Chapter 5 Asynchronous Transfer Mode 1 Chapter 5 Asynchronous Transfer Mode (ATM)

Chapter 5 Asynchronous Transfer Mode

1

Chapter 5

Asynchronous Transfer Mode

(ATM)


2

Introduction

ATM Protocol Architecture Logical connections ATM Cells Service categories ATM Adaptation Layer (AAL)


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ATM Protocol Architecture

Fixed-size packets called cells Streamlined: minimal error and flow control 2 protocol layers relate to ATM functions:

– Common layer providing packet transfers– Service dependent ATM adaptation layer (AAL)

AAL maps other protocols to ATM


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Protocol Model has 3 planes

User Control Management


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Figure 5.1


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Logical 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


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Figure 5.2


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Advantages of Virtual Paths

Simplified network architecture Increased network performance and

reliability Reduced processing and short

connection setup time Enhanced network services


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VPC/VCC Characteristics

Quality of Service (QoS) Switched and semi-permanent virtual

channel connections Cell sequence integrity Traffic parameter negotiation and usage

monitoring (VPC only) virtual channel identifier

restriction within a VPC


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Control Signaling

A mechanism to establish and release VPCs and VCCs

Methods to request for a VC:– Semi-permanent VCCs– Request as needed


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ATM Cells

Fixed size 5-octet header 48-octet information field Small cells reduce delay for high-priority

cells Fixed size facilitate switching in

hardware


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Header Format

Generic flow control Virtual path identifier (VPI) Virtual channel identifier (VCI) Payload type Cell loss priority Header error control


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Figure 5.4

CLP = Cell Loss Priority bit


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Generic 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


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Header Error Control

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

For error detection In some cases, error correction of

single-bit errors in header


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Service 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)


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Figure 5.8


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ATM Adaptation Layer (AAL)

Support non-ATM protocols– Maps to ATM cells

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


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Applications of AAL and ATM

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

VBR voice and video General data services IP over ATM Multiprotocol encapsulation over ATM

(MPOA) LAN emulation (LANE)


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AAL Protocols

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

• Supports specific applications using AAL

– Segmentation and Reassembly Layer (SAR)

• Packages data from CS into cells and unpacks at other end


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Figure 5.9


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Figure 5.10(CBR source)

(connectionless or connection-oriented)

(higher-layer connection-oriented services)

CBR Service

VBR Service

Connection-oriented Service


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AAL Type 1

Constant-bit-rate source SAR simply packs bits into cells and unpacks

them at destination Sequence Number (SN) used in

reassembling SAR PDUs Sequence Number Protection (SNP) provides

error detection/correction No CS PDU since CS sublayer primarily for

clocking and synchronization


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AAL Type 3/4

May be connectionless or connection oriented

May be message mode or streaming mode


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AAL Type 5

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

protocols


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Figure 5.13

Chapter 5 Asynchronous Transfer Mode 1 Chapter 5 Asynchronous Transfer Mode (ATM)

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