OSI reference Model

     In 1978 ISO developed a framework that allows devices of different manufacturers to communicate with different operating systems. In 1984 this framework become an international standard called the ISO OSI model which is the Open Systems Interconnection model. OSI model divides the process of sending a message from one device to another device over a computer network into 7 ordered layers.

  1. Physical layer
  2. Data link layer
  3. Network layer
  4. Transport layer
  5. Session layer
  6. Presentation layer
  7. Application layer

These 7 layers work together for computer communication over a computer network.

     Let us use this diagram to understand the model. When device A sends a message to device B, at the sending side the message is moved from layer seven to layer one. At layer one, the entire message is converted to a form that can be transmitted to the receiving side.  On the receiving side, the message is moved from layer one to layer seven.

     On the sending side, each layer performs its assigned task and gives the message to the layer below it. At the receiving side each layer takes a message from the layer below it performs its assigned task and then gives that message to the layer above it.

1. Application Layer and its functions.

    We will understand first the functions of an application layer. Suppose user A wants to send a message to user B. Message can be sent through email or some messenger like, Gmail, Skype, or some messenger application.

i. File access and shared database access.

     To send the message user A use the services provided by an application layer. The application layer enables user A to have access to network and network applications like Google Chrome, Firefox, Internet Explorer, and Skype to log in and write a message. These network applications use application layer protocol.

ii. E-mail and web surfing.

     To send E-mail, to transfer a file, and for web surfing. For example, for web surfing HTTP or HTTPS protocol is used, To transfer a file FTP protocol is used and to send a mail SMTP protocol is used.  

     So, the functions of the application layer are to provide user interface and support for services like,  

  1. File access
  2. Shared database access
  3. Sending mail
  4. Web surfing.

     Application layer gives this message written by user to presentation layer. Note that the data unit at application layer is APDU i.e., Application Layer Protocol Data Unit.  

2. Presentation Layer and its functions.

     In the OSI model layer below application layer is a presentation layer. Presentation layer is responsible for syntax and semantics of the data or information exchanged between two devices.

i. Translation

     The data received from application layer is in the form of characters and numbers. This data must be converted to bit stream before transmission. As different computers use a different encoding system, presentation layer converts this data into a common format like ASCII or EBCDIC.

ii. Encryption and decryption of data.

     Another important function of presentation layer is to encrypt and decrypt the data.  To ensure the privacy and secure transmission of data sender’s data link layer converts the data into intermediate code or secret code that is called encryption of data. At the receiver side when data is received by presentation layer presentation layer decrypts the data that means convert the intermediate code into original format

iii. Data compression.

     Third important function of presentation layer is to reduce the size of data so that the data will be smoothly transmitted through the medium. The process is called data compression. So, the functions of presentation layer are:

  1. Translation
  2. Encryption and decryption of data.
  3. Data compression.

3. Session Layer and its functions.

     Next layer in the OSI model that is layer below presentation layer is session layer. Session Layer is a network dialogue controller.

i. Connection Management.

     It is responsible for establishing a connection, managing a connection between two users for sending and receiving data, and then releasing the connection between two users. That means the session layer is responsible for authenticating the users on the network.

iii. Synchronization of data.

     Another important function of the session layer is synchronization. The session layer allows the process to add checkpoints to the stream of bits. For example, if user A is sending a file of 500 megabytes, It is advised to insert checkpoints after every 100 megabytes of data to ensure that every 100 megabytes of data is received correctly and acknowledged independently. This is called file checkpointing.

 

     So, the session layer is responsible for

  1. connection management
  2. dialogue control and
  3. synchronization of data.

4. Transport Layer and its functions.

i. Segmentation.

     The transport layer is responsible for the reliability of communication. To control the reliability, the transport layer divides the data received from the above layers into transmittable segments and assigns sequence numbers to each segment. This is called segmentation.

ii. Service point addressing.

     Another important function of the transport layer is process-to-process delivery. Computers often run several programs at the same time transport layer transports a message from one process running on one device to another process running on another device.

For example, if user A use a Gmail application running on device A, the message must be received by a Gmail application running on device B. Process is always identified with a unique address called the port address or service point address. The transport layer adds the port address in the header of each segment to be transmitted.

iii. Flow and Error control.

      The transport layer is also responsible for flow and error control. The transport layer decides how much data the sender can send before it receives acknowledgment or confirmation of sent data from the receiver.

For example, if user A is sending data from a server to a mobile phone, the speed of the server is much higher than the speed of a mobile phone. The transport layer controls the flow of data by reducing the speed of the server so that a mobile phone can accept the data.

 So, the important functions of the transport layer are :

  1. Segmentation
  2. Service point addressing
  3. Flow and Error control.

.  

