Chapter 3Chapter 3Asynchronous Transfer Asynchronous Transfer Mode (ATM)Mode (ATM)

Synchronous vs. Synchronous vs. AsynchronousAsynchronous

ATM- cell switching technology (asynchronous)ATM- cell switching technology (asynchronous) TDM – circuit switching technology TDM – circuit switching technology

(synchronous)(synchronous) ATM is more efficient than TDM. (why?)ATM is more efficient than TDM. (why?) TDM: dedicated time slots.TDM: dedicated time slots. ATM: time slots are available on demand with ATM: time slots are available on demand with

information identifying the destination of the information identifying the destination of the transmission contained in the header of each transmission contained in the header of each ATM cell.ATM cell.

ATMATM Multi-speed network environment that provides a Multi-speed network environment that provides a

variety of complex network services variety of complex network services Can carry voice, data, video separately or Can carry voice, data, video separately or

simultaneouslysimultaneously Can be used in LANs, MANs, or WANsCan be used in LANs, MANs, or WANs Fixed-length packets (cells)Fixed-length packets (cells) Allows multiple logical connections to be multiplexedAllows multiple logical connections to be multiplexed Minimal error and flow control capabilitiesMinimal error and flow control capabilities Connection-oriented virtual channelConnection-oriented virtual channel

Cell Switched ATMCell Switched ATM Similar to frame relaySimilar to frame relay Difference? Difference?

Frame relay switches variable length frames within frame relay Frame relay switches variable length frames within frame relay cloud from source to destinationcloud from source to destination

ATM switches fixed-length cells (48 byte information field, 5 byte ATM switches fixed-length cells (48 byte information field, 5 byte header)header)

Based on packet switching (connection-oriented)Based on packet switching (connection-oriented) Cell sequence integrity preserved via virtual channelCell sequence integrity preserved via virtual channel VCC – virtual channel connection – is set up between end users, VCC – virtual channel connection – is set up between end users,

variable rate, full duplexvariable rate, full duplex VCC also used for controlVCC also used for control

Information field is carried transparently through the network, with Information field is carried transparently through the network, with minimal error controlminimal error control

ATM NetworkATM Network(integrated voice, video, and data services)(integrated voice, video, and data services)

Protocol ArchitectureProtocol Architecture

ATM Protocol LayersATM Protocol Layers

ATM Applications

ATM AdaptiveLayer (AAL)

ATMLayer

PhysicalLayer

ATMLayer

PhysicalLayer

ATM Applications

ATM AdaptiveLayer (AAL)

ATMLayer

PhysicalLayer

ATM End Station ATM Switch ATM End Station

ATM Physical LayerATM Physical Layer Transports cells via a communications channel Transports cells via a communications channel

(either optical or electrical)(either optical or electrical) LAN support: 25-155 Mbps copper or fiberLAN support: 25-155 Mbps copper or fiber WAN support: SONET rates over fiberWAN support: SONET rates over fiber Physical Medium Sublayer: bit transfer, bit Physical Medium Sublayer: bit transfer, bit

alignment, and copper/fiber conversionsalignment, and copper/fiber conversions Transmission Convergence Sublayer: bit/cell Transmission Convergence Sublayer: bit/cell

conversion at sending and receiving nodesconversion at sending and receiving nodes

ATM LayerATM Layer Handles functions of the network layerHandles functions of the network layer Connection-oriented without Connection-oriented without

acknowledgementsacknowledgements Two possible interfaces:Two possible interfaces:

UNI – User-Network Interface: Boundary UNI – User-Network Interface: Boundary between an ATM network and hostbetween an ATM network and host

NNI – Network-Network Interface: Between NNI – Network-Network Interface: Between two ATM switchestwo ATM switches

UNI/NNI InterfaceUNI/NNI Interface

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

Maps higher-layer information into ATM Maps higher-layer information into ATM cells to be transported over an ATM cells to be transported over an ATM networknetwork

Collects information from ATM cells for Collects information from ATM cells for delivery to higher layersdelivery to higher layers

ATM Adaptation LayerATM Adaptation Layer The AAL relays ATM cells between ATM The AAL relays ATM cells between ATM

Layer and higher layer (Segmentation Layer and higher layer (Segmentation and Reassembly (SAR) process) and Reassembly (SAR) process)

When relaying information received from the When relaying information received from the higher layers, it segments the data into ATM higher layers, it segments the data into ATM cells.cells.

