Data Link and Physical Layers

Data Link Layer
  • Reliable data transfer across physical link
  • Responsible of physical addressing
  • Organize the data into frames
  • Puts frames on the physical medium
  • Check frames for errors
  • Hop-to-hop addressing
  Physical Address
  • Unique identifier
  • Not bound to geographical location
  • Used for hop-to-hop addressing
  • Burned on the NIC
  • Cannot be changed
  • Vendor oriented
  • Common example is the MAC address
  • Sometimes called hardware address
  Data Link Layer
  • Received data packages from network layer
  • Convert data into the frames including the following:
    • Data packages
    • Sender and the receiver physical addresses
    • Error checking and control information
  • Checking and control ensures the frame has been delivered without troubles
  • If part of the frame is lost during transmission then:
    • Data link layer protocols of the receiver would see that
    • Data link layer at the receiver requests data retransmit from the sender
  • Switches and bridges devices work in data link layer
  Frame Check Sequence (FCS)
  • Error checking mechanism is creating the unique frame check sequence (FCS)
  • FCS number based on the values of all of the preceding fields in the frame
  • The receiver’s end receives data with the FCS value
  • Receiver compares FCS to the data fields in frame
  • In unlucky scenario of failed comparison the following is done:
    • Data is assumed corrupted
    • Receiver requests data resend
  • If the comparison is a success, receiver sends the message of successful data transmission back to the sender
  • If receiver did not receive the “success” message during specified time, automatically the sender resends the data frames
  Data Link Layer Sublayers
  • Another function of Data Link layer protocols is to control the flow when manny different nodes are sending data at the same time
  • The Data Link layer has been divided into two sublayers be the IEEE as follows:
    • The top sublayer is called Logical Link Control (LLC) communicating only with Network layer and providing the following functions:
      • Hop-to-hop reliability
      • Hop-to-hop flow control
    • The bottom sublayer is Media Access Control (MAC) responsible for the following:
      • Adding physical address to the frame
      • Communication with the Physical layer
  Physical Layer
  • Transmits bit stream over the physical link
  • Sends data signals to media and receives it
  • Adapts to the transmission media
    • Cables
      • Copper (coaxial, twisted pair)
      • Fiber optic (single mode, multi mode)
    • Wireless
  • Responsible for the representation of 1s and 0s
  • NIC, Hub, and Repeater work in Physical Layer
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