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Duties of IEEE 802.2 Logical Link Control (LLC)

In IEEE's version of Ethernet, the top sublayer of the Data link layer is Logical Link Control(LLC).

LLC' performs its functions in the software part of the Data Link Layer. LLC defines how to multiplex multiple network layer protocols in the data link layer frames. So, any logical operation in the Data Link Layer is happening in LLC.


Logical Link Control sublayer performs its multiplexing using Service Access Point (SAP) identifiers.


Let us check the IEEE definition of Logical Link Control(LLC). Logical Link Control sublayer comes under IEEE 802.2. There are two types of IEEE 802.2 frames. They are:


  1. Service Access Point(SAP)

  2. Subnetwork Access Protocol(SNAP)

Logic Link Control (LLC) has a few duties in the OSI reference model. Let us check them.


Duties of Logical Link Control(LLC)

  1. Multiplex multiple network layer protocols into frames.

  2. The second duty is Sequencing and flow control.


IEEE 802.2 uses the SAP(Service Access Point) or SNAP(Subnetwork Access Protocol) field to differentiate between encapsulated layer three payloads.


LLC is the top part of the Data Link Layer. So, it interacts with the Network layer.



IEEE 802.2 (LLC) concerns Ethernet, Token Ring, and FDDI. To read more about the MAC sublayer, visit the link below.


Though LLC can do the sequencing and flow control, the Transport layer does it better. Let me explain how different the LLC and Transport Layer are in sequencing and flow control. LLC is limited to the same LAN. It does not work outside the LAN. To read more about different layers in the OSI Reference Model, click the link below.

The Major Duties of Data Link Layer

The second layer in the OSI Reference model is the Data Link Layer. It lies between the Physical and Network layers. The data link layer is responsible for assigning physical addresses for the devices connected to the network.


The physical address is also known as the hardware address because it is a built-in address.


The difference between a logical address and a hardware address is critical to know. Logical addresses (The best example is an IP address) are not permanent. But, a hardware address (MAC address) is permanent.

The primary function of the Data Link Layer is to deliver frames between the source and destination. However, you must understand that the scope of the Data Link Layer is in the same LAN only.


So, the frame delivery happens between devices connected to the same Local Area Network.


Let us check how the Data Link Layer creates a frame. The data link layer takes bits from the Physical layer and converts them to frames. All communications done in the Data link layer are in frames.




Subdivision in Data-link Layer

The Data link layer has two divisions in the IEEE version (of Ethernet). They are:


1. Logical Link Control (LLC)
2. Media Access Control (MAC)

Let us check the devices that work in the Data Link Layer.

Data-link Layer devices

  1. Switch
  2. NIC
  3. Bridge

Responsibilities of Data Link Layer

  1. Define Physical Addresses.

  2. Communication in the same network in the form of Frames.

  3. Data Link Layer defines the hardware topology of the network.


Major Data Link Layer Protocols


  1. CIF : Cells in Frames Ethernet
  2. FDDI : Fiber Distributed Data Interface
  3. GARP : Generic Attribute Registration Protocol
  4. GMRP : Multicast Registration Protocol.
  5. GVRP :GARP VLAN Registration Protocol
  6. LLC :Logical Link Control.
  7. SNAP :SubNetwork Access Protocol

The Seven Layer Structure

The seven layer structure is:

  1. Physical layer

  2. Data link layer

  3. Network layer

  4. Transport layer

  5. Session layer

  6. Presentation layer

  7. Application layer

Importance of The Physical Layer

The physical layer is the first layer in the OSI model that deals with the physical transmission medium. The data transferred via the physical layer is in the form of electrical signals.

In real life, we can treat the physical layer as the road where the vehicles are passing. There is a reason why I chose the road to explain the Physical Layer. The road is the one which is carrying the vehicles and linking the cities.


The physical layer is the lowest layer in the OSI Reference Model.


Like that example, the physical layer transmits the data signal in electrical signals and connecting nodes.

We can consider the road as the physical layer and vehicles as the electric signals. Just replace the city with a node. I guess you understand the concept.



The major cables using in physical layer

The physical layer is all about the connections. So, which cables do we use in this layer to form a network?


  1. Coaxial cable

  2. Twisted pair cable

  3. Optical fiber cable



Devices Working in the Physical Layer

Sometimes, interviewers may ask about the devices working in the physical layer. They are:

  1. Internet gateway

  2. Hub

  3. Repeater



The Functions and Services Performed by the Physical Layer are:

1. Bit-by-bit delivery

2. Modulation

3. Encoding and Signaling:

4. Bit synchronization

5. Physical Network Topology

In the Physical Layer, data flows in the form of an electric pulse, and the cables are responsible for the safe delivery of the signals. The physical layer is responsible for varying the waveform for the data transfer. This process is known as modulation.



The physical layer is also responsible for various types of encoding and decoding of signals.

The physical layer cares about the transmission mode. It is also the physical layout of the network( topology).




The seven layer structure of the OSI Reference Model is:
  1. Physical layer

  2. Data link layer

  3. Network layer

  4. Transport layer

  5. Session layer

  6. Presentation layer

  7. Application layer

Benefits of the OSI Reference Model

The International Organization for Standardization (ISO) developed a seven-layer model called the Open Systems Interconnection (OSI) Reference Model to help vendors and network experts learn how data transmits between two devices in a network.


ISO developed the OSI Reference model.


OSI Reference Model helps networking device manufacturers to implement new network standards and policies.

It also helps network administrators reduce the complexity of networks and makes it easier to troubleshoot networking issues.



What is The Importance of the OSI Reference Model?

The introduction of the OSI reference model was a revolution. Before it, device manufacturers did not have a common standard. So, there was no hope of interoperability between devices from different companies.


OSI Model Brings a common standard.


Now you can see how critical was the introduction of the OSI reference model.



Seven Layers of OSI Reference Model

There are seven layers in the OSI Reference Model. Each layer has a specific role in data communication. A network engineer must learn the importance and functions of each layer in detail.

Seven layers of the OSI reference model are listed below.


  1. Physical layer

  2. Data link layer

  3. Network layer

  4. Transport layer

  5. Session layer

  6. Presentation layer

  7. Application layer


What is The Need For the OSI Reference Model?

I explained the need for a common standard before. Let me explain it in detail. There was no common standard before the Open System Interconnection (OSI) Reference Model.

It forced the user to stick with a single vendor for all his networking needs because devices from different vendors were incompatible. The International Organization for Standardization (ISO) introduced the seven-layer OSI architecture to overcome it.



OSI Reference Model tells how to connect and configure a network. It helps vendors to make products with universal standards.



Benefits of the OSI Reference Model

The benefits of the OSI Reference Model in a computer network are listed below.


  1. OSI Reference Model promotes interoperability by defining the process of connecting two layers.

  2. By dividing a network into layers, we split complex network activities into simple components.

  3. Helps vendors to follow a modular design.