The Data Link and Physical layers Explained

The data-link layer provides physical addressing, converts data frames into signals, and loads them on the connected media. The physical layer moves signals between devices in the network.

The data-link layer

The data-link layer is the second layer in the OSI and TCP/IP models. It provides physical addressing for network components. A physical address is called Media Access Control (MAC) address. It takes data packets from the network layer and converts them into frames. A frame includes the fields and components the data link layer uses to communicate with devices on the same LAN segment.

The data link layer is also responsible for reading signals from the physical layer and reassembling them into the original data link layer frame. It also performs error detection and discards bad frames. It typically does not perform error correction.

Institute of Electrical and Electronic Engineers (IEEE) 802.2, 802.3, and 802.5; ANSI’s FDDI; Ethernet II; Asynchronous Transfer Mode (ATM), Frame Relay, High-Level Data Link Control (HDLC), Point-to-Point Protocol (PPP), Synchronous Data Link Control (SDLC), Serial Line Internet Protocol (SLIP), and X.25 are examples of data link layer protocols and standards.

Network interface controllers or cards (NICs), bridges, and switches are the primary networking components that function at the data link layer.

Basic functions of the data-link layer

On the sending device, it performs the following tasks.

  • It takes data packets from the network layer and converts them into frames.
  • It attaches a header and trailer to each frame.
  • In the header, it puts the source and destination MAC address.
  • In the trailer, it put a CRC value.
  • Based on the connected media type, it converts the frame into signals and loads them on the media.

On the receiving device, it performs the following tasks.

  • It picks signals from the connected media and converts them into frames.
  • It uses the CRC value of each frame to confirm the frame is good.
  • It deletes all bad frames.
  • For all good frames, it checks their destination MAC address.
  • If the destination MAC address belongs to it, it removes the header and trailer from the frame and gives it to the network layer.

The physical layer

The physical layer is the first layer. It moves data signals in the network. It is responsible for converting binary signals into electric or radio signals and vice versa. For example, the device is connected to a copper cable, it defines how binary 1s and 0s are converted into an electrical signal by using different voltage levels. Or if the device is connected to a fiber cable, the physical layer defines how 1s and 0s are represented using an LED or laser with different light frequencies.

Basic functions of the physical layer

  • It defines standards for interfaces used on networking devices.
  • It defines standards for network cable types and connectors used on each end of the cable
  • It defines pin patterns for connections
  • It converts physical layer signals ( electric and radio) into binary and binary to physical layer signals.

ComputerNetworkingNotes CCNA Study Guide The Data Link and Physical layers Explained