When relaying information received from the When relaying information received from the ATM Layer, it must reassemble the payloads ATM Layer, it must reassemble the payloads into a format the higher layers can into a format the higher layers can understand.understand.

ATM Adaptation LayerATM Adaptation Layer Different AALs were defined in supporting different Different AALs were defined in supporting different

traffic or service expected to be used. The service traffic or service expected to be used. The service classes and the corresponding types of AALs were as classes and the corresponding types of AALs were as follows:follows: Class A - Constant Bit Rate (CBR) service:Class A - Constant Bit Rate (CBR) service:

AAL1 supports a connection-oriented service in which the bit rate AAL1 supports a connection-oriented service in which the bit rate is constant. is constant.

Examples of this service include 64 Kbit/sec voice, fixed-rate Examples of this service include 64 Kbit/sec voice, fixed-rate uncompressed video and leased lines for private data networksuncompressed video and leased lines for private data networks

Class B - Variable Bit Rate (VBR) service:Class B - Variable Bit Rate (VBR) service: AAL2 supports a connection-oriented service in which the bit rate AAL2 supports a connection-oriented service in which the bit rate

is variable but requires a bounded delay for delivery. is variable but requires a bounded delay for delivery. Examples of this service include compressed packetized voice or Examples of this service include compressed packetized voice or

video. The requirement on bounded delay for delivery is video. The requirement on bounded delay for delivery is necessary for the receiver to reconstruct the original necessary for the receiver to reconstruct the original uncompressed voice or video.uncompressed voice or video.

ATM Adaptation LayerATM Adaptation Layer Class C - Connection-oriented data service:Class C - Connection-oriented data service:

For connection-oriented file transfer and in For connection-oriented file transfer and in general, data network applications where a general, data network applications where a connection is set up before data is transferred, connection is set up before data is transferred, this type of service has variable bit rate and does this type of service has variable bit rate and does not require bounded delay for delivery. not require bounded delay for delivery.

Two AAL protocols were defined to support this Two AAL protocols were defined to support this service class, and have been merged into a service class, and have been merged into a single type, called AAL3/4. But with its high single type, called AAL3/4. But with its high complexity, the AAL5 protocol is often used to complexity, the AAL5 protocol is often used to support this class of service.support this class of service.

ATM Adaptation LayerATM Adaptation Layer Class D - Connectionless data service:Class D - Connectionless data service:

Examples of this service include datagram traffic and in general, Examples of this service include datagram traffic and in general, data network applications where no connection is set up before data network applications where no connection is set up before data is transferred.data is transferred.

Either AAL3/4 or AAL5 can be used to support this class of Either AAL3/4 or AAL5 can be used to support this class of service. service.

Operation Administration and Maintenance (OA&M)Operation Administration and Maintenance (OA&M) Defined for supervision, testing, and performance monitoring. It Defined for supervision, testing, and performance monitoring. It

uses loop-back for maintenance and ITU TS standard CMIP, with uses loop-back for maintenance and ITU TS standard CMIP, with organization into 5 hierarchical levels:organization into 5 hierarchical levels:

Virtual Channel (F5 - Between VC endpoints)Virtual Channel (F5 - Between VC endpoints) Virtual Path (F4- Between VP endpoints)Virtual Path (F4- Between VP endpoints) Transmission Path (F3- Between elements that perform assembling, Transmission Path (F3- Between elements that perform assembling,

disassembling of payload, header, or control)disassembling of payload, header, or control) Digital Section (F2 Between section end-points, performs frame Digital Section (F2 Between section end-points, performs frame

synchronization)synchronization) Regenerator Section (F1- Between regeneration sections)Regenerator Section (F1- Between regeneration sections)

