Chapter 2 Protocols and the TCP/IP Suite 1 Chapter 5 Asynchronous Transfer Mode (ATM)

transcript

Chapter 2 Protocols and the TCP/IP Suite1

Chapter 5Chapter 5

Asynchronous Transfer Mode(ATM)

Introduction Introduction

ATM Protocol ArchitectureLogical connectionsATM CellsService categoriesATM Adaptation Layer (AAL)

ATM Protocol ArchitectureATM Protocol Architecture

Fixed-size packets called cellsStreamlined: minimal error and flow

control2 protocol layers relate to ATM functions:

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

(AAL)AAL maps other protocols to ATM

Protocol Model has 3 planesProtocol Model has 3 planes

UserControlmanagement

Figure 5.1Figure 5.1

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

Advantages of Virtual PathsAdvantages of Virtual Paths

Simplified network architectureIncreased network performance and

reliabilityReduced processing and short connection

setup timeEnhanced network services

Table 5.1Table 5.1

VCC UsesVCC Uses

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

VPC/VCC CharacteristicsVPC/VCC Characteristics

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

channel connectionsCell sequence integrityTraffic parameter negotiation and usage

monitoring(VPC only) virtual channel identifier

restriction within a VPC

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

Control SignalingControl Signaling

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

ATM CellsATM Cells

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

cellsFixed size facilitate switching in hardware

Header FormatHeader Format

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

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

Table 5.3Table 5.3

Header Error ControlHeader Error Control

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

error detection in some cases, error correction of single-

bit errors in header2 modes:

– error detection– Error correction

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)

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

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

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)LAN emulation (LANE)

AAL ProtocolsAAL 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

AAL Type 1AAL Type 1

Constant-bit-rate sourceSAR simply packs bits into cells and

unpacks them at destinationOne-octet header contains 3-bit SC field to

provide an 8-cell frame structureNo CS PDU since CS sublayer primarily

for clocking and synchronization

AAL Type 3/4AAL Type 3/4

May be connectionless or connection oriented

May be message mode or streaming mode

AAL Type 5AAL Type 5

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

protocols

Chapter 2 Protocols and the TCP/IP Suite 1 Chapter 5 Asynchronous Transfer Mode (ATM)

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