1

ATM Tutorial

Paul Chen

April, 2000

2

Outlines

1. ATM Basics and Reference Model

2. Concepts of VP and VC

3. ATM Protocol Stack

4. QoS Concept

5. Network Congestion and Policing

6. ATM and SONET

7. ATM OAM Principle

8. ATM Adaptation Layer (AAL)

9. ATM Network Management and MIB

10. References

3

Standards Based

Why Interest in ATM?

Both LAN and WAN TechnologyBoth LAN and WAN Technology

One International

Standard

Voice/Data/Video

Mbit Gbit Scaleable in Speeds

Sc

ale

ab

le i n

D

ist a

nc

e

Ca

rrie

s M

ult

iple

Tra

ffic

Ty

pe

s

ATM

4

• ATM is the foundation technology for Broadband-ISDN

• B-ISDN is the universe of services that will be made possible by the use of ATM technology

ATM and B-ISDN Relationship

VIDEOVIDEO

DATADATA

VOICEVOICE

5

Broadband Protocol Model

Management Plane

Cont

rol

Plan

e User Plane

Signaling(VBR)

CBRe.g.DS1 DS3

Voice

Other VBRe.g.

VBR Video

CO (VBR)e.g.

Frame RelayX.25

OtherServices

Upper Layer 2

AAL

ATM

SONET/SDH PDH

Laye

r2

Laye

r 1

Serv

ices

or Hig

her

Laye

r Pr

otoc

ols

6

• Adaptation Layer (AAL): Inserts/extracts information into 48 byte payload

• ATM Layer: Adds/removes 5 byte header to payload

• Physical Layer: Converts to appropriate electrical or optical format

End Station

ATM Switch

End Station

ATMCells

AAL

ATM

PHY

PHY

ATM

PHY

PHY

ATM

AAL

Functions B-ISDN Layers

7

History of ATM

ITU-TITU-T Launches Launches

B-ISDN B-ISDN ProjectProject

53 Byte Cell 53 Byte Cell Standardized Standardized (June 1989)(June 1989)

ATM ATM Forum Forum

CharteredChartered

ATM ATM ProductsProducts

IntroducedIntroduced

Public WAN Public WAN Services Services

Launched Launched

Anchorage Accord Anchorage Accord Interoperability Interoperability based on ATMF based on ATMF Specifications Specifications

(April 1996)(April 1996)

1985 19891987 1991 1993 1995 1997

8

Comparison of ATM with other Technologies

CONVENTIONALLAN

CONVENTIONALTELECOM

ATM

TRAFFIC TYPE DATA VOICEDATA, VOICE,

VIDEOTRANSMISSION

UNITVARIABLEPACKET

FIXED FRAME FIXED CELL

RATE UP TO G BPS UP TO G BPS M BPS TO G BPS

CONNECTIONTYPE

CONNECTIONLESS

CONNECTION-ORIENTED

CONNECTION-ORIENTED

DELIVERY OFTRAFFIC BEST EFFORT GUARANTEED

DEFINEDCLASSES

ACCESS SHARED DEDICATED DEDICATED

9

Anatomy of an ATM Cell

VCI

HEC

VCI

VPI

CLPPTIVCI

VPI

GFC (UNI) OR VPI (NNI)

48Bytes

Byte 5

Byte 4Byte 3

Byte 2

Byte 1

Header

Payload

8 7 6 5 4 3 2 1

10

Virtual Circuits

First we have the cable...First we have the cable...

Next, ATM Addressing Defines Paths...Next, ATM Addressing Defines Paths...

Then Channels.Then Channels.

• VP’s

• VC’s

11

SONET and ATM Channels

Transport Overhead

Path Overhead

VT1.5 DS1

STS-1

28 VT1.5

STS-1(DS3)

STS-1(DS3)

Transport Overhead

Path Overhead

12

VP

VP

• VPI: Virtual Path Identifier– 4,096 at NNI and 256 at UNI

• VCI: Virtual Channel Identifier– 65,536

• Both used to route cells through network– Unique on link-by-link basis– Interpreted at each switch

Virtual Paths & Virtual Channels

VCs

VCs

VCs

VCs

VP

VP

PhysicalTransmission

Link

13

• ATM is virtual connection-oriented; there must always be a virtual connection established before cells can be sent

