LLC (Logical Link Control) IEEE 802.2

In IEEE's version of Ethernet, the top sublayer of Data link layer is Logical Link Control(LLC). LLC's function is performed in software. LLC defines how to multiplex multiple network layer protocols in the data link layer frames. Logical Link Control sublayer performs its multiplexing by using Service Access Point (SAP) identifiers. Logical Link Control sublayer is defined 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)

Major Duties of Logical Link Control(LLC)
  1. Multiplex multiple network layer protocols in to frames

  2. Sequencing and flow control

IEEE 802.2 uses SAP(Service Access Point) or SNAP(Subnetwork Access Protocol) field to differentiate between encapsulated layer 3 payloads. IEEE 802.2 (LLC) concerns Ethernet, Token Ring and FDDI. To read more about MAC sublayer, visit the link below.
MAC Sublayer IEEE 802.3

Though sequencing and flow control can be done in Logical Link Control Layer, normally this process is done at Transport layer. To read more about different layers in OSI Reference Model, click the link below.
OSI Reference Model

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Data Link Layer Provide Physical Addresses

The second layer in OSI Reference model is Data Link Layer. Data link layer is responsible for providing physical addresses for the devices connected to the network. Physical address is also called hardware address because it is built in address (not a logical address like IP address). Data link layer is responsible for taking bits from Physical layer and convert them to frames. All communications done in Data link layer is in the form of frames.


Sub division in Data-link Layer

In IEEE version of Ethernet, Data link layer is subdivided in to:

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

Data-link Layer devices

Networking devices functioning at Data link layer are:

  1. Switch
  2. NIC
  3. Bridge

Responsibilities of Data Link Layer
  1. Define Physical Addresses
  2. Communication in the same network in form of Frames
  3. Define 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

Seven Layer Structure

The seven layer structure of OSI Reference Model is listed below.
  1. Physical layer

  2. Data link layer

  3. Network layer

  4. Transport layer

  5. Session layer

  6. Presentation layer

  7. Application layer

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Physical layer

Physical layer is the first layer in OSI model which deals with the physical transmission medium. The data transferred via physical layer is in the form of electrical signals. In a real life we can treat the physical layer as road where the vehicles are passing.Because the road is the one which is carrying the vehicles and linking the cities. Like that the physical layer is transmitting the data signal in the form of electrical signal and connecting nodes.

The major cables using in physical layer are:



1. Coaxial cable

2. Twisted pair cable

3. Optical fiber cable


Devices working in physical layer:



1. Internet gateway

2. Hub

3. Repeater



The major 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 Physical Layer data is transmitting in the form of electric pulse and the cables are responsible for the safe delivery of the signals. Physical Layer is also responsible for varying the wave form for the ease of data transfer. This process is know as modulation. Physical layer is also responsible for various types of encoding and decoding of signals.Physical layer also caring the transmission mode and physical lay out of the network( topology)



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OSI Reference Model

The International Organization for Standardization (ISO) developed a seven layer model called Open Systems Interconnection (OSI)Reference Model to help vendors and network experts to learn how data is transferred between networking devices. OSI Reference Model helps networking device manufactures to implement new network standards and policies. OSI Reference Model helps network administrators to reduce the complexity of networks and makes it easy to troubleshoot networking issues.


Seven Layers of OSI Reference Model

There are seven layers in OSI Reference Model and each layer has specific role in data communication. Seven layers of 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 of OSI Reference Model

Prior to the introduction of Open System Interconnection (OSI) Reference Model, each vendors have their own manufacturing standards. It forced the user to stuck with single vendor for all his/her networking needs because devices from different vendors were not compatible. To overcome this issue International Organization for Standardization (ISO) introduced the seven layer OSI architecture. OSI Reference Model tells how to connect two layers in networks and helped vendors to make products in universal standards.

Benefits of OSI Reference Model

The major benefits of OSI Reference Model in a computer network are listed below.
  1. OSI Reference Model promotes interoperability between vendors by defining the process of connecting two layers.

  2. By dividing a network in to layers, we split complex network activities in to a set of simple components.

  3. Helps vendors to follow modular design

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