ATM Adaptation LayerATM Adaptation LayerAAL AAL

TypesTypesConnection Connection

ModeModeBit RateBit Rate Timing relationship Timing relationship

between source & between source & destinationdestination

AAL1AAL1 Connection Connection orientedoriented

ConstantConstant YesYes

AAL2AAL2 Connection Connection orientedoriented

VariableVariable YesYes

AAL3AAL3 ConnectionlessConnectionless VariableVariable NoNo

AAL4AAL4 ConnectionlessConnectionless VariableVariable NoNo

AAL5AAL5 ConnectionlessConnectionless VariableVariable NoNo

Virtual ConnectionsVirtual Connections Virtual Channel Connection (VCC) – Full duplex Virtual Channel Connection (VCC) – Full duplex

virtual circuit with logical connection between virtual circuit with logical connection between source and destination – can be PVC or SVCsource and destination – can be PVC or SVC

Virtual Path Connection (VPC) – Semi-permanent Virtual Path Connection (VPC) – Semi-permanent (or customer controlled or network controlled) (or customer controlled or network controlled) connection that provides a logical collection of connection that provides a logical collection of virtual channels that have the same endpointvirtual channels that have the same endpoint

A single virtual path supports multiple virtual A single virtual path supports multiple virtual channels (analogy – highway = VPC, lane = VCC)channels (analogy – highway = VPC, lane = VCC)

VCI vs VPIVCI vs VPI VPI – Virtual Path Identifier – identified in VPI – Virtual Path Identifier – identified in

cell’s header. Cannot establish a virtual cell’s header. Cannot establish a virtual channel before virtual pathchannel before virtual path

VCI – Virtual Channel Identifier – only VCI – Virtual Channel Identifier – only have local significance – different virtual have local significance – different virtual paths reuse VCIs (but VCIs on same paths reuse VCIs (but VCIs on same path must be unique)path must be unique)

What is so special about What is so special about a virtual path?a virtual path?

ATM is connection-oriented, so circuit must be established ATM is connection-oriented, so circuit must be established before transmissionbefore transmission As route established, VPIs and VCIs are assignedAs route established, VPIs and VCIs are assigned

VPI and VCI info suffices for addressing infoVPI and VCI info suffices for addressing info Simplified network architecture (based on VC or VP)Simplified network architecture (based on VC or VP) Increased network performance and reliability (fewer, Increased network performance and reliability (fewer,

aggregated entities because of simplified network aggregated entities because of simplified network architecture)architecture)

Reduces processing and short connection setup timeReduces processing and short connection setup time User may define closed user group or closed networks of User may define closed user group or closed networks of

virtual channel bundlesvirtual channel bundles

ATM PVC and SVCATM PVC and SVC PVC allows direct connectivity between sites, and is similar to a PVC allows direct connectivity between sites, and is similar to a

leased line. leased line. Advantages:Advantages:

guarantees availability of a connection guarantees availability of a connection does not require call setup procedures between switches. does not require call setup procedures between switches.

Disadvantages:Disadvantages: requires manual setup between the source and the destination.requires manual setup between the source and the destination. no network resiliency is available with PVC.no network resiliency is available with PVC.

SVC is created and released dynamically and remains in use only as SVC is created and released dynamically and remains in use only as long as data is being transferred. In this sense, it is similar to a long as data is being transferred. In this sense, it is similar to a telephone call. Dynamic call control requires a signaling protocol telephone call. Dynamic call control requires a signaling protocol between the ATM endpoint and the ATM switch. between the ATM endpoint and the ATM switch.

Advantages:Advantages: connection flexibility connection flexibility call setup that can be handled automatically by a networking device. call setup that can be handled automatically by a networking device.

