OSI model was designed by the International Organization for Standardization to provide a framework on which to build a suite of open system protocols. The vision was that this set of protocol would be used to develop an international network that would not be dependent on proprietary systems. It is used for data network design, operation specification, and troubleshooting. Open Systems Interconnection (OSI) is a standard reference model for communication between two end users in a network. The model is used in developing products and understanding networks. It has seven layers which are as follows:-

7. Application Layer

6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer


OSI ModelOSI Model

  7. Application layer:- It is the top most layer of OSI model. It provides the interface to user processes for communication and data transfer in a network. It provides standardized services such as virtual terminal, file and job transfer operations.

Protocols:– It use many protocol few of which are as follows:-

  • Domain name service protocol (DNS):- DNS is used to resolve internet names to IP addresses.
  • Hypertext transfer protocol (HTTP):- HTTP is used to transfer files that make up the web pages of the www.
  • Simple mail transfer protocol (SMTP):- SMTP is used for the transfer of email messages and attachment.
  • File transfer protocol (FTP):- FTP is used for interactive file transfer between systems.

6. Presentation Layer:- This layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). It is sometimes called the syntax layer. Key functions of the presentation layer are as follows:

  1. Masks the differences of data formats between dissimilar systems.
  2. Specifies architecture-independent data transfer format.
  3.  Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data

­ 5. Session Layer:- This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination. Key functions of the session layer are as follows:

  1. Controls establishment and termination of logic links between users.
  2.  Manages user sessions and dialogues.
  3. Reports upper layer errors

 4. Transport Layer:- This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer. Key functions of the transport layer are as follows:

  1. Manages end-to-end message delivery in network.
  2. Provides reliable and sequential packet delivery through error recovery and flow control mechanisms.
  3. Provides connectionless oriented packet delivery.

3. Network Layer:- This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding. Its key functions are-

  1. Determines how data are transferred between network devices
  2. Routes packets according to unique network device addresses.
  3. Provides flow and congestion control to prevent network resource depletion.

2. Data link layer: – This layer provides synchronization for the physical level and does bit-stuffing for strings of 1’s in excess of 5. It furnishes transmission protocol knowledge and management. Key functions of Data Link Layer are-

  1. Defines procedures for operating the communication links.
  2. Frames packets.
  3. Detects and corrects packets transmit errors.

1. Physical Layer:- This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier. Key functions of Physical layer are:

  1. Defines physical means of sending data over network devices.
  2. Interfaces between network medium and devices
  3. Defines optical, electrical and mechanical characteristics


Transmission Control Protocol / Internet Protocol created by DOD (Department of Defense) to insure and preserve the data integrity as well maintain the communication. TCP/IP also has a network model. TCP/IP was on the path of development when the OSI standard was published and there was interaction between the designers of OSI and TCP/IP standards. The TCP/IP model is not same as OSI model. OSI is a seven-layered standard, but TCP/IP is a four layered standard. The OSI model has been very influential in the growth and development of TCP/IP standard, and that is why much OSI terminology is applied to TCP/IP.

TCP/IP is basic communication protocol in private network. It is two layer program and the layers are classified as: 

  1. Higher layer – TCP
  • Lower layer – IP

TCP– It manages the assembling of message or file into smaller packet that transmits it over the internet and is received by ICP layer that reassembles it into original message.

IP– It handles the address part of each packet so that it gets at the right destination, this implies each gateway computer on the network checks this address to see where to forward the message.

  • TCP/IP is a client server model of communication.
  • TCP/IP communication is point to point.


4. Application Layer – is the top most layer of four layer TCP/IP model. Application layer is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with transport layer services to use the network.

It includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol)DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.

3. Transport Layer – is the third layer of the four layer TCP/IP model. The position of the Transport layer is between Application layer and Internet layer. The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data.

The main protocols included at Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

 2. Internet Layer – is the second layer of the four layer TCP/IP model. The position of Internet layer is between Network Access Layer and Transport Layer. Internet layer pack data into data packets known as IP datagrams, which contain source and destination address (logical address or IP address) information that is used to forward the datagrams between hosts and across networks. The Internet layer is also responsible for routing of IP datagrams.

Packet switching network depends upon a connectionless internetwork layer. This layer is known as internet layer, is the linchpin that holds the whole design together. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data packets may appear in a different order than they were sent. It is the job of the higher layers to rearrange them in order to deliver them to proper network applications operating at the Application layer.

The main protocols included at Internet layer are IP (Internet Protocol)ICMP (Internet Control Message Protocol)ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol)

1. Network Access Layer  is the first layer of the four layer TCP/IP model. Network Access layer defines details of how data is physically sent through the network, including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire.

The protocols included in Network Access layer are EthernetToken RingFDDI, X.25, Frame Relay etc.

The most popular LAN architecture among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media. An Access Method determines how a host will place data on the medium.

IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from network traffic. When a host wants to place data on the wire, it will check the wire to find whether another host is already using the medium. If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance, they will collide with each other, destroying the data. If the data is destroyed during transmission, the data will need to be retransmitted. After collision, each host will wait for a small interval of time and again the data will be retransmitted.

Differences between OSI and TCP/IP

1. The OSI model originally distinguishes between service, interval and protocols.2. The OSI model is a reference model.

3. In OSI model, the protocols came after the model was described.

4. In OSI model, the protocols are better hidden.

5. The OSI model has 7 layers. 

6. The OSI model supports both connection-less and connection-oriented communication in the network layer, but only connection -oriented communication in transport layer.

1. The TCP/IP model doesn’t clearly distinguish between service, interval and protocol.2. The TCP/IP model is an implementation of the OSI model.

3. In TCP/TP model, the protocols came first, and the model was really just a description of the existing protocols.

4. In TCP/IP model, the protocols are not hidden.

5. The TCP/IP model has only 4 layers.

6. The TCP/IP model supports both connection-less and connection-oriented communication in the transport layer, giving users the choice.

This article on OSI and TCP/IP was submitted by Ramanjeet Kaur of IT 6th Semester (Batch 2009) of CTIT. The resources used by her are mentioned below:

Book Referred – Cisco Certified Network Associate (Todd Lammle)
Links Visited:-

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s