11

Asynchronous Transfer Mode

(ATM)

22

Introduction Introduction

ATM Protocol ArchitectureLogical connectionsATM CellsService categoriesATM Adaptation Layer (AAL)

33

ATM Protocol ArchitectureATM Protocol Architecture

Fixed-size packets called cellsStreamlined: minimal error and flow

control (high data rates)2 protocol layers relate to ATM functions:

– Common layer providing packet transfers– Service dependent ATM adaptation layer

(AAL)AAL maps other protocols to ATM

4

ATM Layers

55

Protocol Model has 3 planesProtocol Model has 3 planes

User: Provides for user information transfer, along with error and flow control.

Control: Performs call control and connection control functions.

Management: Performs management functions and coordination between all the planes.

6

Architecture of an ATM Network

77

Logical ConnectionsLogical Connections

VCC (Virtual Channel Connection): a logical connection analogous to virtual circuit in X.25

VPC (Virtual Path Connection): a bundle of VCCs with same endpoints

88

Figure 5.2Figure 5.2

9

TP, VPs, and VCs

10

Example of VPs and VCs

11

Connection Identifiers

12

Figure 19-13

Routing with a Switch

13

Virtual Connection Identifiers in UNIs and NNIs

1414

Advantages of Virtual PathsAdvantages of Virtual Paths

Simplified network architectureIncreased network performance and

reliabilityReduced processing and short connection

setup timeEnhanced network services

1515

Table 5.1Table 5.1

1616

VCC UsesVCC Uses

Between end usersBetween an end user and a network entityBetween 2 network entities

1717

Figure 5.3Figure 5.3

1818

VPC/VCC CharacteristicsVPC/VCC Characteristics

Quality of Service (QoS) : Cell loss ratioSwitched and semi-permanent virtual

channel connectionsCell sequence integrityTraffic parameter negotiation and usage

monitoring(VPC only) virtual channel identifier

restriction within a VPC

1919

Control SignalingControl Signaling

A mechanism to establish and release VPCs and VCCs

4 methods for VCCs:– Semi-permanent VCCs– Meta-signaling channel– User-to-network signaling virtual channel– User-to-user signaling virtual channel

2020

Control SignalingControl Signaling

3 methods for VPCs– Semi-permanent– Customer controlled– Network controlled

21

Multiplexing Using Different Packet Sizes

22

Multiplexing Using Cells

23

ATM Multiplexing

2424

ATM CellsATM Cells

Fixed size5-octet header48-octet information fieldSmall cells reduce delay for high-priority

cellsFixed size facilitate switching in hardware

2525

Header FormatHeader Format

Generic flow controlVirtual path identifier (VPI)Virtual channel identifier (VCI)Payload typeCell loss priorityHeader error control

2626

Figure 5.4Figure 5.4

27

Figure 19-28 ATM HeaderATM Header

28

Virtual Connection Identifiers in UNIs and NNIs

2929

Generic Flow ControlGeneric Flow Control

Control traffic flow at user-network interface (UNI) to alleviate short-term overload conditions

When GFC enabled at UNI, 2 procedures used:– Uncontrolled transmission

– Controlled transmission Every connection either subject to flow control or not Flow control is from subscriber to network

– Controlled by network side

30

Payload Type (PT)Payload Type (PT)

Indicates the type of the information in the information field.

3 bits.– 1st bit indicates user information.– 2nd bit indicates congestion.– 3rd bit indicates SDU (Service Data Unit).

31

Payload Type (Cont…)Payload Type (Cont…)

PT Coding

Interpretation

000 User Data, congestion not experienced, SDU type = 0

001 User Data, congestion not experienced, SDU type = 1

010 User Data, congestion experienced, SDU type = 0

011 User Data, congestion experienced, SDU type = 1

100 OAM segment associated cell

101 OAM end to end associated cell

110 Resource management cell

111 Reserved for future function

32

Cell Loss Priority (CLP)Cell Loss Priority (CLP)

Provides guidance to the network in the event of congestion.

A value ‘0’ indicates a cell of relatively higher priority.

A value ‘1’ indicates a cell of relatively lower priority.

3333

Header Error ControlHeader Error Control

8-bit field calculated based on remaining 32 bits of header

In some cases, error correction of single-bit errors in header

2 modes: – Error detection– Error correction

3434

Figure 5.5Figure 5.5

3535

Figure 5.6Figure 5.6

36

Definition of TermsDefinition of Terms COS – Class of Service – sets a priority of data

delivery, based upon the class. Higher priority data get delivered before lower priority data (example – which should have higher priority – streaming video or email?)

