Data Link Layer of the OSI Model

By Dan DiNicolo, August 25th, 2005 Posted in CCNA Study Guide Chapter 01. Subscribe to our RSS Feed

The Data Link Layer of the OSI model acts as an interface between the Network and Physical layers. The main responsibilities of the Data Link layer include:

• Data framing and physical addressing. When data is passed to the Data Link layer, it is framed for transmission using various LAN and WAN protocols. This allows network protocols to be transmitted over different network technologies including Ethernet, Token Ring, and Frame Relay as examples. Hardware or Media Access Control (MAC) addressing is used to uniquely identify hosts at the Data Link layer. Since they make forwarding decisions based on MAC addresses, bridges and switches are examples of equipment found at this layer.
• Flow control, error checking, and frame sequencing. Data Link layer devices are capable of transmitting flow control codes that identify whether upper layer protocols are capable of receiving data at the current rate. Error checking is provided in the form of a Cyclic Redundancy Check (CRC), a simple mathematical calculation performed on each frame to ensure it hasn’t been corrupted in transit. Frame sequencing reorders frames that were received in a different order than they were sent.

Interacting with Network layer protocols. When a host receives a frame, the frame header contains information on which Network layer protocol the data will be passed to. The Data Link layer helps to make network technologies independent of the upper layer protocols in use.

Examples of Data Link layer protocols:

• Ethernet (802.3): Contention-based LAN technology
• Token Ring (802.5): Token-passing LAN technology
• Wireless LAN (802.11): Wireless LANs
• Frame Relay: Packet-switched WAN technology
• ISDN: Digital dial-up connections

Tip: Remember that the protocol data unit (PDU) of the Data Link layer is referred to as a frame.

The Data Link layer is actually comprised of two sub-layers (defined by the Institute of Electronics and Electrical Engineers – the IEEE), called Media Access Control (MAC) and Logical Link Control (LLC).

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