Lect1. - 01/06/05

Lect1..ppt - 01/06/05

CDA 6505 Network Architecture

and Client/Server Computing

Lecture 5

Asynchronous Transfer Mode(ATM)

byZornitza Genova Prodanoff

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Outline

•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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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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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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VCC Uses

• Between end users• Between an end user and a network entity• Between 2 network entities

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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

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

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Control Signaling

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

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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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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

• error detection• in some cases, error correction of single-bit errors

in header• 2 modes:

– error detection– Error correction

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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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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

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Applications of AAL and ATM

• Circuit emulation (e.g., 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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Applications of AAL and ATM

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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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AAL Type 1

• Constant-bit-rate source• 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

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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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