Asynchronous Transfer Mode (ATM)

• Background• Physical and ATM Layers

• AALs• Applications

• Introduction

IntroductionAsynchronous Transfer Mode (ATM) technology delivers important advantages over existing LAN and WAN technologies, including the promise of scalable bandwidths at unprecedented price and performance points and Quality of Service (QoS) guarantees, which facilitate new classes of applications such as multimedia. However, ATM is the most complex technology ever developed by the networking industry. In fact, the deployment of ATM networks requires the overlay of a highly complex, software intensive, protocol infrastructure. This infrastructure is required to both allow individual ATM switches to be linked into a network, and for such networks to internetwork with the vast installed base of existing local and wide area networks.

ATM Quick Highlights

• The Telecom Industry’s thrust into multi-media data networking•Fixed-sized “cell” (53 bytes)• Built to provide Quality-of-service• Connection-oriented• Designed to run over SONET/SDH

4

B-ISDN (contd.)

Physical layer TCPMD

ATM Layer

ATM adaptation layer(AAL) CS

SAR

User PlaneControl Plane

Management PlaneLayer ManagementPlane Management

Data Transfer

User and Control

Packets

Cells, VC’s

Physical Medium

Upper Layers Upper Layers

ConvergenceSub layer

Segmentation andReassembly Sub layer

TransmissionConvergenceSub layer (Framing)

Physical MediumDependent Sub layer

Why?• Why a small cell instead of a large packet?• Queue delays tend to grow as packet size grows. A small cell helps maintain streamlined flows• No/little performance loss due to padding large fields• Small cells better for voice• No need for in-route fragmentation

Why?

•Why a fixed cell size instead of variable-sizepackets?• Switch architecture can be optimized to the fixed size, so switching can be done in hardware• Scalable parallel switch designs

Why?

• Why 53 bytes?– US wanted 64 payload bytes, Europe wanted 32– Compromised on 48– +5 header = 53

ATM + and –+• QoS• Multimedia Support• Hardware Switching -> High Speed• Connection-Oriented (-?)-• IP Support• LAN arena dominated by huge installed Ethernet base• Ethernet growing toward MAN, WAN• Connection-Oriented (+?)

End Station End Station

Switch

UserApplication

UserApplication

ATMAdaptationLayer (AAL)

ATMAdaptationLayer (AAL)

ATMLayer

ATMLayer

ATMLayer

PhysicalLayer

PhysicalLayer

PhysicalLayer

PhysicalLayer

53-ByteCells

53-ByteCells

ATM Architecture

ATM• Physical and ATM Layers

• Cells, Formats, and Addressing• Virtual Circuits• Switches and Media• Interfaces

ATM Cell Header(48 bytes PDU)5 bytes

HEC8 bits

PTI3 bits

VCI16 bits

VPI8 bits

GFC4 bits

CLP (1)

User-Network Interface (UNI)

5 bytes ATM Header

HEC8 bits

PTI3 bits

VCI16 bits

VPI12 bits

CLP (1)

Network-Network Interface (NNI)

5 bytes ATM Header

(48 bytes PDU)

(48 bytes PDU)

Reserved VCI’s0-15: ATM Mgmt16-31: ATM Forum3&4- OAM5- qsaal, 16- ilmi17- LECS, 18- pnni

Header Error Control (HEC)HEC covers the header only, not the payload -- the goal is to ensure correct delivery

First Four Cell Header Bytes

bbbbbbbbb bbbbbbbbb bbbbbbbbb bbbbbbbbb

x8 + x2 + x + 1 Remainder+ 01010101

HEC

If P(bit error) = 10-8, then P(Undetected header error) = ~10-20:

HEC also assists in synchronizing:- Look at 53-byte sequences until you find one where the HEC field works correctly- If this holds up for D sequences in a row, assume you’ve synched- p(bad synch) = ~2-8D

UNI Header Fields

HEC8 bits

PTI3 bits

VCI16 bits

VPI8 bits

GFC4 bits

CLP (1)

5 bytes ATM Header

(48 bytes PDU)

GFC - General Flow ControlOnly used between host and network. Overwritten by first switch.VPI - Virtual Path IDVCI - Virtual Circuit IDPTI - Payload type IDCLP - Cell Priority (ID’s cells for deletion when congestion experienced)HEC -Header Checksum (all 1-bit errors corrected, 90% of multi-bit errors detected)

