Physical Transmission of Data, Error Notifications and Network Topology offered by The Data Link layer of the OSI Reference Model. The Layer 2 ensures the delivery of messages to a particular device on the network using MAC or Hardware Addresses and also decodes that data from the layer 3 for the Physical layer into bits to forward. The Data Link layer of the OSI Reference Model splits the messages which are called Data Frame and inserts a modified header that includes the MAC address of source and destination. The customized header shapes like a capsule which encloses the unique data in the similar way that navigational devices, engines and other devices were attached to the lunar modules of the Apollo project. This variety of equipments was helpful only during some definite phases of space flight and were stripped off the module and canceled when action was complete. Data transmission in networks is similar as the above example.
In the above picture the Data Link layer with the Ethernet and IEEE specifications. IEEE 802.2 employed in combination with and added structure to the other IEEE standards.
Routers or Layer 3 Switches work at the Network layer, only concern about the network and select the best path for this purpose but never care about the particular host. So the Data Link layer is responsible for the real unique identification of each particular host that exists in a LAN. The Data Link Layer employs Hardware Addresses to transmit packets to each host on the LAN, as transmit between routers. Between routers, every transmitting packet always covered with a frame which includes the control information at this layer but that data is removed at the destined router and the actual packet is left. Packet framing remain continues for every leap at least the packet is lastly sent to the destination host. Packet never modified itself along the route; it is only enclosed with the type of control information that’s required for it for transmit on to the multiple types of media. Switches and bridges work at the Data Link layer and sift the LAN by utilizing MAC Addresses.
There are two sub-layers of the Data Link Layer defined by The IEEE Ethernet:
Media Access Control (MAC) IEEE 802.3 Standard, Describes how packets are positioned on the media. Argument media access is “first come, first served” access where everyone shares the same bandwidth thus the name. MAC or Physical addressing, line discipline, error warning, frame delivery, flow control and logical topologies are defined here. Logical topology is the hint way from a physical topology.
Logical Link Control (LLC) IEEE 802.2 Standard, Works on the identification of Network layer protocols and encapsulate them. LLC header provides the information to the Data Link layer what to do with packet when frame is accepted. In this sub-layer, host receives a frame then read its LLC header to find destination of the packet. Sequencing of control bits offered by LLC, while flow control also offered by LLC.
This article is about The Data Link Layer of the OSI Reference Model according to the CCNA 640-802 Cisco Certified Network Associate course 2011.