CITS3002 Computer Networks  
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Responsibilities of the Physical and Data Link Layers

The Physical and Data Link layers have strongly related responsibilities.
Unlike other adjacent layers, it's not possible to simply replace an implementation of one layer with another.

Physical Layer responsibilities:

  • Provides the physical topology of the network
  • Provides the transfer medium (such as copper or optical cable)
  • [TX] Encodes data into a transmission signal supported by the medium
  • [RX] Decodes data into a transmission signal supported by the medium
  • [TX+RX] Generates or amplifies re-transmitted signals along the medium
  • [RX] Monitors (only detects) transmission errors
  • [RX] Detects arriving signal levels to synchronize speed and timing

Data Link Layer responsibilities:

  • [TX] Constructs data frames using the format of the Physical Layer
  • [TX] Calculates Cyclic Redundancy Codes (CRC) information
  • [RX] Checks for arriving errors using CRC information
  • [RX] Initiates and arbitrates the link to reduce interruption and contention
  • [RX] Examines arriving and passing device addresses in data
  • [RX+TX] Acknowledges receipt of a frame
  • [RX+TX] Retransmits data if there is an error


CITS3002 Computer Networks, Lecture 2, The Physical Layer, Errors, and the Data Link Layer, p1, 4th March 2020.