HEC8 bits

PTI3 bits

VCI16 bits

VPI12 bits

CLP (1)

5 bytes ATM Header

(48 bytes PDU)

NNI Header Fields

VPI - Virtual Path ID

VCI - Virtual Circuit ID

PTI - Payload type ID

CLP - Cell Priority (used to ID cells for deletion when congestion experienced)

HEC -Header Checksum (all 1-bit errors corrected, 90% of multi-bit errors

PTI Field Codes000 User Data Cell Type 0 No congestion experienced001 User Data Cell Type 1 No congestion experienced010 User Data Cell Type 0 Congestion experienced011 User Data Cell Type 1 Congestion experienced100 Maintenance info between adjacent switches101 Maintenance info between source and destination switches110 Resource management cell (for ABR congestion control)111 Reserved

MeaningPTIUsed by AAL5to denote end ofmessage

0 b b

Explicit ForwardCongestionIndicator (EFCI)Set byCongestedswitch

Three types of ATM services exist: Three types of ATM services exist: permanent virtual circuits (PVC), permanent virtual circuits (PVC), switched virtual circuits (SVC), and switched virtual circuits (SVC), and connectionless service.connectionless service.

PVC allows direct connectivity PVC allows direct connectivity between sites. between sites.

PVC guarantees availability of a PVC guarantees availability of a connection and does not require call connection and does not require call setup procedure between switches. setup procedure between switches.

Disadvantages of PVCs include static Disadvantages of PVCs include static connectivity and manual setup.connectivity and manual setup.

ATM servicesATM services

An SVC is created and released An SVC is created and released dynamically and remains in use only as dynamically and remains in use only as long as data is being transferred.long as data is being transferred.

In this sense, it is similar to a telephone In this sense, it is similar to a telephone call. Dynamic call control requires a call. Dynamic call control requires a signaling protocol between the ATM signaling protocol between the ATM endpoint and the ATM switch. endpoint and the ATM switch.

The advantages of SVCs include The advantages of SVCs include connection flexibility and call setup that connection flexibility and call setup that can be handled automatically by a can be handled automatically by a networking device. Disadvantages networking device. Disadvantages include the extra time and overhead include the extra time and overhead required to set up the connection.required to set up the connection.

Virtual circuitVirtual circuit ATM networks are fundamentally ATM networks are fundamentally

connection-oriented, which means that a connection-oriented, which means that a virtual channel (VC) must be set up across virtual channel (VC) must be set up across the ATM network prior to any data transfer.the ATM network prior to any data transfer.

Two types of ATM connections exist: Two types of ATM connections exist: virtual virtual patpaths, which are identified by virtual path hs, which are identified by virtual path identifiers, and identifiers, and virtual channelsvirtual channels, which are , which are identified by the combination of a VPI and a identified by the combination of a VPI and a virtual channel identifier (VCI)virtual channel identifier (VCI)..

Virtual path and virtual Virtual path and virtual channelchannel

VC connection msgVC connection msg

ATM BroadcastingATM Broadcasting

ATM switchingATM switching The basic operation of an ATM switch is The basic operation of an ATM switch is

straightforward: The cell is received straightforward: The cell is received across a link on a known VCI or VPI across a link on a known VCI or VPI value. The switch looks up the value. The switch looks up the connection value in a local translation connection value in a local translation table to determine the outgoing port (or table to determine the outgoing port (or ports) of the connection and the new ports) of the connection and the new VPI/VCI value of the connection on that VPI/VCI value of the connection on that link. The switch then retransmits the link. The switch then retransmits the cell on that outgoing link with the cell on that outgoing link with the appropriate connection identifiers.appropriate connection identifiers.

c ATMSw1

ATMSw4

ATMSw2

ATMSw3

ATMDCC

ab

de

VP3 VP5

VP2

VP1

a

bc

deSw = switch

Digital Cross ConnectOnly switches virtual paths

ATM over SONETATM over SONET ATM designed to run over SONET ATM designed to run over SONET

OC-3cOC-3c• • Basic: 155.52 Mbps gross rateBasic: 155.52 Mbps gross rate• • New generation runs at OC-12 (622 New generation runs at OC-12 (622