• Connections can be established:›› Administratively as PVCs

– Lowest common denominator for Interoperability for devices not supporting UNI 3.x signaling

›› Dynamically as SVCs– Implies ATM signaling capability

ATM Connections

14

• ATM protocol is connection-oriented– once connection is set up, cells are quickly switched in hardware

by using VPI/VCI at very high speeds

• Uses fixed cell length– Allows switch hardware to be optimized around a fixed length cell

• Uses SONET as physical layer interface– Scales to high speed and is defined and deployed at Gigabit rates

ATM Switches are easily Scaleable in Speed

15

Logical ATM Switch Fabric

ATM Switch Ingress PathATM Switch Ingress Path

PHY receive

termination

Connection Lookup

OAM Processing

PolicingBuffering, Queueing

& Scheduling

from interface

to queue

AT

M L

ayer P

rocessin

g

Ph

ysical/TC

L

ayer P

rocessin

g

ATM Switch Egress PathATM Switch Egress Path

Fabric receive

termination

Connection Lookup

Buffering, Queueing

& SchedulingOAM

Policing(EFCI)

from queue

to interface

AT

M L

ayer P

rocessin

g

Intern

al L

oo

pb

ack

16

CPN 1

NN 1

CPN 2

NN 2

CPN 3

VP2VC8VC11

VC8VC11

VP3VP2 VP3

VC21VC11

VP5 VP5 VP8 VP8

VC21 VC2 VC2VC11

VC7

VC7

VP3

VP3

VP6VP6

VP5

VP5

VP2 VP2

VP1VP1

VC9

VC2

VC2

VC9

User/NetworkInterface

(UNI)

User/NetworkInterface

(UNI)

Network NodeInterface

(NNI)

Link 1 Link 1 Link 1Link 2

Link 2Link 3

Link 4

Link 1

Link 1Link 2

Link 2Link 3

Concept – VPs and VCs in the Network

Routing Concept in an ATM Network

17

ATM Protocol Stack

Service AccessPoint (SAP)

AAL - SAP

Common Part Convergence Sublayer (CPCS)• Builds header and trailer records onto user data frame• Assures integrity at the frame level

Segmentation and Reassembly (SAR) • Converts CPCS frames into cells • Adds cell headers and trailers to provide integrity at the cell level

Transmission Convergence Sublayer• HEC generation and checking • Cell delineation• Transmission frame adaptation • Decoupling of Cell Rate (ITU systems)

Physical Media Dependant Sublayer• Encoding for transmission • Transmission (Electrical/Optical)• Timing and synchronization

Service Specific Functions (SSCS)• Provide additional functions as required for specific services (can be null)

Service Access Point (SAP)Cell

Switching

Sublayer Boundary

Layer 2(Link)

ISOModel(OSI)

Layer 3(Network)

MAC

Layer 1(Physical)

PhysicalLayer

ATM Layer

Higher Layers

(Not part of ATM)

ATM Adaptation Layer (AAL)

18

• QoS is associated with a VCC that specifies an average bandwidth as well as a maximum bandwidth

• QoS is provisioned for a VPC or VCC (VCCs within the VPC may have a lower QoS than the VPC)

• QoS parameters include:– Cell Transfer Delay (Network Latency)– Cell Delay Variation (Jitter)– Cell Transfer Capacity (Speed - average and peak allowed rates)– Cell Error Ratio– Cell Loss Ratio– Cell Misinsertion Rate

Concept - QoS

19

• Constant Bit Rate (CBR)– Includes traffic where a continuous stream of bits at a

predefined constant rate is transported through the network (e.g., T1 circuit, voice)

– Low latency, low jitter, low errors and cell loss

• Realtime Variable Bit Rate (rt-VBR)– Like CBR in the sense that we want low latency, low jitter,

low errors and cell loss but the rate the bits can transmit varies (e.g., compressed video, voice)

• Non-realtime Variable Bit Rate (nrt-VBR)– Like rt-VBR except ‘some’ latency and jitter might not cause a

problem (e.g., one-way TV distribution)

Concept - ATM Service Categories

20

• Unspecified Bit Rate (UBR)– Provides ‘best effort’ delivery of data. Good for end-to-end

applications that have flow control (high cell loss, high jitter and latency)

