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Computer Networks: Introduction

Computer Networks: Introduction

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Computer Networks: Introduction

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  1. Computer Networks:Introduction Ivan Marsic Rutgers University Chapter 1 – Introduction

  2. Topic:Introduction to Data Networking  Goals  Communication Media  Protocols  Reliable Transmission

  3. User Goals and Tunable Knobs Visible network properties: Delivery Correctness Fault tolerance Timeliness Cost Customer Tunable network parameters: Network topology Communication protocols Network architecture Components Physical medium Network Engineer

  4. Topology vs. Robustness Paul Baran, 1964

  5. Internet Map: Major ISPs

  6. Fully Interconnected Network New York City, 1888

  7. Early Telephone Switching Offices

  8. 1924: First Mobile Telephone The first version of a mobile radio telephone being used in 1924.

  9. Exploiting Locality Saul Steinberg, “A View of the World from Ninth Avenue,” cover of The New Yorker March 29, 1976

  10. Distortion of Signals threshold "0"/"1"

  11. Packet Error Rate Approximation PER = packet error rate BER = bit error rate n = packet length [in bits]

  12. Packet Transmission (1) Example: Sender sends a 6-bit packet “101101” to the receiver

  13. Transmission Link Capacity Effect of link speed:  Link 2 can transmit 10 times more bits per unit of time  or, Link 2 can transmit the same message in a 10 times shorter period

  14. Wireless Communication Point source Interfering sources Interference pattern

  15. Radio Signal Propagation Ray tracing simulation in a closed office environment. Signal intensity map for a room with a doorway and a metal desk door desk

  16. Transmission / Interference Range

  17. Protocols

  18. Statistical Multiplexing

  19. 3-Layer Protocol Stack Protocol at layer i depends only on the protocols at i1 (not at i1!)

  20. Layer 1 / 3-Layer Protocol Stack Link Layer: Protocol modules at layer 1 (bottom layer) exchange packets over the link

  21. Layer 2 / 3-Layer Protocol Stack Network Layer: Protocol modules at layer 2 (middle layer) route packets from source to destination (possibly over many links)

  22. Layer 3 / 3-Layer Protocol Stack Applications:  Network games  Internet telephony  Email End-to-End Layer: Protocol modules at layer 3 (top layer) create illusion of different link types (tailored to application-specific needs)

  23. Protocol Layers at Hosts/Switches

  24. Bit Stuffing for Transparency

  25. Visit for more details on the OSI Reference Architecture ISO OSI Protocol Stack • Application services (SIP, FTP, HTTP, Telnet, …) • Data translation (MIME) • Encryption (SSL) • Compression • Dialog control • Synchronization • Reliable (TCP) • Real-time (RTP) • Source-to-destination (IP) • Routing • Address resolution • Wireless link (WiFi) • Wired link (Ethernet) • Radio spectrum • Infrared • Fiber • Copper

  26. Packet Nesting Across Layers

  27. How Headers Guide Packets

  28. Error Detection and Correction

  29. Interleaving

  30. Packet Transmission (2)

  31. Transmission and Propagation Delays Transmission delay: Propagation delay:

  32. Fluid Flow Analogy

  33. What Contributes to RTT

  34. Topic:Reliable Transmission via Retransmission  Stop-and-Wait  Go-Back-N  Selective Repeat

  35. Automatic Repeat reQuest (ARQ) Stop-and-wait ARQ Transmit a frame and wait for acknowledgement (ACK) If positive ACK from receiver, send next frame If ACK does not arrive after a certain period of time (Timeout), retransmits the frame Simple, low efficiency Go-back-N ARQ Transmit frames continuously, no waiting The receiver only ACKs the highest-numbered frames received in sequence ACK comes back after a round-trip delay If timeout, the sender retransmits the frames that are not ACKed and N–1 succeeding frames that were transmitted during the round-trip delay (N frames transmitted during a round-trip delay) Need buffer at sender, does not have to buffer the frames at the receiver, Moderate efficiency and complexity. Less efficient when the round-trip delay is large and data transmission rate is high Selective-repeat ARQ Transmit continuously, no waiting The receiver ACKs all successfully received frames The sender only retransmits (repeats) the unACKed frames when their timers expire Most efficient, but most complex, buffer needed at both sender & receiver, needs per-frame timer

  36. Stop-and-Wait with Errors

  37. Stop & Wait Sender Utilization Stop & Wait sender utilization, under error-free transmission: Probability of successful transmission, with error rate pe: Expected sender utilization for Stop & Wait, under errors:

  38. Sliding Window – Keeping the Pipe Full • Goal: Sender should be busy sending packets (as long as it has packets ready to send) • Sender utilization as a metric of protocol performance • “Keeping the pipe full”

  39. Sliding Window ARQ

  40. Go-back-N ARQ

  41. Selective Repeat ARQ

  42. Acknowledgements: GBN vs. SR

  43. Topic:Broadcast and Wireless Links  ALOHA  Hidden and Exposed Stations  Carrier Sensing Multiple Access  CSMA/CD, CSMA/CD

  44. Transmission Cone

  45. Transmission Cone, Collision& Vulnerable Period (b) Collision occurs if two (or more) transmission cones overlap. (a)

  46. Parameter  Ratio of propagation delay vs. packet transmission time:

  47. Parameter  Ratio of propagation delay vs. packet transmission time

  48. ALOHA and Slotted ALOHASender’s State Diagram

  49. ALOHA Packet Transmission

  50. When transmission cones of ALOHA stations overlap?