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CSE 30264 Computer Networks

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  1. CSE 30264Computer Networks Prof. Aaron Striegel Department of Computer Science & Engineering University of Notre Dame Lecture 7 – February 2, 2010

  2. Today’s Lecture • Project 1 • Reference Client • Switching and Forwarding • Chapter 3.2 Application Transport Network Data Physical Read Chapter 4.1 for Tuesday CSE 30264

  3. Project 1 • Reference Client CSE 30264

  4. Switching and Forwarding Outline Cell Switching Segmentation and Reassembly CSE30264

  5. Frame / Packet Length • How long should yourframe be? • Similar to THT concept • Is it a fixed size or variable? • Consistency? • Frame is the atomic unit of the network • Cannot pre-empt it • Medium becomes free afterblock is finished being transferred CSE 30264

  6. Variable vsFixed Length Packets • No optimal length • Small • High header-to-data overhead • Large • Low utilization for small messages • Fixed-length easier to switch in hardware • Simpler • Enables parallelism Each packet means more overhead CSE30264

  7. Variable Location - Lookup <body class="natBodynatViewBody "> <a name="PageTop"></a><table class="natPageBox"cellpadding="0" cellspacing="0" border="0" width="100%"><tr><td class="natBoxBorderTopLeft"> </td><td class="natBoxBorderTopRight"> </td></tr><tr> <td class="natBoxBorderLeft"> <div class="natBodyContents"> <!-- sidebar left --><table class="natLayoutTablenatHeaderArt" id="natHeaderArt" cellpadding="0" cellspacing="0" border="0" width="100%"> <tr><td class="natTopBarnatTopLeft " valign="top"> CSE 30264

  8. Fixed Location - Lookup Look in the first two bytes of the message from the client CSE 30264

  9. Big vs Small Packets • Small improves flexibility • Finer-grained scheduling • Share capacity easier between flows • Less time waiting to forward • Can send when whole packet is there • Example • Maximum packet = 4KB, Link speed=100 Mb/s • Transmission time = 4096 x 8/100 = 327.68us • High priority packet may sit in the queue 327.68us • Maximum packet = 48 bytes + Overhead, same link speed • Transmission time = 53 x 8/100 = 4.24us for ATM vs. CSE30264

  10. ATM – Asynchronous Transfer Mode • Evolved from phone network • Predictable • Deterministic • Connection-oriented • Setup / teardown of virtual circuits • Quality of Service (QoS) Synchronous vs. Asynchronous CSE 30264

  11. Cell Switching (ATM) • Used in both WAN and LAN settings • Specifications • ATM Forum • Signalling Q.2931 • Packets are called cells • 5-byte header + 48-byte payload • Commonly transmitted over SONET • Other physical layers possible CSE30264

  12. Big vsSmall Redux • Small improves latency (for voice) • voice digitally encoded at 64Kbps (8-bit samples at 8KHz) • need full cell’s worth of samples before sending cell • example: 1000-byte cells implies 125ms per cell (too long) • smaller latency implies no need for echo cancelers • ATM compromise: 48 bytes = (32+64)/2 CSE30264

  13. Cell Format Numbers are in bits • User-Network Interface (UNI) • host-to-switch format • GFC: Generic Flow Control (still being defined) • VCI: Virtual Circuit Identifier • VPI: Virtual Path Identifier • Type: management, congestion control, AAL5 (later) • CLP: Cell Loss Priority • HEC: Header Error Check (CRC-8) • Network-Network Interface (NNI) • switch-to-switch format • GFC becomes part of VPI field CSE30264

  14. UNI from earlier Cell Format • Type • BOM: beginning of message • COM: continuation of message • EOM: end of message • SSM: single-segment message • SEQ: sequence of number • MID: multiplexing id • Length: number of bytes of PDU in this cell How do we take messages (bigger data) and map it down? CSE30264

  15. Encapsulation Convergence Sublayer Protocol Data Unit CSE30264

  16. Segmentation and Reassembly • ATM Adaptation Layer (AAL) • AAL 1 and 2 designed for applications that need guaranteed rate (e.g., voice, video) • AAL 3/4 designed for packet data • AAL 5 is an alternative standard for packet data AAL AAL ■ ■ ■ ■ ■ ■ ATM ATM CSE30264

  17. 0 ─ 24 8 8 8 16 < 64 KB 8 16 CPI Btag BASize User data Pad 0 Etag Len AAL 3/4 • Convergence Sublayer Protocol Data Unit (CS-PDU) • CPI: common part indicator (version field) • Btag/Etag: beginning and ending tag • BAsize: hint on amount of buffer space to allocate • Length: size of whole PDU CSE30264

  18. < 64 KB 0 ─ 47 bytes 16 16 32 Data Pad Reserved Len CRC-32 AAL5 • CS-PDU Format • Pad: trailer always falls at end of ATM cell • Length: size of PDU (data only) • CRC-32 • Cell Format • End-of-PDU bit in Type field of ATM header CSE30264

  19. Virtual Paths • 8-bit VPI and 16-bit VCI • Two-level hierarchy of virtual connections CSE30264

  20. Path Aggregation VPI 0, VCI 17 VPI 0, VCI 17 VPI 25 VPI 0, VCI 25 VPI 0, VCI 25 VPI 0, VCI 3089 VPI 0, VCI 3089 CSE 30264