Mbps),Mbps), OC-48 (2.4 Gbps)OC-48 (2.4 Gbps)

ATM interfacesATM interfacesPrivate Public Public NNI

Public UNI

Private NNI

Private UNI

Private

UNI

Private UNI

B-ICI

Broadband Inter-Carrier Interface (B-Broadband Inter-Carrier Interface (B-ICI)ICI)

Public Network-to-Network InterfacePublic Network-to-Network Interface NNINNI Switch-to-switch interface protocolSwitch-to-switch interface protocol Two versions: Public and private Two versions: Public and private

(similar,more flexibility in private version)(similar,more flexibility in private version) NNI Includes:NNI Includes: Routing protocol (Link-sate/OSPF)Routing protocol (Link-sate/OSPF) Signaling protocol for link setup/teardownSignaling protocol for link setup/teardown

UNIUNI Protocol for interfacing with user Protocol for interfacing with user

equipment equipment Two versions: Public and privateTwo versions: Public and private

ATM ADAPTATION LAYER ATM ADAPTATION LAYER (AAL)(AAL)

Specifically, the AAL receives packets Specifically, the AAL receives packets from upper-level protocols and breaks from upper-level protocols and breaks them into the 48-byte segments that form them into the 48-byte segments that form the payload field of an ATM cell. the payload field of an ATM cell.

AALprotocol model consists of a AALprotocol model consists of a Segmentation and Reassembly (SAR) Segmentation and Reassembly (SAR) sublayer and Service-Specific sublayer and Service-Specific Convergence Sublayers (CPCS and Convergence Sublayers (CPCS and SSCS).SSCS).

•The ATM Adaptation Layer (AAL) provides support for higher-layer services such as signaling, circuit emulation, voice, and video. AALs also support packet-based services, such as IP, LANs, and Frame Relay

AALAAL

AAL TypesAAL Types

AAL 1AAL 1 AAL1, a connection-oriented service, is suitable for AAL1, a connection-oriented service, is suitable for

handling constant bit rate sources (CBR), such as voice handling constant bit rate sources (CBR), such as voice and videoconferencing.and videoconferencing.

The sequence number field (SN) and sequence number The sequence number field (SN) and sequence number protection (SNP) fields provide the information that the protection (SNP) fields provide the information that the receiving AAL1 needs to verify that it has received the receiving AAL1 needs to verify that it has received the cells in the correct order. The rest of the payload field is cells in the correct order. The rest of the payload field is filled with enough single bytes to equal 48 bytes.filled with enough single bytes to equal 48 bytes.

AAL1 requires timing synchronization between the source AAL1 requires timing synchronization between the source and destination and, for that reason, depends on a media and destination and, for that reason, depends on a media that supports clocking, such as SONET. The standards for that supports clocking, such as SONET. The standards for supporting clock recovery are currently being defined.supporting clock recovery are currently being defined.

AAL 1AAL 1

AAL 1 AAL 1 SAR PDU (non-pointer type)

Adds sequence # with protection (checksum)Adds parity bit (even) over header

AAL1 SAR PDU(Pointer Type)

• Pointer field gives offset to start of next message (0-92 bytes)

Structured mode AAL1 SAR Structured mode AAL1 SAR and CSand CS

Unstructured mode AAL1 SAR and Unstructured mode AAL1 SAR and CSCS

AAL 2AAL 2 Designed to support Variable Bit Rate (“Bandwidth on Demand”)Designed to support Variable Bit Rate (“Bandwidth on Demand”) Provides for partial payloads to support low rate dataProvides for partial payloads to support low rate data Error protection over full PDUError protection over full PDU Simple flag to indicate position in messageSimple flag to indicate position in message Also AAL 2 was designed to multiplex a number of such low Also AAL 2 was designed to multiplex a number of such low

variable bit rate data streams on to a single ATM connection.variable bit rate data streams on to a single ATM connection.

AAL2 OperationAAL2 OperationCPS packet

CPS packetCPS packet Channel identifier (CID)Channel identifier (CID): CPS can multiplex several streams : CPS can multiplex several streams

onto a single ATM connection. The CID identifies each onto a single ATM connection. The CID identifies each channel. CID values are allocated as follows: the 0 value is channel. CID values are allocated as follows: the 0 value is used as padding, and the 8 to 255 values are valid CID used as padding, and the 8 to 255 values are valid CID values used to identify channels.values used to identify channels.

