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Chapter 5

Chapter 5. Asynchronous Transfer Mode (ATM). Introduction . ATM Protocol Architecture Logical connections ATM cell structure Service levels/categories ATM Adaptation Layer (AAL). Introduction . ATM evolved from B-ISDN development efforts Frame Relay: high-speed WAN (1.5+ Mbps)

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Chapter 5

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  1. Chapter 5 Asynchronous Transfer Mode (ATM)

  2. Introduction • ATM Protocol Architecture • Logical connections • ATM cell structure • Service levels/categories • ATM Adaptation Layer (AAL) Chapter 5 ATM

  3. Introduction • ATM evolved from B-ISDN development efforts • Frame Relay: high-speed WAN (1.5+ Mbps) • ATM: very high speed WAN (155 Mbps and 622Mbps) • ATM, like Frame Relay, was built on the assumption that the underlying physical media was reliable and flexible • minimal error and flow control capabilities • even more streamlined, therefore faster, than Frame Relay • Specifications developed by ITU-T and ATM Forum Chapter 5 ATM

  4. ATM Protocol Architecture • Fixed-size packets called cells • “cell switching” like packet switching • 2 primary protocol layers relate to ATM functions: • Common layer providing packet transfers, logical connections (ATM) • Service dependent ATM adaptation layer (AAL) • AAL maps other protocols to ATM • like IP (AAL5) Chapter 5 ATM

  5. Protocol Model has 3 planes • User – provides for user information transfer and associated controls (flow control, congestion control) • Control – performs call control and connection control functions (signaling) • Management – provides plane management and layer management and coordination functions Chapter 5 ATM

  6. ATM Protocol Reference Model Map data to the ATM cell structure Framing, cell structure & Logical Connections Various data rates (155.52 Mbps, 622.08 Mbps) over various physical media types (Fiber Optic, SONET, UTP, etc.) Chapter 5 ATM

  7. User Plane Layers User information User information AAL AAL ATM ATM ATM ATM PHY PHY PHY PHY … End system End system Network Chapter 5 ATM

  8. User Plane Layers User information User information Chapter 5 ATM

  9. Logical Connections • VCC (Virtual Channel Connection): a logical connection analogous to a virtual circuit in X.25, or Frame Relay data link connection • full-duplex flow between end users • user-network control signaling • network-network management/routing • VPC (Virtual Path Connection): a bundle of VCCs with the same end points (not necessarily same end-users) • and switched along the same path Chapter 5 ATM

  10. ATM Connection Relationships Virtual Channel: basic logical communications channel Virtual Path: groups of “common” virtual channels Physical Transmission Path: physical communications link Chapter 5 ATM

  11. Advantages of Virtual Paths • Simplified network architecture – allows separation of functionality into into individual logical connections and related groups of logical connections • Increased network performance and reliability – network consists of fewer aggregated entities • Reduced processing and short connection setup time – complex setup tasks are in virtual paths, simplifies setup of new virtual channels over existing virtual path • Enhanced network services – supports user-specified closed groups/networks of VC bundles Chapter 5 ATM

  12. Virtual Path/Virtual Channel Terminology Virtual Channel (VC) A generic term used to describe unidirectional transport of cells associated by a common unique identifier Virtual Channel Identifier (VCI) A unique numerical tag for a particular VC link Virtual Channel Link A means of unidirectional transport of cells between the point where a VCI is assigned and where it is translated or terminated Virtual Channel Connection (VCC) A concatenation of VC links that extends between two connected ATM end-points Chapter 5 ATM

  13. Virtual Path/Virtual Channel Terminology Virtual Path (VP) A generic term which describes unidirectional transfer of cells that are associated with a common unique identifier Virtual Path Identifier (VPI) Identifies a particular VP Virtual Path Link A group of VC links identified by a common identifier between the point where the identifier (VPI) is assigned and where it is translated or terminated Virtual Path Connection (VPC) A concatenation of VP links that extends between ATM end-points where the VCIs are assigned and where they are translated or terminated Chapter 5 ATM

  14. ATM VPC/VCC a VP3 VP5 a b ATM Sw 1 b ATM Sw 2 c ATM Sw 3 ATM DCC c d VP6 e VP2 VP1 d ATM Sw 4 e Sw = switch DCC = Cross-connect switch Chapter 5 ATM