Disadvantages:Disadvantages: the extra time and overhead required to set up the connection.the extra time and overhead required to set up the connection. network management and troubleshootingnetwork management and troubleshooting

ATM Connection ATM Connection RelationshipsRelationships

ATM Quality of Service (QoS)ATM Quality of Service (QoS) CBR: Constant Bit Rate CBR: Constant Bit Rate

Guaranteed transmission rateGuaranteed transmission rate VBR: Variable Bit Rate VBR: Variable Bit Rate

rtVBR and nrtVBRrtVBR and nrtVBR Peak Cell Rate (PCR), Sustained Cell Rate (SCR), and Max Peak Cell Rate (PCR), Sustained Cell Rate (SCR), and Max

Burst Size (MBR)Burst Size (MBR) UBR: Unspecified Bit RateUBR: Unspecified Bit Rate

No guarantee, best effortNo guarantee, best effort

CBR

VBR

UBR

PCR

SCR

ServiceBit Rate

Call Establishment Using VPsCall Establishment Using VPs

VP/VC CharacteristicsVP/VC Characteristics Quality of service based on VCCQuality of service based on VCC Switched and semi-permanent channel Switched and semi-permanent channel

connectionsconnections Call sequence integrity – packets arrive in orderCall sequence integrity – packets arrive in order Traffic parameter negotiation and usage Traffic parameter negotiation and usage

monitoringmonitoring VPC onlyVPC only

Virtual channel identifier restriction within VPC – some Virtual channel identifier restriction within VPC – some VCCs reserved for network managementVCCs reserved for network management

ATM CellsATM Cells Fixed size – 53 bytesFixed size – 53 bytes 5 octet header5 octet header 48 octet information field48 octet information field Small cells reduce queuing delay for high Small cells reduce queuing delay for high

priority cellspriority cells Small cells can be switched more efficientlySmall cells can be switched more efficiently Easier to implement switching of small cells in Easier to implement switching of small cells in

hardwarehardware

ATM Cell FormatATM Cell Format

Header FormatHeader Format Generic flow control (GFC)Generic flow control (GFC)

Only at user to network interface (UNI)Only at user to network interface (UNI) Controls flow between user and local ATM switchControls flow between user and local ATM switch 2 sets of procedures: Uncontrolled and Controlled 2 sets of procedures: Uncontrolled and Controlled

transmissiontransmission Virtual path identifierVirtual path identifier

8 bits in UNI cell, 12 bits in NNI cell8 bits in UNI cell, 12 bits in NNI cell Addressing info for cell routingAddressing info for cell routing

Virtual channel identifierVirtual channel identifier 16 bit16 bit Additional addressing info for cell routingAdditional addressing info for cell routing

Header FormatHeader Format Payload type – 3 bits (user info or network Payload type – 3 bits (user info or network

management)management) first bit: user data (0) or control data (1)first bit: user data (0) or control data (1) second bit: 0 = no congestion, 1 = congestion), second bit: 0 = no congestion, 1 = congestion), third bit: whether the cell is the last cell (1) in a series third bit: whether the cell is the last cell (1) in a series

of AAL5 frameof AAL5 frame Cell loss priority (CLP)Cell loss priority (CLP)

Indicates whether the cell should be discarded (1) if it Indicates whether the cell should be discarded (1) if it encounters extreme congestion as it moves through encounters extreme congestion as it moves through the networkthe network

Header error control (HEC)Header error control (HEC) Checksum on the first 4 bytes of the headerChecksum on the first 4 bytes of the header Correct an isolated bit error and detect multiple bit Correct an isolated bit error and detect multiple bit

errorserrors

ATM Service CategoriesATM Service Categories ATM is designed to transfer many different types ATM is designed to transfer many different types

of traffic simultaneously, including real-time voice, of traffic simultaneously, including real-time voice, video, and bursty TCP trafficvideo, and bursty TCP traffic

Way in which data flow is handled depends on the Way in which data flow is handled depends on the characteristics of the traffic flow and requirements characteristics of the traffic flow and requirements of the application (ex. Real-time video must be of the application (ex. Real-time video must be delivered within minimum variation in delay)delivered within minimum variation in delay)

Primary service categories – real time service, Primary service categories – real time service, non-real time servicenon-real time service

ATM Service CategoriesATM Service Categories Real timeReal time

Constant bit rate (CBR)Constant bit rate (CBR) Real time variable bit rate (rt-VBR)Real time variable bit rate (rt-VBR)

Non-real timeNon-real time 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)

Real Time ServicesReal Time Services If want to avoid or decrease variation of delay (jitter), use CBR or rt-VBRIf want to avoid or decrease variation of delay (jitter), use CBR or rt-VBR CBR CBR

