork controlled

ATM CellsATM Cells

ATM Header FieldsATM Header Fields

generic flow controlgeneric flow control Virtual path identifierVirtual path identifier Virtual channel identifierVirtual channel identifier payload typepayload type cell loss prioritycell loss priority header error controlheader error control

Generic Flow Control (GFC)Generic Flow Control (GFC) control traffic flow at user to network interface control traffic flow at user to network interface

(UNI) to alleviate short term overload(UNI) to alleviate short term overload two sets of procedurestwo sets of procedures

uncontrolled transmissionuncontrolled transmission controlled transmissioncontrolled transmission

every connection subject to flow control or notevery connection subject to flow control or not if subject to flow controlif subject to flow control

may be one group (A) defaultmay be one group (A) default may be two groups (A and B)may be two groups (A and B)

flow control is from subscriber to networkflow control is from subscriber to network

GFC - Single Group of GFC - Single Group of ConnectionsConnections

1.1. If TRANSMIT=1 send uncontrolled cells any If TRANSMIT=1 send uncontrolled cells any time. If TRANSMIT=0 no cells may be senttime. If TRANSMIT=0 no cells may be sent

2.2. If HALT received, TRANSMIT=0 until If HALT received, TRANSMIT=0 until NO_HALTNO_HALT

3.3. If TRANSMIT=1 & no uncontrolled cell to send:If TRANSMIT=1 & no uncontrolled cell to send:1.1. If GO_CNTR>0, TE may send controlled cell and If GO_CNTR>0, TE may send controlled cell and

decrement GO_CNTRdecrement GO_CNTR2.2. If GO_CNTR=0, TE may not send controlled cellsIf GO_CNTR=0, TE may not send controlled cells

4.4. TE sets GO_CNTR to GO_VALUE upon TE sets GO_CNTR to GO_VALUE upon receiving SET signalreceiving SET signal

Use of HALTUse of HALT

to limit effective data rate on ATMto limit effective data rate on ATM should be cyclicshould be cyclic to reduce data rate by half, HALT issued to reduce data rate by half, HALT issued

to be in effect 50% of timeto be in effect 50% of time done on regular pattern over lifetime of done on regular pattern over lifetime of

connectionconnection

Two Queue ModelTwo Queue Model

uses two counters each with current & uses two counters each with current & initial values:initial values: GO_CNTR_AGO_CNTR_A GO_VALUE_AGO_VALUE_A GO_CNTR_BGO_CNTR_B GO_VALUE_BGO_VALUE_B

Header Error ControlHeader Error Control

Effect of Effect of Error in Error in

Cell Cell HeaderHeader

Impact of Random Bit ErrorsImpact of Random Bit Errors on HEC Performanceon HEC Performance

Transmission of ATM CellsTransmission of ATM Cells

I.432 specifies several data rates:I.432 specifies several data rates: 622.08Mbps622.08Mbps 155.52Mbps155.52Mbps 51.84Mbps51.84Mbps 25.6Mbps25.6Mbps

two choices of transmission structure:two choices of transmission structure: Cell based physical layerCell based physical layer SDH based physical layerSDH based physical layer

Cell Based Physical LayerCell Based Physical Layer

no framing imposedno framing imposed vontinuous stream of 53 octet cellsvontinuous stream of 53 octet cells cell delineation based on header error cell delineation based on header error

control fieldcontrol field

Cell Delineation State Cell Delineation State DiagramDiagram

Impact of Random Bit Errors Impact of Random Bit Errors on Cell Delineation on Cell Delineation

PerformancePerformance

Acquisition Time v Bit Error Acquisition Time v Bit Error RateRate

SDH Based Physical LayerSDH Based Physical Layer

imposes structure on ATM streamimposes structure on ATM stream eg. for 155.52Mbpseg. for 155.52Mbps use STM-1 (STS-3) frameuse STM-1 (STS-3) frame

can carry ATM and STM payloadscan carry ATM and STM payloads specific connections can be circuit specific connections can be circuit

switched using SDH channelswitched using SDH channel SDH multiplexing techniques can combine SDH multiplexing techniques can combine

several ATM streamsseveral ATM streams

STM-1 Payload for SDH-STM-1 Payload for SDH-Based ATM Cell TransmissionBased ATM Cell Transmission