QOS – Quality of Service – involves establishing certain parameters for a specific transmission – e.g. amount of bandwidth required for a given priority data transmission, max. amount of latency tolerated, etc

Both are required to deliver real-time voice and video traffic

3737

Service CategoriesService Categories

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

Non-real-time service– Non-real-time variable bit rate (nrt-VBR)– Available bit rate (ABR)– Unspecified bit rate (UBR)– Guaranteed frame rate (GFR)

38

Real Time ServicesReal Time Services If want to avoid or decrease variation of delay (jitter), use CBR or rt-VBR CBR Fixed data rate continuously available Commonly used for uncompressed audio and video

– Video conferencing– Interactive audio– A/V distribution and retrieval

rt-VBR Best for time sensitive applications– Tightly constrained delay and delay variation

rt-VBR applications transmit at a rate that varies with time– e.g. compressed video – Produces varying sized image frames– Original (uncompressed) frame rate constant – So compressed data rate varies

Can statistically multiplex connections

39

Non-Real Time Non-Real Time

Intended for applications with bursty traffic and limited constraints on delay and delay variation

Greater flexibility, greater use of multiplexing

40

nrt-VBRnrt-VBR

May be able to characterize expected traffic flow

Improve QoS in loss and delayEnd system specifies:

– Peak cell rate – Sustainable or average rate – Measure of how bursty traffic is

e.g. Airline reservations, banking transactions

41

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

Unused capacity of CBR and VBR traffic made available to UBR

For application that can tolerate some cell loss or variable delays– e.g. TCP based traffic

Cells forwarded on FIFO basisBest efforts serviceText/Data/image transfer, messagingRemote Terminal (telecommuting)

42

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

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

Resources allocated to give at least MCRSpare capacity shared among ABR and

UBR sourcese.g. LAN interconnection

43

Guaranteed Frame Rate (GFR)Guaranteed Frame Rate (GFR)

Designed to support IP backbone sub networks

Purpose: optimize handling of frame based traffic passing from LAN through router to ATM backbone– Used by enterprise, carrier and ISP networks– Consolidation and extension of IP over WAN

UNI establishes hand shaking between NIC and switch

4444

Figure 5.8Figure 5.8

45

ATM Layers

46

ATM Layers in End-Point Devices and Switches

47

Functions of the LayersFunctions of the Layers

4848

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

Support non-ATM protocols– e.g., PCM voice, LAPF

AAL Services– Handle transmission errors– Segmentation/reassembly (SAR)– Handle lost and misinserted cell conditions– Flow control and timing control

4949

Applications of AAL and ATMApplications of AAL and ATM

Circuit emulation (e.g., T-1 synchronous TDM circuits)

VBR voice and videoGeneral data servicesIP over ATMMultiprotocol encapsulation over ATM

(MPOA) (IPX, AppleTalk, DECNET)LAN emulation (LANE)

5050

AAL ProtocolsAAL Protocols

AAL layer has 2 sublayers:– Convergence Sublayer (CS)

Supports specific applications using AALservice dependant

– Segmentation and Reassembly Layer (SAR)Packages data from CS into cells and unpacks at

other end

51

AAL Types

5252

Figure 5.9Figure 5.9

5353

Figure 5.10Figure 5.10

5454

AAL Type 1AAL Type 1

Constant-bit-rate source: Circuit Emulation

SAR simply packs bits into cells and unpacks them at destination

One-octet header contains 3-bit SC field to provide an 8-cell frame structure

No CS PDU since CS sublayer primarily for clocking and synchronization

55

Figure 19-23

AAL1

56

AAL 2AAL 2

Variable bit rate source: MPEG Video.Analog Applications such as audio and

video.Timing information but not CBR.Not clearly defined.

5757

AAL Type 3/4AAL Type 3/4

May be connectionless or connection oriented

May be message mode or streaming mode

58

Figure 19-25 AAL3/4

5959

Figure 5.11Figure 5.11

6060

Figure 5.12Figure 5.12

6161

AAL Type 5AAL Type 5

Streamlined transport for connection oriented protocols– Reduce protocol processing overhead– Reduce transmission overhead– Ensure adaptability to existing transport

protocols

62

Figure 19-26

AAL5

6363

Figure 5.13Figure 5.13

64

ATM Layer

Defines the cell format and connection setup.