Functions of LLC and MAC sublayer

As I discussed earlier that Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies. For the Layer 2 protocols, as with all 802 IEEE standards, Ethernet relies on the two separate sublayers of the data link layer to operate, the Logical Link Control (LLC) and the MAC sublayers. The Figure below illustrates the LLC and MAC Sub-Layers۔


LLC sublayer

Data link layer has further divided into two sublayers. The upper sublayer is Logic Link Control (LLC). So LLC Layer communicates with the upper layers of the OSI model. The LLC get the network protocol data, which is usually an IPv4 packet. LLC also adds control information to help deliver the packet to the destination. The LLC communicate with the upper layers of the application and transition the packet to the lower layers for delivery.

LLC is implemented in software, and its implementation is independent of the hardware. The driver software of computer NIC and be considered as LLC. The driver is a software program and instruction that interacts directly with the computer NIC and other hardware. Driver software also enables communication between the network device, computer, operating system as well as with other network computers and network devices.

MAC sublayer

MAC is the lower sub-layer of the data link layer. It is listed in the IEEE 802.3 standards. The figure above lists common IEEE Ethernet standards. It also shows how the data link layer divided into the LLC and MAC sub-layers. The LLC communicates with the network layer while the MAC sub-layer allows various network access technologies. For instance, the MAC sub-layer communicates with Ethernet LAN technology to send and receive frames over copper or fibre-optic cable. The MAC sub-layer also communicates with wireless technologies such as Wi-Fi and Bluetooth to send and receive frames wirelessly.

As we know that the Media Access Control (MAC) layer is between the Logical Link Control (LLC) sublayer and physical layers, The MAC sublayer has the following key tasks:

Data encapsulation

The data encapsulation process includes frame assembly from the transmission node and disassembly on the receiving node. To form the frame, the MAC layer adds a header and trailer to the network layer PDU.

Data encapsulation provides three key functions:

  • Data Framing-The framing process provides a sequence of one or more characters to recognize a group of bits that make up a frame. These sequences bits provide synchronization between the transmitting and receiving nodes.
  • Addressing -The data link layer is the lowest layer in the OSI model concerned with addressing. Data Link Layer receive Layer 3 PDU. The encapsulation process in the data link layer provides data link layer address as well as labelling information with a particular destination location. Every device on a network has a unique number, generally called a physical address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine get to it properly.
  • Error detection –The data link each frame contains a trailer that used to detect any errors in transmissions.

Media Access Control

The Media Access Control sub-layer second responsibility is to control access to the media. This sub-layer is also responsible for the placement of frames on the media and the removal of frames from the media. The MAC sub-layer communicates directly with the physical layer.