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Understanding Asynchronous Transfer Mode (ATM): Architecture, Cells, and Service Categories

This chapter provides an in-depth look into Asynchronous Transfer Mode (ATM), focusing on its protocol architecture and key concepts such as logical connections, fixed-size cells, and various service categories. The ATM model comprises three planes: user, control, and management, which facilitate effective data transmission. It explores the roles of Virtual Channel Connections (VCCs) and Virtual Path Connections (VPCs) in simplifying network architecture and enhancing performance. Furthermore, the chapter covers aspects of the ATM Adaptation Layer (AAL), highlighting their importance in integrating non-ATM protocols and maintaining data integrity during transmission.

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Understanding Asynchronous Transfer Mode (ATM): Architecture, Cells, and Service Categories

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

  2. Introduction • ATM Protocol Architecture • Logical connections • ATM Cells • Service categories • ATM Adaptation Layer (AAL) Chapter 5 Asynchronous Transfer Mode

  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 Chapter 5 Asynchronous Transfer Mode

  4. Protocol Model has 3 planes • User • Control • Management Chapter 5 Asynchronous Transfer Mode

  5. Figure 5.1 Chapter 5 Asynchronous Transfer Mode

  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 Chapter 5 Asynchronous Transfer Mode

  7. Figure 5.2 Chapter 5 Asynchronous Transfer Mode

  8. Advantages of Virtual Paths • Simplified network architecture • Increased network performance and reliability • Reduced processing and short connection setup time • Enhanced network services Chapter 5 Asynchronous Transfer Mode

  9. 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 Chapter 5 Asynchronous Transfer Mode

  10. Control Signaling • A mechanism to establish and release VPCs and VCCs • Methods to request for a VC: • Semi-permanent VCCs • Request as needed Chapter 5 Asynchronous Transfer Mode

  11. 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 Chapter 5 Asynchronous Transfer Mode

  12. Header Format • Generic flow control • Virtual path identifier (VPI) • Virtual channel identifier (VCI) • Payload type • Cell loss priority • Header error control Chapter 5 Asynchronous Transfer Mode

  13. Figure 5.4 CLP = Cell Loss Priority bit Chapter 5 Asynchronous Transfer Mode

  14. 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 Chapter 5 Asynchronous Transfer Mode

  15. 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 Chapter 5 Asynchronous Transfer Mode

  16. 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) Chapter 5 Asynchronous Transfer Mode

  17. Figure 5.8 Chapter 5 Asynchronous Transfer Mode

  18. 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 Chapter 5 Asynchronous Transfer Mode

  19. 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) Chapter 5 Asynchronous Transfer Mode

  20. 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 Chapter 5 Asynchronous Transfer Mode

  21. Figure 5.9 Chapter 5 Asynchronous Transfer Mode

  22. Figure 5.10 (CBR source) CBR Service (connectionless or connection-oriented) VBR Service (higher-layer connection-oriented services) Connection-oriented Service Chapter 5 Asynchronous Transfer Mode

  23. 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 Chapter 5 Asynchronous Transfer Mode

  24. AAL Type 3/4 • May be connectionless or connection oriented • May be message mode or streaming mode Chapter 5 Asynchronous Transfer Mode

  25. AAL Type 5 • Streamlined transport for connection oriented protocols • Reduce protocol processing overhead • Reduce transmission overhead • Ensure adaptability to existing transport protocols Chapter 5 Asynchronous Transfer Mode

  26. Figure 5.13 Chapter 5 Asynchronous Transfer Mode

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