ATM Service CategoriesATM Service Categories

Real time - limit amount/variation of delayReal time - limit amount/variation of delay Constant bit rate (CBR)Constant bit rate (CBR) Real time variable bit rate (rt-VBR)Real time variable bit rate (rt-VBR)

Non-real time - for bursty trafficNon-real time - for bursty traffic Non-real time variable bit rate (nrt-VBR)Non-real time variable bit rate (nrt-VBR) Available bit rate (ABR)Available bit rate (ABR) Unspecified bit rate (UBR)Unspecified bit rate (UBR) Guaranteed frame rate (GFR)Guaranteed frame rate (GFR)

Constant Bit Rate (CBR)Constant Bit Rate (CBR)

fixed data rate continuously availablefixed data rate continuously available tight upper bound on delaytight upper bound on delay uncompressed audio and videouncompressed audio and video

video conferencingvideo conferencing interactive audiointeractive audio A/V distribution and retrievalA/V distribution and retrieval

Real-Time Variable Bit RateReal-Time Variable Bit Rate(rt-VBR)(rt-VBR)

for time sensitive applicationsfor time sensitive applications tightly constrained delay and delay variationtightly constrained delay and delay variation

rt-VBR applications transmit data at a rate that rt-VBR applications transmit data at a rate that varies with timevaries with time eg. compressed video eg. compressed video produces varying sized image framesproduces varying sized image frames original (uncompressed) frame rate constantoriginal (uncompressed) frame rate constant so compressed data rate variesso compressed data rate varies

hence can statistically multiplex connectionshence can statistically multiplex connections

Non-Real-Time Variable Bit Rate Non-Real-Time Variable Bit Rate (nrt-VBR)(nrt-VBR)

if can characterize expected bursty traffic flowif can characterize expected bursty traffic flow eg. airline reservations, banking transactionseg. airline reservations, banking transactions

ATM net allocates resources based on thisATM net allocates resources based on this to meet critical response-time requirementsto meet critical response-time requirements

giving improve QoS in loss and delaygiving improve QoS in loss and delay end system specifies:end system specifies:

peak cell rate peak cell rate sustainable or average rate sustainable or average rate measure of how bursty traffic ismeasure of how bursty traffic is

Unspecified Bit Rate (UBR)Unspecified Bit Rate (UBR)

may be additional capacity over and above that may be additional capacity over and above that used by CBR and VBR trafficused by CBR and VBR traffic not all resources dedicated to CBR/VBR trafficnot all resources dedicated to CBR/VBR traffic unused cells due to bursty nature of VBRunused cells due to bursty nature of VBR

for application that can tolerate some cell loss or for application that can tolerate some cell loss or variable delaysvariable delays eg. TCP based trafficeg. TCP based traffic

cells forwarded on FIFO basiscells forwarded on FIFO basis best effort servicebest effort service

Available Bit Rate (ABR)Available Bit Rate (ABR)

application specifies peak cell rate (PCR) application specifies peak cell rate (PCR) and minimum cell rate (MCR)and minimum cell rate (MCR)

resources allocated to give at least MCRresources allocated to give at least MCR spare capacity shared among all ARB spare capacity shared among all ARB

sourcessources eg. LAN interconnectioneg. LAN interconnection

ATM Bit Rate ServicesATM Bit Rate Services

SummarySummary

Asynchronous Transfer Mode (ATM)Asynchronous Transfer Mode (ATM) architecture & logical connectionsarchitecture & logical connections ATM Cell formatATM Cell format transmission of ATM cellstransmission of ATM cells ATM servicesATM services