• Available Bit Rate (ABR)– Has guaranteed delivery but not delivery time (Limited cell loss,

high jitter and latency)– ABR has many other parameters and a technique (RM cell) to

manage this resource beyond the scope of this presentation

Concept - ATM Service Categories

21

ApplicationArea

CBR rt-VBR nrt-VRB ABR UBR

Critical Data xx x xxx x N/S

LAN interconnectionLAN emulation

x x xx xxx xx

DataTransport/interworking(IP-FR-SMDS)

x x xx xxx xx

Circuit Emulation-PABX xxx xx N/S N/S N/S

POTS/ISDN-video conference

xxx N/S N/S

Compressed Video/Audio x xxx xx xx x

1-way TV Distribution x xx xxx N/S N/S

Interactive Multimedia xxx xxx xx xx x

Application Areas for ATM Service Categories

Score to indicate the ‘advantage’:Optimum: xxx Good: xx Fair: x N/S: Not SuitableNot Quoted: Presently considered not applicable with advantage (might be shown in the future)

22

• When congestion happens on ATM networks - Cell gets discarded!!!

• One mechanism to control congestion is to ‘police’ packets as they enter the network (i.e., UNI) or are passed between Nodes (i.e., NNI)

• Types of policing controls include:– Generic Flow Control (GFC) - Instructs the ATM network to employ

a flow control algorithm for cells in this connection.– Call Admission Control (CAC) - Done during establishing a

channel; makes sure the requested bandwidth exists and that the QoS can be provided.

– Traffic Policing and Shaping - Usage/Network Parameter Control (UPC/NPC)

Concept - Network Congestion & Policing

23

• Checks the rate of the cells at the input to ensure that arriving cells meet the ‘traffic profile’ (bandwidth and QoS) specifications. If not, it can discard the cell or mark the cell as eligible for discard.

• Algorithm to determine if a cell should be discarded is called ‘Leaky bucket’.

– Explicit Forward Congestion Indicator (EFCI) - If node is becoming congested, it marks the cells. The end station then hopefully throttles its traffic.

Concept -

Network Congestion & Policing

24

• Pre-established connections• Permanent• No signaling required

PVC - Manual Set Up

VPI/VCI14/1055

14/1055

87/45

25/125

9/47

9/47

Console orNMS GUI

25

• Uses UNI 3.0/3.1 signaling– VPI/VCI = 0/5

• Automatic• Transparent to User

SVC - Automatic Set Up

Terminal A

Terminal B

Connect to B

Connect to B

Connect to B

OK

OK

OK

OK

26

ATM Network Interfaces

Service Provider

CustomerPremises

WAN

FUNI

B-ICIWAN

Public NNI

PublicUNI

PrivateNNI

PrivateUNIPrivate

UNI

Router

RemoteRemoteSiteSite

27

• Network Node Interface (NNI)– The interface at a network node which is used to

interconnect with another network node

• SONET/SDH interface is preferred– ANSI T1.105-1995 and ITU-T G.707 March 1996

• Provides ATM mapping to 10 Gbps and beyond

• Let’s review SONET

Public NNI

28

STS- 1 Frame with ATM

3 Bytes 87 Bytes

51.84 Mbps

3 Bytes

6 Bytes

Section Section OverheadOverhead

Line Line OverheadOverhead

FFIIXX SSttuuffff

1X9 Byte

FFIIXX SSttuuffff

1X9 Byte

PPaatthh

OOvveerrhheeaadd

1X9 Byte

ATM ATM PayloadPayload48.38448.384MbpsMbps

29

STS-3c Frame Structure

9 Bytes 261 Bytes

155.52 Mbps

3 Bytes

6 Bytes

Section Section OverheadOverhead

Line Line OverheadOverhead

PPaatthh

OOvveerrhheeaadd

ATM PayloadATM Payload149.76 Mbps149.76 Mbps

H H …

…

H

H

ATM Cell

53 Bytes

30

STS-48c Frame Structure

144 Bytes 4160 Bytes

2.48832 Gbps

3 Bytes

6 Bytes

Section Section OverheadOverhead

Line Line OverheadOverhead

PPaatthh

OOvveerrhheeaadd

ATM PayloadATM Payload2.39616 Gbps2.39616 Gbps

H H

… H

FFiixxeedd SSttuuffff

…15

Bytes

4176 Bytes

31

Fault Management Example

LOS- Loss of SignalAIS- Alarm Indication SignalRDI- Remote Defect IndicationFERF- Far End Receive Failure