Length indicator (LI)Length indicator (LI): Its value is one less than the number : Its value is one less than the number of bytes in the CPSpacket payload. The default maximum of bytes in the CPSpacket payload. The default maximum length of the CPS-packet payload is 45 bytes. length of the CPS-packet payload is 45 bytes.

Header error control (HEC)Header error control (HEC): It use the pattern : It use the pattern xx5 + 5 + xx2 + 1. 2 + 1. The receiver uses the contents of the HEC to detect errors in The receiver uses the contents of the HEC to detect errors in the header.the header.

User-to-user-indication (UUI)User-to-user-indication (UUI): used for transferring : used for transferring information between the peer CPS users. The CPS transports information between the peer CPS users. The CPS transports this information transparently.this information transparently.

AAL2 OperationAAL2 Operation (CPS) PDU format(CPS) PDU format

CPS-PDUCPS-PDU

Parity (P)Parity (P): A 1-bit field used to detect errors in the STF.: A 1-bit field used to detect errors in the STF.

Sequence numbers (SN)Sequence numbers (SN): A 1-bit field used to number : A 1-bit field used to number modulo 2 the successive CPSPDUs.modulo 2 the successive CPSPDUs.

Offset field (OSF)Offset field (OSF): The CPS-PDU payload can carry CPS : The CPS-PDU payload can carry CPS packets in a variety of different arrangements. To extract packets in a variety of different arrangements. To extract the CPS-packets from the CPS-PDU payload, a 6-bit the CPS-packets from the CPS-PDU payload, a 6-bit offset offset field (OSF) field (OSF) is used to indicate the start of a new CPS-is used to indicate the start of a new CPS-packet in the CPS-PDU payload. Specifically, OSF gives the packet in the CPS-PDU payload. Specifically, OSF gives the number of bytes between the end of the STF and the start number of bytes between the end of the STF and the start of the first CPS-packet in the CPS-PDU payload. of the first CPS-packet in the CPS-PDU payload.

AAL2 OperationAAL2 Operation

AAL 3/4AAL 3/4 AAL3/4 supports both connection-oriented and AAL3/4 supports both connection-oriented and

connectionless data. It was designed for network service connectionless data. It was designed for network service providers and is closely aligned with Switched providers and is closely aligned with Switched Multimegabit Data Service (SMDS). AAL3/4 is used to Multimegabit Data Service (SMDS). AAL3/4 is used to transmit SMDS packets over an ATM network.transmit SMDS packets over an ATM network.

Originally 2 separate AALs:Originally 2 separate AALs:

– – AAL3: Connection-oriented packet svcs (X.25)AAL3: Connection-oriented packet svcs (X.25)– – AAL4: Connectionless svcs (IP)AAL4: Connectionless svcs (IP)

• • Eventually combined into a single type for all data serviceEventually combined into a single type for all data service

AAL3/4 CS PDUAAL3/4 CS PDU

AAL3/4 SAR PDUAAL3/4 SAR PDU

AAL3/4 OperationAAL3/4 Operation

AAL5AAL5 AAL 5 is used for the transfer of data. Due to its simplicity, AAL 5 is used for the transfer of data. Due to its simplicity,

it is the most popular adaptation layer.it is the most popular adaptation layer.

AAL5 is a Simple Efficient Adaptation Layer (SEAL). The AAL5 is a Simple Efficient Adaptation Layer (SEAL). The Common Part (CP) AAL5 supports Variable Bit Rate (VBR) Common Part (CP) AAL5 supports Variable Bit Rate (VBR) traffic, both connection-oriented and connectionless.traffic, both connection-oriented and connectionless.

It is used to transfer most non-SMDS data, such as It is used to transfer most non-SMDS data, such as classical IP over ATM and LAN Emulation (LANE).classical IP over ATM and LAN Emulation (LANE).

