Data Link Layer of the OSI Model

Logical Link Control

The Logical Link Control sub-layer is the upper layer of the two, and has responsibility for separating Network layer protocols from the underlying network technology. Because of this, network protocols and technologies can be changed fairly easily. The LLC is primarily concerned with providing Service Access Points (SAPs) between the MAC sub-layer and Network layer protocols. For example, the frame header might contain SAP addresses stating that the contents are destined for layer 3 protocols like IP (SAP address 06) or IPX (SAP address E0). The flow control and frame sequencing mechanisms described earlier are also the responsibility of the LLC. The LLC is defined in the IEEE 802.2 standard.

You will often see Data Link layer SAPs referred to as DSAPs and SSAPs. DSAPs (Destination Service Access Points) are the interfaces used by the LLC to interact with the Network layer. Conversely, SSAPs (Source Service Access Points) are the interfaces used by the Network layer to interact with the LLC.

Figure: LLC and Network Layer Interaction 

Media Access Control

The Media Access Control sub-layer is primarily responsible for MAC addressing and the hardware functions of the Data Link layer. This sub-layer handles the transmitting and receiving of data over the network, converting data to (and from) bits according the rules of the technology in use. On an Ethernet network, this would involve adding information such as the source and destination hardware addresses, and then following the rules of Carrier Sense Multiple Access with Collision Detection (CSMA/CD) transmission. Ethernet (including CSMA/CD) and other Data Link layer technologies will be discussed in detail in Chapter 2. The MAC sub-layer is defined by IEEE standards such as 802.3 (Ethernet), 802.5 (Token Passing), and others.

Tip: Bridges and switches exist at the Data Link layer of the OSI model.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.