PTEPTE

LOS

LTELTE ATM SwitchATM SwitchSTESTERepeaterRepeater

ATM SwitchATM Switch

Terminal VP VCADMX

RDI-L (formally Line FERF)

RDI-P (formally STS Yellow)

VP-RDI

VC-RDI

Line AIS (AIS-L) STS Path AIS (AIS-P) VP-AIS VC-AIS

32

• Fault Management, using AIS, RDI, continuity check and loopback OAM cells.

• Performance management, using forward monitoring and backward reporting OAM cells.

• Activation/deactivation of performance monitoring and/or continuity check, using activation/deactivation OAM cells.

• System management OAM cells for use by end-systems only.

ATM-Operation and Maintenance Principles

33

• Operations, Administration and Maintenance (OAM)• ATM allows the maintenance/test operation to be

performed on a VPC or VCC.• These operations are performed on a selected basis;

they can span segments or can be end-to-end.• Types of maintenance/test operations:

– Performance Monitoring - a VPC or VCC is monitored to ensure the connection is not congested or has degraded (forward and backward monitoring are provided)

– Failure detection (AIS, RDI)– PM and Failure Reporting (RDI, PM results)– Facility Protection of VPCs– Fault Isolation (continuity checks and loopbacks)

Concept - OAM

34

• Physical Layer Mechanism– F1: SONET Section Level

– F2: SONET Line Level

– F3: SONET Path Level

• ATM Layer Mechanism– F4: Virtual Path Level

• End to end F4 flow• Segment F4 flow

– F5: Virtual Channel Level• End to end F5 flow• Segment F5 flow

I.610- Operation and Maintenance Flows

35

Fault Management Example Using F1 - F5 Flows

PTEPTE

LOSLTELTE ATM SwitchATM SwitchATM SwitchATM Switch

VP VCADMX

F2 (RDI-L)

F3 (RDI-P)

F4 (VP-RDI)

F5 (VC-RDI)

F1 F2 (AIS-L) F4 (VP-AIS)

STESTERepeaterRepeater

Terminal

F5 (VC-AIS)F3 (AIS-P)

36

Example of Mechanismfor OAM Flows

VCCendpoint

VCCendpoint

VC cross-connectVP cross-connect

Physical layer connecting point

AAL

ATMPL

AAL

ATMPLPL PL

ATMPL

ATMPL

ATMPL

ATMPL

Virtual channel OAM cell indicated by PT identifier F5

Virtual path connection uses VCI(=3/4) for OAM F4 VPC - OAM F4

VCI 1 VCI 1 VCI 2 VCI 2

VPI 1 VPI 1 VPI 2 VPI 2 VPI 3 VPI 3

Transmission path F3 Trans path F3 Trans path F3

F1, F2 F1, F2 F1, F2 F1, F2

37

Layered Model of AIS & RDI

(Layer to layer indications) (Peer to peer indications)

VP

PATHPATH

LINELINE

SECTION

PHYSICAL

VCVCVC-AIS (F5)VC-AIS (F5)

VC-RDI (F5)VC-RDI (F5)

VP-RDI (F4)VP-RDI (F4)

RDI-P (F3)RDI-P (F3)

RDI-L (F2)RDI-L (F2)

(BIP-8 PM, F1)(BIP-8 PM, F1)

VP-AIS (F4)VP-AIS (F4)

AIS-P (F3)AIS-P (F3)

AIS-L (F2)AIS-L (F2)

(F1)(F1)

38

The AAL process is the most important feature of the ATM The AAL process is the most important feature of the ATM Communications process...Communications process...

How the Adaptation process is carried out depends on the type How the Adaptation process is carried out depends on the type

of service to be transported...of service to be transported...

The ATM Adaptation Layer

AAL TYPE SERVICE TYPE COMMENTSAAL1 Constant Bit Rate

CBRIsochronous Traffic like DS0,DS1s, DS3s to carry Voice

AAL2 Variable Bit RateVBR

For data services, compressedAudio / Video, etc.