  15. ATM Connection Relationships Chapter 5 ATM

  16. VPC/VCC Characteristics • Quality of Service (QoS) • Switched and semi-permanent virtual channel connections • Cell sequence integrity • Traffic parameter negotiation and usage monitoring • average rate, peak rate, burstiness, peak duration, etc. • (VPC only) virtual channel identifier restriction within a VPC Chapter 5 ATM

  17. Control Signaling • A mechanism to establish and release VPCs and VCCs (per ITU-T Rec. I.150) • 4 methods for VCCs: • Semi-permanent VCC: no control signaling required • Meta-signaling channel: permanent, low data rate channel for setting up signaling channels • User-to-network signaling virtual channel: set up between user and network • User-to-user signaling virtual channel: set up between users within a VPC, allowing users to set up and tear down VCCs, without network intervention Chapter 5 ATM

  18. ATM Cells • Fixed size • 5-octet header • 48-octet information field • Small cells may reduce queuing delay for high-priority cells (essential for low delay) • Fixed size facilitates more efficient switching in hardware (essential for very high data rates) Chapter 5 ATM

  19. ATM Cell Format (p. 98) Chapter 5 ATM

  20. Header Format • Generic flow control (more ->) • Virtual path identifier (VPI) • Virtual channel identifier (VCI) • Payload type (3 bits: identifies cell as user data or network management cell, presence of congestion, SDU type) • Cell loss priority (0: high; 1: low) • Header error control (more ->) Chapter 5 ATM

  21. Generic Flow Control • Used to control traffic flow at user-network interface (UNI) to alleviate short-term overload conditions • Note: not employed in network core • When GFC is enabled at the UNI, two procedures are used: • Uncontrolled transmission: not subject to flow control • Controlled transmission: flow control constraints (using GFC mechanism) are in force Chapter 5 ATM

  22. Header Error Control • 8-bit field - calculated based on the other 32 bits in the header • CRC based on x8 + x2 + x + 1 -> generator is 100000111 • error detection • in some cases, error correction of single-bit errors in header • 2 modes: • Error detection • Error correction Chapter 5 ATM

  23. HEC Operation at Receiver Based on recognition of fact that bit errors occur in bursts. Chapter 5 ATM

  24. ATM 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 ATM

  25. ATM Bit Rate Service Levels Chapter 5 ATM

  26. ATM Adaptation Layer (AAL) • Support higher-level protocols and/or native applications • e.g., PCM voice, LAPF, IP • AAL Services • Handle transmission errors • Segmentation/reassembly (SAR) • Handle lost and misinserted cell conditions • Flow control and timing control Chapter 5 ATM

  27. ATM Adaptation Layer (AAL) Chapter 5 ATM

  28. 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) Chapter 5 ATM

  29. AAL Protocol and Services • Basis for classification: • requirement for a timing relationship between source and destination • requirement for a constant bit rate data flow • connection or connectionless transfer Chapter 5 ATM

  30. AAL Protocols • AAL layer has 2 sublayers: • Convergence Sublayer (CS) • Supports specific applications/protocols using AAL • Users attach via the Service Access Point (like a port number) • Common part (CPCS) and application service-specific part (SSCS) • Segmentation and Reassembly Sublayer (SAR) • Packages data from CS into ATM cells and unpacks at other end Chapter 5 ATM

  31. AAL Protocols and PDUs Chapter 5 ATM

  32. AAL Protocol Descriptions Chapter 5 ATM

  33. Segmentation and Reassembly PDUs Chapter 5 ATM

  34. 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 structure is defined since CS sublayer primarily for clocking and synchronization Chapter 5 ATM

  35. AAL Type 1 Chapter 5 ATM

  36. AAL Type 2 • Intended for use with applications with variable bit-rate service on multiple channels (multiplexing), or low bit rate, short-frame traffic • Intended for variable bit rate applications that generate bursty data and demand low loss • Originally, connectionless (AAL4) or connection (AAL3) oriented, now combined into single format (AAL 3/4) • Provides comprehensive sequencing and error control mechanisms AAL Type 3/4 Chapter 5 ATM

  37. AAL 3/4 Chapter 5 ATM

  38. AAL Type 5 • Streamlined transport for connection oriented protocols • Reduce protocol processing overhead • Reduce transmission overhead • Ensure adaptability to existing transport protocols • primary function is segmentation and reassembly of higher-level PDUs Chapter 5 ATM

  39. AAL5 Chapter 5 ATM

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