Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706
Home Page
Data Communications and Networks I • 4003-420-01/4005-740-01 • Fall Quarter 2012
Course Page

4003-420-01/4005-740-01 Data Communications and Networks I
Module 3. Packet Transmission -- Lecture Notes

Prof. Alan Kaminsky -- Fall Quarter 2012
Rochester Institute of Technology -- Department of Computer Science

Key Organizing Principle For This Course

  • What do I need to know about topic X to write network applications?

  • What issues related to topic X affect network application design?

Data Transmission

  • Continuous transmission

  • Packetized transmission

  • Rationale for packetized transmission
    • Because network applications usually transmit data sporadically anyway
    • To provide fairer access to a shared data link among multiple computers
    • To limit the damage if a transmission error occurs

Transmission Errors

  • Definition of transmission errors

  • Sources of transmission errors

  • Nature of transmission errors
    • Independent errors
    • Correlated errors (burst errors)

  • Bit error rate (BER)

  • Calculating the probability of transmission errors
    • Pr[no errors]
    • Pr[1 error]
    • Pr[2 or more errors]

Error Detection and Correction Techniques

  • Parity
    • Even parity
    • Odd parity
    • Error detection abilities of parity
    • (Also used to detect errors in computer memories -- "parity memory")

  • Checksum
    • For example, the standard Internet Checksum (RFC 1071)
    • Error detection abilities of checksum

  • Cyclic redundancy check (CRC)
    • For example, CRC-CCITT (polynomial x16 + x12 + x5 + 1)
    • Error detection abilities of CRC

  • Hamming code
    • Invented in 1950 by Richard W. Hamming (1915-1998)
    • The original (7,4) Hamming code
    • Error detection and correction abilities of the Hamming code
    • (Also used to detect and correct errors in computer memories -- "ECC memory")

Local Area Networks

  • LAN communication channels
    • Point-to-point channels
    • Broadcast channels

  • LAN topologies
    • Bus
    • Star (tree)
    • Ring

  • Channel access methods
    • IEEE 802.4/Token Bus, IEEE 802.5/Token Ring
      • Token passing
      • Deterministic
    • IEEE 802.3/Ethernet
      • Carrier sense multiple access/collision detection (CSMA/CD)
      • Random
    • IEEE 802.11/Wireless Ethernet
      • Carrier sense multiple access/collision avoidance (CSMA/CA)
      • Random

Packet Formats

  • IEEE 802.3/Ethernet station addresses (MAC addresses) -- 48 bits

  • Ethernet frame format
    • Destination MAC address (6 bytes)
    • Source MAC address (6 bytes)
    • Frame type (2 bytes)
    • Data -- payload (46 .. 1500 bytes)
    • CRC (4 bytes)

  • IEEE 802.3 frame format
    • Destination MAC address (6 bytes)
    • Source MAC address (6 bytes)
    • Data length (2 bytes)
    • Data (46 .. 1500 bytes)
      • Protocol identifier ("SNAP header") (8 bytes)
      • Payload
    • CRC (4 bytes)

  • Packet overhead

Subscriber Access

  • Modems

  • Dialup access
    • Slow by today's standards, up to 53 Kbps
    • Symmetric data rates (the same data rate both incoming and outgoing)

  • Asymmetric digital subscriber line (ADSL) access
    • Download speeds up to 6.144 Mbps
    • Upload speeds up to 640 Kbps
    • Asymmetric data rates

  • Cable access
    • Speeds comparable to ADSL (marketing hype notwithstanding)
    • Asymmetric data rates

Data Link Layer Issues Affecting Network Application Design

  • Bit error issues

  • Channel access issues

  • Packet overhead issues

  • Symmetric vs. asymmetric data rate issues

Data Communications and Networks I • 4003-420-01/4005-740-01 • Fall Quarter 2012
Course Page
Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706
Home Page
Copyright © 2009 Alan Kaminsky. All rights reserved. Last updated 17-Sep-2009. Please send comments to ark­@­cs.rit.edu.