02. Data Link Layer of OSI Model (Layer2)

OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it. The 7x Layer model starts from Physical till Application Layer. Data Link Layer is the second layer in the 7Layer OSI Model after Physical Layer. This is the place where most LAN, Ethernet and wireless LAN technologies are defined.

Data Link controls the Layer2 Network boundary. The data link layer provides functional and procedural methods of transferring data between two points. It works like traffic control system which works without concern to their ultimate destination. When devices attempt to use a medium simultaneously, frame collisions occur. Protocols like Data link specify how devices detect and recover from such collisions and may provide mechanisms to reduce or prevent them.

PDU at Data Link Layer is called Frame.

OSI Model Layers Waqas Karim

Sub-Layers of Data Link Layer

Data Link Layer has two further Sublayers which perform different functions. These layers are:

i. LLC (Logical Link Control) Sub-Layer

This LLC sublayer is primarily concerned with multiplexing protocols to be sent over Media Access Control (MAC) sublayer. The LLC does this by splitting up the data to be sent into smaller frames and adding descriptive information to these frames, called headers.

ii. MAC (Media Access Control) Sub-Layer

Media Access Control Layer controls how a computer on the network gains access to the data  … and permission to transmit it.

The MAC sublayer is also responsible for the actual access to the network cable, or communication medium.

 

Data Link Layer Protocols

The OSI Model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. Each layer on the OSI Model has some protocols associated with it. Some important protocols on Data Link layer are listed in below:

OSI Model Data LInk Layer Waqas Karim Pic1

 

 Network Equipment/Components at Data Link Layer

Similar to protocols, each layer has associated equipment with it. Some important Equipment that operate at Data Link Layer of OSI Model are listed in below:

OSI Model Data LInk Layer Waqas Karim Pic2

 

Summary

  • Data Link Layer is the 2nd Layer in OSI 7 Layer Model
  • It performs very important functions like L2 Switching, Logical Link Control, Media Access Control, Physical Addressing, Encapsulation, Error Control, …
  • Important Protocols at Data Link Layer include Ethernet, HDLC, ARP, PPP, Frame Relay, VLAN, STP, L2 QoS, LLDP, CDP, L2TP, NDP, VTP, MAC, …
  • Equipment operating at Data Link Layer include L2 Switches, Bridges, Access Points, Modems, NIC, …

 

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