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Chapter 5 Asynchronous Transfer Mode (ATM) Chapter 5 Asynchronous Transfer Mode 1 Introduction ATM Protocol Architecture Logical connections ATM Cells Service categories ATM Adaptation Layer (AAL) Chapter 5 Asynchronous Transfer Mode 2 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 3 Protocol Model has 3 planes User Control Management Chapter 5 Asynchronous Transfer Mode 4 Figure 5.1 Chapter 5 Asynchronous Transfer Mode 5 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 6 Figure 5.2 Chapter 5 Asynchronous Transfer Mode 7 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 8 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 9 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 10 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 11 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 12 Figure 5.4 CLP = Cell Loss Priority bit Chapter 5 Asynchronous Transfer Mode 13 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 14 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 15 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 16 Figure 5.8 Chapter 5 Asynchronous Transfer Mode 17 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 18 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 19 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 20 Figure 5.9 Chapter 5 Asynchronous Transfer Mode 21 Figure 5.10 (CBR source) CBR Service (connectionless VBR Service or connection-oriented) (higher-layer Connection-oriented connection-oriented services) Service Chapter 5 Asynchronous Transfer Mode 22 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 23 AAL Type 3/4 May be connectionless or connection oriented May be message mode or streaming mode Chapter 5 Asynchronous Transfer Mode 24 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 25 Figure 5.13 Chapter 5 Asynchronous Transfer Mode 26