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UNIT I (contd..)

UNIT I (contd..). Asynchronous Transfer Mode (ATM). Introduction. ATM Protocol Architecture Logical connections ATM Cells Service categories ATM Adaptation Layer (AAL). ATM Protocol Architecture. Fixed-size packets called cells Streamlined: minimal error and flow control

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UNIT I (contd..)

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  1. UNIT I (contd..) Asynchronous Transfer Mode (ATM)

  2. Introduction • ATM Protocol Architecture • Logical connections • ATM Cells • Service categories • ATM Adaptation Layer (AAL)

  3. 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

  4. Protocol Model has 3 planes • User • Control • management

  5. Figure 5.1

  6. 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

  7. Figure 5.2

  8. Advantages of Virtual Paths • Simplified network architecture • Increased network performance and reliability • Reduced processing and short connection setup time • Enhanced network services

  9. Table 5.1

  10. VCC Uses • Between end users • Between an end user and a network entity • Between 2 network entities

  11. Figure 5.3

  12. 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

  13. 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

  14. Control Signaling • 3 methods for VPCs • Semi-permanent • Customer controlled • Network controlled

  15. 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

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

  17. Figure 5.4

  18. 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

  19. Table 5.3

  20. 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

  21. Figure 5.5

  22. Figure 5.6

  23. Figure 5.7

  24. 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)

  25. Figure 5.8

  26. 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

  27. 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)

  28. 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

  29. Figure 5.9

  30. Figure 5.10

  31. 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

  32. AAL Type 3/4 • May be connectionless or connection oriented • May be message mode or streaming mode

  33. Figure 5.11

  34. Figure 5.12

  35. AAL Type 5 • Streamlined transport for connection oriented protocols • Reduce protocol processing overhead • Reduce transmission overhead • Ensure adaptability to existing transport protocols

  36. Figure 5.13

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