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Computer networks

Free The data link layer lessons

7 lessons, about 1½ hours in all. Each teaches one idea with a visual, then gives you a problem or a few questions on it. Read the lessons here, then create a free account to practise and keep your progress.

Start free with lesson 1See the full The data link layer path

Stage 0Detecting and fixing errors

  1. Parity and checksumsAdd a little redundancy to a frame so the receiver can tell when bits have been damaged.12 min
  2. Cyclic redundancy checkDivide the frame by a generator polynomial, append the remainder, and see which errors slip through.14 min
  3. Hamming codes: fixing a bitPlace check bits at the powers of two so that the pattern of failed checks points at the damaged bit.14 min

Stage 1Flow control and reliability

  1. Stop-and-wait and why it is slowSee the simplest reliable protocol, and work out how little of a long link it uses.13 min
  2. Sliding windows: go-back-N and selective repeatKeep several frames in flight, and compare resending everything after a loss with resending only the lost frame.15 min

Stage 2Sharing the wire

  1. Ethernet and CSMA/CDSee the Ethernet frame, and how stations sharing one wire listen, collide, back off and try again.14 min
  2. ARP and how switches learnFind the MAC address that goes with an IP address, and see how a switch builds its table from the frames it sees.13 min

All of The data link layer is free

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