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Data link layer

(also layer 2, data link, DLL, protocol layer)

Data link layer definition

The data link layer is the second layer in the Open System Interconnection (OSI) model. The data link layer is responsible for transferring data between adjacent network nodes in wide area networks (WAN) or all nodes in local area networks (LAN.)

Real data link layer functions

  • Framing: The data link layer organizes data into frames for transmission to the physical layer and the network layer. Packets received from the network layer are divided into smaller frames and sent bit by bit to the physical layer. Signals from the physical layer are assembled into frames and sent to the network layer.
  • Addressing: The data link layer adds the source and destination MAC/physical addresses to each frame’s header for node-to-node delivery.
  • Error control: Error detection bits are added to frame headers in the data link layer. Based on these bits, receiving nodes can perform error checks on data frames.
  • Flow control: The data link layer must ensure that the sender’s sending speed does not exceed the receiver’s processing speed. Doing so prevents an overflow in the receiver’s buffer and subsequent data loss.
  • Access control: The data link layer controls which devices have access to which communication channels to avoid frame collision and data loss.

Examples of data link layer protocols

  • ARCnet
  • ATM
  • CDP (Cisco Discovery Protocol)
  • CAN (Controller Area Network)
  • Econet
  • Ethernet
  • FDDI (Fiber Distributed Data Interface)
  • Frame Relay
  • HDLC (High-Level Data Link Control)
  • IEEE 802.2
  • IEEE 802.11
  • LLDP (Link Layer Discovery Protocol)
  • LocalTalk
  • MPLS (Multiprotocol Label Switching)
  • NDP (Nortel Discovery Protocol)
  • PPP (Point-to-Point Protocol)
  • Profibus
  • SpaceWire
  • SPB (Shortest Path Bridging)
  • StarLan
  • Token Ring
  • UDLD (Unidirectional Link Detection)