Efficiency:Efficiency: AAL3/4: 4 bytes per message + 4 bytes per cell => 44 AAL3/4: 4 bytes per message + 4 bytes per cell => 44

User Data Bytes / CellUser Data Bytes / Cell AAL5: 8 bytes per message => 48 User Data Bytes / AAL5: 8 bytes per message => 48 User Data Bytes /

Cell, 8% improvementCell, 8% improvement

AAL5 CS PDUAAL5 CS PDU

AAL5 CS PDUAAL5 CS PDU Padding (Pad)Padding (Pad): It can be between 0 and 47 bytes long, and : It can be between 0 and 47 bytes long, and

is added so that the entire CPS-PDU including the padding is added so that the entire CPS-PDU including the padding and the remaining fields in the trailer becomes an integer and the remaining fields in the trailer becomes an integer multiple of 48 bytes. multiple of 48 bytes.

CPS user-to-user indication (CPS-UU)CPS user-to-user indication (CPS-UU): A 1-byte field used to : A 1-byte field used to transfer transparently CPS user-to-user information.transfer transparently CPS user-to-user information.

Common part indicator (CPI)Common part indicator (CPI): A 1-byte field to support : A 1-byte field to support future AAL 5 functions.future AAL 5 functions.

LengthLength: A 2-byte field used to indicate the length in bytes : A 2-byte field used to indicate the length in bytes of the CPS-PDU payload . of the CPS-PDU payload .

CRC-32CRC-32: This 4-byte field contains the FCS calculated by : This 4-byte field contains the FCS calculated by the transmitting CPS over the entire contents of the CPS-the transmitting CPS over the entire contents of the CPS-PDU The pattern used is: PDU The pattern used is: xx32 + 32 + xx26 + 26 + xx23 + 23 + xx22 + 22 + xx16 + 16 + xx12 + 12 + xx11 + 11 + xx10 + 10 + xx8 + 8 + xx7 + 7 + xx5 + 5 + xx4 + 4 + xx2 + 2 + x x + 1.+ 1.

AAL5 SARAAL5 SAR Simply breaks CS PDU into 48-byte chunks and passes Simply breaks CS PDU into 48-byte chunks and passes

them to ATM Layer.them to ATM Layer. No overhead bytes added.No overhead bytes added.

AAL5 OperationAAL5 Operation

LANELANELAN EmulationLAN Emulation

Role of LANERole of LANE Emulate a local area network on top Emulate a local area network on top

of an ATM network.of an ATM network.

A technology that network designers A technology that network designers can deploy to internetwork their can deploy to internetwork their legacy LANs (for example, Ethernet legacy LANs (for example, Ethernet and Token Ring LANs), with ATM-and Token Ring LANs), with ATM-attached devices. attached devices.

LANE BasicsLANE Basics LANE uses MAC encapsulation (OSI Layer LANE uses MAC encapsulation (OSI Layer

2) 2) All devices attached to an emulated LAN All devices attached to an emulated LAN

(ELAN) appear to be on one bridged (ELAN) appear to be on one bridged segment.segment.

Each Emulated LANs (ELAN) is independent Each Emulated LANs (ELAN) is independent of each other.of each other.

Switch Switch one ELAN traffic one ELAN traffic Router Router inter-ELAN traffic inter-ELAN traffic Network will become much faster Network will become much faster

LANE componentsLANE components LAN Emulation Client (LEC)LAN Emulation Client (LEC)

End systemsEnd systems Network interface card (NIC)Network interface card (NIC) LAN switch with ATM uplinkLAN switch with ATM uplink RouterRouter

Different functionsDifferent functions ATM address resolutionATM address resolution Data forwardingData forwarding Control FunctionControl Function

Performs MAC level emulationPerforms MAC level emulation

LANE componentsLANE components LAN Emulation Server (LES)LAN Emulation Server (LES)

One LES per ELAN One LES per ELAN Implements the control function for a Implements the control function for a

particular ELAN. (central control point)particular ELAN. (central control point) Each LES is identified by a unique ATM Each LES is identified by a unique ATM

addressaddress Registering & resolving of MAC address Registering & resolving of MAC address

to ATM address to ATM address

LANE componentsLANE components LAN Emulation Server (LES)LAN Emulation Server (LES)

LANE componentsLANE components LAN Emulation Configuration Server (LECS)LAN Emulation Configuration Server (LECS)

Maintains a database of ELANs and the ATM Maintains a database of ELANs and the ATM addresses of the LESs that control the ELANs addresses of the LESs that control the ELANs

Assigns individual LEC to different ELANs Assigns individual LEC to different ELANs

LANE componentsLANE components Broadcast and unknown server Broadcast and unknown server

(BUS) (BUS) One BUS per ELANOne BUS per ELAN

Central point for distributing Central point for distributing broadcasts and multicasts broadcasts and multicasts

LANE VCCsLANE VCCs Configuration Direct VCC (LECS)Configuration Direct VCC (LECS)

bidirectional point-to-point VCC set up bidirectional point-to-point VCC set up by the LEC to the LECS.by the LEC to the LECS.

