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Understanding Asynchronous Transfer Mode (ATM) Protocols and Their Applications

This document offers a comprehensive overview of Asynchronous Transfer Mode (ATM) protocols, covering key aspects such as ATM protocol architecture, logical connections, cell structure, and service categories. It explains the importance of fixed-size cells, the functions of the ATM Adaptation Layer (AAL), and highlights various applications of ATM including circuit emulation and data services. Additionally, it discusses virtual channel connections (VCC) and virtual path connections (VPC), emphasizing their advantages, Quality of Service (QoS) mechanisms, and control signaling methods within ATM networks.

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Understanding Asynchronous Transfer Mode (ATM) Protocols and Their Applications

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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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