AAL3 Connection-Orientedfor VBR DataTransfer

Bursty data over long periods

AAL4 Connectionless VBRData Transfer

For short, bursty data (SMDS…)

AAL5 Simplified AAL Mainly for point-to-point

39

Classes of ATM Service

22

RequiredRequired

3/4, 53/4, 5

VariableVariable

3/43/4

ConstantConstant

CONNECTION ORIENTEDCONNECTION ORIENTED

11

CLASS BCLASS B CLASS CCLASS C CLASS DCLASS DCLASS ACLASS A

Not RequiredNot Required

Connection ModeConnection Mode CONNECTION-LESSCONNECTION-LESS

Timing Relation BetweenTiming Relation BetweenSource & DestinationSource & Destination

Bit RateBit Rate

AAL TypesAAL Types

40

The AAL Process

CS-PDU CS-PDU

SAR-PDUSAR-PDU

SAR ProcessSAR Process

These two sublayers convert the user information into 48-byte cell payloads. Each sublayer produces a Protocol Data Unit (PDU). The CS-PDU is variable length while the SAR-PDU is always 48 bytes.

AAL is dividedAAL is dividedinto twointo twosublayers:sublayers:

1) CONVERGENCESUBLAYER

2) SEGMENTATION &REASSEMBLY SUBLAYER

CS ProcessCS Process

SAR-PDU SAR-PDU

CS-PDU

USER INFORMATIONUSER INFORMATION

41

AAL-1 Processing

Header Payload

CSI CRC

SN Field4 Bits

SNP Field4 Bits

PDU Payload (47 Octets)

1 2 3 4 1 2 3 4 Sequence

CountParity

SN: Sequence NumberSNP: Sequence Number ProtectionCSI: Convergence Sublayer Indicator

42

AAL-2 Processing

CPS-Packet

CPS-PacketHeader (3 octets)

CPS-PacketPayload (1 to 45/64 octets)

Start Field(1 Octet)

CPS-PDU Payload( up to 47 octets and pad)

CPS-PDU

Each AAL2 user generates CPS packets with a 3-octet packet header and a variable length payload. The CPS sublayer collects CPS packets from AAL2 users multiplexedonto the same VCC over a specified interval of time, forming CPS-PDU, comprised of 48 octets worth of CPS packets.

Cell Header(5 octets)

ATM Cell

43

The AAL Process: AAL 3/4 CS-PDU

BASizeCPI BTag ETagPad AL Information

CPI:CPI: Common Point Indicator - 1 ByteCommon Point Indicator - 1 Byte

BTag: Beginning Tag - 1 ByteBTag: Beginning Tag - 1 Byte

BA Size: Buffer Allocation Size - 2 BytesBA Size: Buffer Allocation Size - 2 Bytes

Info Payload: Length of Payload (Max: 65, 535 Bytes)Info Payload: Length of Payload (Max: 65, 535 Bytes)

Pad:Pad: Up to 3 Bytes - used to align CS-PDU lengthUp to 3 Bytes - used to align CS-PDU length

AL:AL: Alignment - 1 ByteAlignment - 1 Byte

ETag: End Tag - 1 ByteETag: End Tag - 1 Byte

Length: 2 BytesLength: 2 Bytes

Length

CS-PDU

44

AAL 3/4

BASizeCPI BTag AAL SEVICE DATA UNIT

AAL - SDUAAL - SDU

MID: Message Identifier CRC: Cyclic Redundancy Check EOM: End of message

Al Length

LengthETag Fill

BOM: Beginning of message COM: Continuation of message EOM: End of message

ConvergenceSublayerProtocol

Data Unit:CS-PDUCS-PDU

Segmentation &Reassembly

Protocol Data Unit:

SAR-PDUSAR-PDU

MID Payload CRCSequence

Type Sequence

Number Length

Indicator Payload

44 Bytes44 Bytes44 Bytes

Payload

EOMCOM

BASIZE: Buffer Allocation Size BTAG: Beginning Tag ETAG: End Tag

BOM

2 BITS 4 BITS 10 BITS 6 BITS 10 BITS

2 Bytes 2 Bytes

45

The AAL Process: AAL5 CPCS-PDU

CRC CPCS-PDU PayloadCPCS-PDU Payload

LENGTH: CPCS-PDU Length CRC: Cyclic Redundancy CheckCPCS: Common Part Convergence Sublayer