LANE VCCsLANE VCCs Control Direct VCC (LES)Control Direct VCC (LES)

This is a bidirectional VCC set up by the LEC This is a bidirectional VCC set up by the LEC to the LESto the LES

Used to forward registration and control Used to forward registration and control information information

Control Distribute VCC (LES)Control Distribute VCC (LES) This is a unidirectional VCC set up from the This is a unidirectional VCC set up from the

LES to the LEC LES to the LEC point-to-multipoint connection with each LEC point-to-multipoint connection with each LEC

as a leafas a leaf Used to forward control information Used to forward control information

LANE VCCsLANE VCCs Control Direct VCC and Control Control Direct VCC and Control

Distribute VCC Distribute VCC

LANE VCCsLANE VCCs Multicast Send VCC (BUS)Multicast Send VCC (BUS)

bidirectional point-to-point VCC set up bidirectional point-to-point VCC set up by the LEC to the BUSby the LEC to the BUS

Multicast Forward VCC (BUS)Multicast Forward VCC (BUS) This is a unidirectional VCC set up from This is a unidirectional VCC set up from

the BUS to the LECthe BUS to the LEC point-to-multipoint connection, with point-to-multipoint connection, with

each LEC as a leafeach LEC as a leaf

LANE VCCsLANE VCCs Multicast Send VCC and Multicast Multicast Send VCC and Multicast

Forward VCC Forward VCC

LANE VCCsLANE VCCs Data Direct VCCData Direct VCC

Bi-Directional VCC between 2 LECs that Bi-Directional VCC between 2 LECs that want to exchange data. Initiated by any want to exchange data. Initiated by any LECLEC

Used for users data trafficUsed for users data traffic

LANE operationLANE operation1.1. Contacting the LECSContacting the LECS

Find the ATM address of the LES.Find the ATM address of the LES. Using the configure direct VCC. Using the configure direct VCC.

2.2. Joining the LESJoining the LES LEC sets up the control-direct VCC and send a join LEC sets up the control-direct VCC and send a join

request to the LES.request to the LES. The LES assigns the LEC with a unique LEC ID The LES assigns the LEC with a unique LEC ID The LEC then registers its own MAC and ATM The LEC then registers its own MAC and ATM

addresses with the LESaddresses with the LES3.3. Joining the BUSJoining the BUS

LEC get the BUS address from the LESLEC get the BUS address from the LES LEC sets up the multicast send VCC with the BUS LEC sets up the multicast send VCC with the BUS

and send a joining requestand send a joining requestThe LEC has become a member of the ELANThe LEC has become a member of the ELAN

Address ResolutionAddress Resolution Unknown destination.Unknown destination. LEC send ARP request to the LESLEC send ARP request to the LES The LES forward the request to all the LECs on The LES forward the request to all the LECs on

its ELAN.its ELAN. LES forward the request to the BUS to broadcast LES forward the request to the BUS to broadcast

the request on all endpoints.the request on all endpoints. The desired LEC will respond and send ARP The desired LEC will respond and send ARP

response to the LESresponse to the LES The LES forward the response to all its LECs The LES forward the response to all its LECs

(using the control distibute VCC) so they learn (using the control distibute VCC) so they learn the new MAC to ATM address binding.the new MAC to ATM address binding.

ConclusionConclusion Transfer Information between nodes in a Transfer Information between nodes in a

global networkglobal network Data, Voice & VideoData, Voice & Video

Use Fixed packet lengths (called cells)Use Fixed packet lengths (called cells) Cells are routed via virtual circuitsCells are routed via virtual circuits Flexible bandwidth and high QOSFlexible bandwidth and high QOS Support Wide range of service requirementsSupport Wide range of service requirements Lower cost and higher bandwidthLower cost and higher bandwidth The ATM will dominate future market!The ATM will dominate future market!

Thank you for your Thank you for your attentionattention