OSI Reference Model

OSI Reference Model

This model is based on a proposal by ISO revised in 1995. The model is called the ISO-OSI(open System Interconnection) as it deals with connecting open systems – that is systems that are open for communication with others.

The principles that were applied to arrive at the seven layers can be briefly summarised as:

  1. A layers should be created where a different abstraction is needed
  2. Each layer should perform a well-defined function
  3. The function of each layer should be chosen with an eye toward defining internationally standardised protocols
  4. The layer boundaries should be chosen to minimize information flows across the interfaces
  5. The number of layers should be large enough that distinct function need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy

OSI model is a reference model used for communication from which the other protocols are derived.

Layers of the OSI Model:

The seven Layers can be easily remembered by the pharase “All People Seem To Need Data Processing”. The seven layers are :

Figure 3.1 – OSI Layers

Purpose of each layer:

Application Layer : Initiates  request or accepts a request

Presentation Layer : Adds formatting, display or encryption information

Session Layer : Adds traffic flow information to determine when the message get sent

Transport layer : Adds error handling information

Network Layer : Sequencing and address information is added to packet

Data Link Layer: Adds error checking information and prepares data for physical layer

Physical Layer : Sends bit stream as signals

Description of each layers.

  1. Physical Layer: Concerned with transmitting with raw bits.

Design issue is that :

  • when one side send a ‘1’ it is received on other side as a ‘1’ not as ‘0’ bit.
  • Bits are converted to signals and for how long a bit last,
  • Whether transmission may proceed in both direction simultaneously
  • How the initial connection is established
  • How to end a connection when both sides are done
  • How many pins the network connector has and purpose of each pin
  • Physical and electrical and timing characteristics of the connector like like voltage, current, frequency, type of cable, layout of pins and more.
  • Defines the functions and services to be performed by physical devices :
    • Modulation – conversion of signals to bits and vice versa.
    • Multiplexing – establishment and termination of connection.
    • Synchronization of bits.
    • Line coding etc.
  • Defines type of line configuration to be use :
    • Point-to-point – dedicated link
    • Point-to-multipoint – shared link
  • Defines type of transmission mode to be use:
    • Simplex – one way communication
    • Half-duplex – both way, but one at a time communication
    • Full duplex – both way, at the same time communication
  • Physical topology : Ring, Bus, Star, Mesh etc.
  • Protocols used: Ethernet, ISDN, and other IEEE 802.
  • Network devices used: Network card, Modem, Repeater, Oscilloscope, Amplifier.
  1. The Data Link layer: the main task is to transform a raw transmission facility into a line that appears free of undetected transmission errors.

Design Issues:

  • Encapsulation of the packet or message received from the network layer to form frames How to keep fast sender synchronised to slow receiver
  • Traffic regulation through which tells the transmitter when the receiver can accept more data
  • Flow control – Imposes a flow control to prevent overwhelming of the receiver.
  • Error control – Adds a method to detect and retransmit damaged or lost frames and also method to prevent duplication of frames.
  • How to detect errors and possible correct
  • Physical addressing – To distribute frames in its own network of device it adds header to the frame which contains MAC address of sender and receiver.
    And to forward the frame to other network of device then the receiver address is the address of the device that connects to adjacent network.
  • Access control – Uses multiple access protocol to specify, who has control over the link at any given time. Detect when the shared channel is free so that a sender can send data on the channel (Known as medium access mechanism (a sub layer of DLL)

Protocols used: Ethernet, CSMA/CD, Token bus, Token ring.

Network devices used: Switch, ISDN router, Hub, NIC.

Network Layer

Purpose: to control the operations of the subnet.

Design Issues:

  • Logical Addressing – Adds a header to the packet received from the transport layer, which consists of logical (network) address to distinguish sender and receiver address over other network, as physical addresses are limited to Local Area Network (LAN).
  • Routing – How the packets are routed from source to destination-they can be static (based on static tables), they can also be determined automatically to avoid failed components or dynamic (determined anew for each packet.
  • Handling congestion control – If there are too many packets in the subnet at the sametime, they will get in one another’s way, forming a bottleneck
  • Quality of service – like delay, transmit time, jitter
  • Problem of heterogeneity – Since protocol on networks may differ, problems like addressing, size of packets etc are to be resolved at this layer.

Network Layer is the third lowest layer in the OSI reference model, which is responsible for packet delivery from source to destination through intermediate routers across multiple networks.

  • Controlling packet switching and data congestion.

Protocols used: IP, ARP, RARP, ICMP, IGMP, IPX, DDP etc.

Network devices used: router, gateway, frame relay device.


Transport Layer:

Purpose: Accept data from above layers and if necessary split it into smaller units. It also ensures that they arrive correctly at the other end.

Fragmentation and reassembling – Splits the message received from session layer into segments

It determines what services are to be provided to session layer, and ultimately to the user of the network.

Design Issues:

Error-free point-to-point channel that delivers messages or bytes in the order in which they were sent. Other service exist that does not guarantee about the order of the delivery, and broadcasting of message to multiple destinations

  • Connection Control – Transport layer can be connectionless or connection-oriented. A connectionless is unreliable with less overhead where each packet is treated as independent packet.
    Connection-oriented is reliable with high overhead, where connection should be established before transmitting a packet.
  • Flow and Error Control – Performs at the level of source and destination i.e. end-to-end, instead of across a single link like data link layer.

Protocols used: TCP, UDP, ARP, RARP, ATP ,NetBIOS/NetBEUI etc.

Network devices used: Gateway, Router.


The Session Layers:

It allows user on different machines to establish a session. Session offer various services, including dialog control(Keeping track of whose turn it is to transmit), token management (preventing two parties same critical operation simultaneously)and synchronization (providing a resume operation from where they left off in the event of crash)

Following are the functions and responsibilities of session layer :

  • Allows communication between local and remote application process in either half duplex or full duplex transmission mode.
  • It sets up, manages and terminates the connection between two application processes.
  • Synchronizes the communication by adding checkpoints into a stream of data.
  • Informs errors of upper layer to the user.

Protocols used: NetBIOS, RPC.

Devices used: Gateway.

The presentation layer


It is concerned with syntax and semantics of information transmitted.

Following are functions performed by the presentation layer :

  • Translation – As data types, formats and encoding methods are different for various operating systems, presentation layer translates it into common dependent format.
  • Compression – Reduces the size of data to be transferred and increases the throughput of the data.
  • Encryption and Decryption – Secret or confidential or important data is first encrypted using algorithm (protocol) and then transferred over the network. Decryption is done at the receiving end to get back the original form of data.

Protocol used: LPP (Lightweight Presentation Protocol)

The Application Layers

Following are the services provided by the application layer :

  • Provides interfaces to user processes.
  • Identifies the communicating users and their authentication.
  • Provides services like browsing, e-mail, file transfer, chatting, directory etc.


Contain numbers of protocols that are commonly used by users

  • DHCP – Dynamic Host Configuration Protocol – can automatically assign Internet addresses to computers and users
  • FTP – File Transfer Protocol – a protocol that is used to transfer and manipulate files on the Internet
  • HTTP – HyperText Transfer Protocol – An Internet-based protocol for sending and receiving webpages
  • IMAP – Internet Message Access Protocol – A protocol for e-mail messages on the Internet
  • IRC – Internet Relay Chat – a protocol used for Internet chat and other communications
  • POP3 – Post Office protocol Version 3 – a protocol used by e-mail clients to retrieve messages from remote servers
  • SMTP – Simple Mail Transfer Protocol – A protocol for e-mail messages on the Internet


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