CPCS-UUCPCS-UU LengthLengthCPICPI PADPAD

PAD: Padding UU: User-to-User IndicationCPI: Common Part Indicator

Unit: octets

0- 47

CPCS-PDUCPCS-PDU

Trailer

1 1 2 41 - 65,535

46

AAL-5

AAL5-SDUs

PADCPCS-PDU Payload

SAR Payload

AAL Service Data Unit (SDU)

AAL5-SAP

1-65,535 octets

CRC LengthCPICPCS-UU0-47 1 1 2 4

CPCS-PDUTrailer

• • •

Header Payload Header Payload Header Payload• • •5 48 5 48 5 48

SAR Payload

SAR Payload

CPCS-PDUsoctets

SAR-PDUs

ATM-SAPCells

Octets

• • •Payload Type=AAL_Indicate

47

• The Role of SNMP - RFC-1157The Role of SNMP - RFC-1157

• Integrated Local Management Interface - ILMIIntegrated Local Management Interface - ILMI

• ILMI-MIB - ATM ForumILMI-MIB - ATM Forum

• AToM-MIB - RFC-2515AToM-MIB - RFC-2515

Network Management for ATM

48

• The Simple Network Management Protocol (SNMP) has become widely accept in the LAN industry as an open standard for managing equipment from multiple vendors

• SNMP is supported by various WAN systems vendors

• Bellcore is using SNMP for their customer network management capabilities

• SNMP V2 addresses some of the bandwidth utilization, and security issues that were present in SNMP V1

• SNMP V3 is being defined in IETF

• ILMI utilizes the SNMP protocol as an ‘interim solution’

• Using RFC1577 - (IP & ARP over ATM) it is now possible to manage ATM networks from a central (or multiple) NMS, using SNMP/UDP/IP packets encapsulated in AAL5

The Role of SNMP

49

Integrated LocalManagement Interface

RemotelyAccessible

Agent

UME UME UME UME

UME UME

RemotelyAccessible

Agent

Beyond the Scopeof ILMI Specification

ILMI(SNMP/AAL)

ILMI(SNMP/AAL) Private

ATM Switch

ILMI(SNMP/AAL)

NETWORK MANAGEMENT NETWORK MANAGEMENT STATION or SUBSYSTEMSTATION or SUBSYSTEM

UME: UNI Management Entity

ATM End-System

Public NetworkATM Switch

Private UNI

Public UNI

Public UNI

50

ILMI PERFORMS THE FOLLOWING TASKS:ILMI PERFORMS THE FOLLOWING TASKS:

Basic Configuration information

PVC Status Indication in FR/ATM Service Interworking

ILMI Connectivity detection and auto neighbor discovery

Address registration for SVC and PNNI

ABR attribute setting for SVC

Auto-configuration of a LAN Emulation Client - LEC

Functions of ILMI

51

MIB specification in ATM UNI 3.X and 4.0MIB specification in ATM UNI 3.X and 4.0

Management information includes:Management information includes:

Physical Layer

ATM Layer

ATM Layer Statistics

Virtual Path (VP) Connections

Virtual Channel (VC) Connections

Network Prefix

Network Address

Service Registry

ILMI MIB

52

Structure of the MIB groups:Structure of the MIB groups:

Configuration

DS3 PLCP

TC Sublayer

Virtual Link Configuration

VP/VC cross-connect

Network Address

AAL Connection Performance Statistics

Reference: RFC 1695 & 2515

ATM Management MIB - ATOM

53

• ATM Forum Documents - www.atmforum.comATM Forum Documents - www.atmforum.com

• GR-1248 - Generic Requirements for Operations on ATM GR-1248 - Generic Requirements for Operations on ATM NEsNEs

• I.363 & 365 - ATM Adaptation Layer (AAL) SublayerI.363 & 365 - ATM Adaptation Layer (AAL) Sublayer

• I.610 - BISDN OAM Principles and FunctionsI.610 - BISDN OAM Principles and Functions

• RFC2761 - Terminology for ATM BenchmarkingRFC2761 - Terminology for ATM Benchmarking

REFERENCES