5. Network Layer and its functions.

i. Inter-networking

     A network layer is a network support layer that takes data in the form of segments and encapsulates those segments into data packets. There is an Inter-network to connect every two devices in the network. As a message travels from user A to user B, it may pass through many intermediate nodes in the sub-network or Inter-network. The network layer is responsible for delivering the data packets from source to destination across multiple links in the sub-network or  Inter-network.

ii. Logical addressing

     Logical addressing is one of the important functions of the network layer. In the network, devices are uniquely identified with a logical address called an IP address.  The network layer is responsible for adding the address of the source and the address of the destination in the data packet.

iii. Routing of data packets.

     Another important function of the network layer is the routing of data packets. As there is a large subnetwork between user A and user B, there may be many available paths to send a data package from user A to user B. In the subnetwork, there are many routers and switches. The network layer is responsible to route or switch a data packet to its final destination. This is called routing.  Routing is an act of forwarding data packets from one router to another router by choosing the best path or shortest path.

 So, the important functions of a network layer are:

  1. Inter-networking
  2. Logical Addressing
  3. Routing of data packets.

6. Data Link Layer and its functions.

i. Framing

     The layer below the network layer is a data link layer. Data Link Layer is responsible for the reliable and error-free transmission of data between two adjacent nodes in the network. The Data Link Layer takes a bit stream from the network layer and divides that bit stream into a manageable unit called a frame.  framing at the data link layer is taking data, in the form of a packet, and encapsulating that packet into a frame.  If the size of the frame becomes too large to manage, the packet may be further divided into small-size frames because small-size frames will make data transmission error control and flow control much more efficient.

ii. Physical addressing.

     The second function of the data link layer is physical addressing. There are basically two types of addressing.

 

  1. Logical addressing and
  2. physical addressing

Logical addressing is done at the network layer and physical addressing is done at the data link layer. Data Link Layer uses MAC (Media Access Control) address to send and receive data frames. MAC address is a 48-bit address. It is embedded in a network interface card by a computer manufacturer. If the frame is to be sent, the data link layer adds the address of the source and destination in the header part of each frame.

iii. Flow and error control

      Another important function of the data link layer is flow control and error control collectively this is called Data Link Control. Flow control is one of the important duties of the data link layer. Flow control is a set of procedures that tells the center how much data it can send before it receives an acknowledgment from the receiver.

For example, in the sub network sender node has a speed of 100 frames per second but receivers would receive 50 frames per second. So, in this case, the data link layer will control the flow of data by reducing the speed of the sender up to 50 frames per second.

Error control is controlling errors in data transmission. Whenever data bits flow from one point to another point in the form of signals,  bits may get corrupted or changed from one to zero or zero to one because of noise or interference.  This is called an error and the data link layer is responsible for controlling this type of error in the data transmission.  

iv. Medium Access Control

     The most important function of the data link layer is access control. Whenever two or more devices are connected to a shared link, which is called a multipoint link. There should be a protocol to control access to the medium. Data Link Layer provides a protocol by which only one node can have access to the medium to send data bits.  Important functions of the data link layer are:

  1. Framing
  2. Physical Addressing
  3. Flow and error control
  4. Medium access control.

7. Physical Layer and its functions.

     In the network support layers, the Physical layer is responsible for moving individual bits from one end to the other end. The physical layer actually deals with the physical connectivity of devices in the network.

i. Representation of bits

     The first important function of the physical layer is representation of bits.  The data at the physical layer is in the form of bit stream i.e., 1’s and 0’s. To be transmitted, these data must be converted to signals. This is called encoding. So, the physical layer is responsible for defining the type of encoding. Signals may be in the form of electrical signals if the medium is a coaxial cable or twisted pair cable. It may be an optical signal if the medium is an optical fiber cable.  The physical layer also decides the type of transmission medium between sender and receiver it may be a wireless medium or wired medium.

ii. Synchronization of bits.

     Another important function of the physical layer is the synchronization of bits. We have seen frame synchronization at the data link layer. The sender and receiver must not only be synchronized with the bit rate but there must also be a bit-level synchronization. That means the sender’s and receiver’s clocks must be synchronized.

iii. Defining line configuration

     The physical layer is also concerned with the connection of devices with the medium i.e. it defines the line configuration whether the configuration is point-to-point or point-to-multipoint. Also decides the physical network topology which defines how devices are connected to the network.

iv. Deciding physical topology and transmission modes

     Finally, we must understand physical layer also defines the direction of transmission between two devices whether the transmission mode is

  1. Simplex
  2. Half duplex or
  3. Full duplex

So, the important functions of the physical layer are:

  1. Representation of bits
  2. Synchronization of bits.
  3. Defining line configuration
  4. Deciding physical topology and transmission modes
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