Video Description

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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