OSI Reference Model

OSI Reference Model


  1. Representation of Bits: Data in this layer consists of stream of bits. The bits must be encoded into signals for transmission. It defines the type of encoding i.e. how 0’s and 1’s are changed to signal.
  2. Data Rate: This layer defines the rate of transmission which is the number of bits per second.
  3. Synchronization: It deals with the synchronization of the transmitter and receiver. The sender and receiver are synchronized at bit level.
  4. Interface: The physical layer defines the transmission interface between devices and transmission medium.
  5. Line Configuration: This layer connects devices with the medium: Point to Point configuration and Multipoint configuration.
  6. Topologies: Devices must be connected using the following topologies: Mesh, Star, Ring and Bus.
  7. Transmission Modes: Physical Layer defines the direction of transmission between two devices: Simplex, Half Duplex, Full Duplex.
  8. Deals with baseband and broadband transmission.


Physical Layer in ISO-OSI Model

DATA LINK Layer – OSI Model

Data link layer is most reliable node to node delivery of data. It forms frames from the packets that are received from network layer and gives it to physical layer. It also synchronizes the information which is to be transmitted over the data. Error controlling is easily done. The encoded data are then passed to physical.

Error detection bits are used by the data link layer. It also corrects the errors. Outgoing messages are assembled into frames. Then the system waits for the acknowledgements to be received after the transmission. It is reliable to send message.


  1. Framing: Frames are the streams of bits received from the network layer into manageable data units. This division of stream of bits is done by Data Link Layer.
  2. Physical Addressing: The Data Link layer adds a header to the frame in order to define physical address of the sender or receiver of the frame, if the frames are to be distributed to different systems on the network.
  3. Flow Control: A flow control mechanism to avoid a fast transmitter from running a slow receiver by buffering the extra bit is provided by flow control. This prevents traffic jam at the receiver side.
  4. Error Control: Error control is achieved by adding a trailer at the end of the frame. Duplication of frames are also prevented by using this mechanism. Data Link Layers adds mechanism to prevent duplication of frames.
  5. Access Control: Protocols of this layer determine which of the devices has control over the link at any given time, when two or more devices are connected to the same link.

Data Link Layer in ISO-OSI Model

Network Layer – OSI Model

The main aim of this layer is to deliver packets from source to destination across multiple links (networks). If two computers (system) are connected on the same link then there is no need for a network layer. It routes the signal through different channels to the other end and acts as a network controller.

It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.

Network Layer in ISO-OSI Model


  1. It translates logical network address into physical address. Concerned with circuit, message or packet switching.
  2. Routers and gateways operate in the network layer. Mechanism is provided by Network Layer for routing the packets to final destination.
  3. Connection services are provided including network layer flow control, network layer error control and packet sequence control.
  4. Breaks larger packets into small packets.

Transport Layer – OSI Model

The main aim of transport layer is to be delivered the entire message from source to destination. Transport layer ensures whole message arrives intact and in order, ensuring both error control and flow control at the source to destination level. It decides if data transmission should be on parallel path or single path

Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer and ensures that message arrives in order by checking error and flow control.


Transport Layer in ISO-OSI Model


  1. Service Point Addressing : Transport Layer header includes service point address which is port address. This layer gets the message to the correct process on the computer unlike Network Layer, which gets each packet to the correct computer.
  2. Segmentation and Reassembling : A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission.
  3. Connection Control : It includes 2 types :
    • Connectionless Transport Layer : Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.
    • Connection Oriented Transport Layer : Before delivering packets, connection is made with transport layer at the destination machine.
  4. Flow Control : In this layer, flow control is performed end to end.
  5. Error Control : Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.

Session Layer – OSI Model

Its main aim is to establish, maintain and synchronize the interaction between communicating systems. Session layer manages and synchronize the conversation between two different applications. Transfer of data from one destination to another session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.


Session Layer in ISO-OSI Model


  1. Dialog Control : This layer allows two systems to start communication with each other in half-duplex or full-duplex.
  2. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into stream of data. Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages is recommended. This ensures that 50 page unit is successfully received and acknowledged. This is beneficial at the time of crash as if a crash happens at page number 110; there is no need to retransmit 1 to100 pages.

Presentation Layer – OSI Model

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.


Presentation Layer in ISO-OSI Model


  1. Translation : Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
  2. Encryption : It carries out encryption at the transmitter and decryption at the receiver.
  3. Compression : It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.

Application Layer – OSI Model

It is the top most layer of OSI Model. Manipulation of data (information) in various ways is done in this layer which enables user or software to get access to the network. Some services provided by this layer includes: E-Mail, transferring of files, distributing the results to user, directory services, network resource etc.


Application Layer in ISO-OSI Model


  1. Mail Services : This layer provides the basis for E-mail forwarding and storage.
  2. Network Virtual Terminal : It allows a user to log on to a remote host. The application creates software emulation of a terminal at the remote host. User’s computer talks to the software terminal which in turn talks to the host and vice versa. Then the remote host believes it is communicating with one of its own terminals and allows user to log on.
  3. Directory Services : This layer provides access for global information about various services.
  4. File Transfer, Access and Management (FTAM) : It is a standard mechanism to access files and manages it. Users can access files in a remote computer and manage it. They can also retrieve files from a remote computer.

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