Fixed data rate continuously availableFixed data rate continuously available Commonly used for uncompressed audio and videoCommonly used for uncompressed audio and video

Video conferencingVideo conferencing Interactive audioInteractive audio A/V distribution and retrievalA/V distribution and retrieval

rt-VBR rt-VBR Best for time sensitive applicationsBest for time sensitive applications Tightly constrained delay and delay variationTightly constrained delay and delay variation Applications transmit at a rate that varies with time (e.g. compressed video)Applications transmit at a rate that varies with time (e.g. compressed video)

Produces varying sized image framesProduces varying sized image frames Original (uncompressed) frame rate constant (isochronous)Original (uncompressed) frame rate constant (isochronous) So compressed data rate variesSo compressed data rate varies

Can statistically multiplex connectionsCan statistically multiplex connections

Non-Real Time Non-Real Time Intended for applications with bursty Intended for applications with bursty

traffic and limited constraints on delay traffic and limited constraints on delay and delay variationand delay variation

Greater flexibility, greater use of Greater flexibility, greater use of statistical multiplexingstatistical multiplexing

nrt-VBRnrt-VBR May be able to characterize expected traffic May be able to characterize expected traffic

flowflow Improve QoS in loss and delayImprove 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

e.g. Airline reservations, banking e.g. Airline reservations, banking transactionstransactions

UBRUBR Unused capacity of CBR and VBR traffic Unused capacity of CBR and VBR traffic

made available to UBRmade available to UBR For application that can tolerate some For application that can tolerate some

cell loss or variable delayscell loss or variable delays e.g. TCP based traffice.g. TCP based traffic

Cells forwarded on FIFO basisCells forwarded on FIFO basis Best efforts serviceBest efforts service

ABRABR Application specifies peak cell rate Application specifies peak cell rate

(PCR) it will use and minimum cell rate (PCR) it will use and minimum cell rate (MCR) it requires(MCR) it requires

Resources allocated to give at least Resources allocated to give at least MCRMCR

Spare capacity shared among ABR and Spare capacity shared among ABR and UBR sourcesUBR sources

e.g. LAN interconnectione.g. LAN interconnection

Guaranteed Frame Rate (GFR)Guaranteed Frame Rate (GFR) Designed to support IP backbone Designed to support IP backbone

subnetworkssubnetworks Purpose: optimize handling of frame based Purpose: optimize handling of frame based

traffic passing from LAN through router to traffic passing from LAN through router to ATM backboneATM backbone Used by enterprise, carrier and ISP networksUsed by enterprise, carrier and ISP networks Consolidation and extension of IP over WANConsolidation and extension of IP over WAN

UNI establishes handshaking between NIC UNI establishes handshaking between NIC and switchand switch

ATM vs Frame RelayATM vs Frame Relay Frame relay uses variable length framesFrame relay uses variable length frames ATM fixed length cellsATM fixed length cells ATM has higher overhead, but faster ATM has higher overhead, but faster

speed and traffic management (better speed and traffic management (better suited for video and voice)suited for video and voice)

ATM vs SONETATM vs SONET SONET is a transport mechanism, SONET is a transport mechanism,

transporting data over fiber.transporting data over fiber. Can act as a transport carrier for ATM (or Can act as a transport carrier for ATM (or

FDDI, or ISDN, etc.)FDDI, or ISDN, etc.) ATM is a technology and protocol ATM is a technology and protocol

designed to use SONET as its carrier designed to use SONET as its carrier serviceservice

Why is ATM so Efficient?Why is ATM so Efficient? Minimal error and flow controlMinimal error and flow control

Reduces overhead of processing ATM cellsReduces overhead of processing ATM cells Reduces number of required overhead bitsReduces number of required overhead bits

Fixed size simplified processing at each ATM Fixed size simplified processing at each ATM node (can be switched more efficiently – more node (can be switched more efficiently – more efficient use of router)efficient use of router)

Small cells reduce queuing delaySmall cells reduce queuing delay Minimal addressing info on each cellMinimal addressing info on each cell Efficient traffic managementEfficient traffic management