FREE online courses on Internet Network of Networks - What is a Protocol - OSI Seven-Layer Model

International Standards organization (ISO), developed  Open Systems Interconnection (OSI) networking suite. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model), and a set of protocols.

 

+----------------+

|                      Application                    |

|                     Presentation                  |

|                        Session                      |

+----------------+

+----------------+

 |                      Transport                      |

+----------------+

 

+----------------+

 |                    Data Link                         |

+----------------+

 

+----------------+

 |                        Physical                      |

+----------------+

Figure : The Sever Layers of OSI Model

The seven layers of the OSI Basic Reference Model are:

1.  The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. This layer defines the size of ethernet coaxial cable, the type of BNC connector used, and the termination method.

2.  The Data Link Layer describes the logical organization of the data bits transmitted on a particular medium. This layer defines the framing, addressing and checksumming of  the ethernet packets.

3.  The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. This layer defines the addressing and routing structure of the Internet.

4.  The Transport Layer describes the quality and nature of the data delivery. This layer defines if and how re-transmissions will be used to ensure data delivery.

5.  The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. This layer describes how request and reply     packets are paired in a remote procedure call.

6.  The Presentation Layer describes the syntax of data being transferred. This layer describes how floating point numbers can be exchanged between hosts with different math formats.

7.  The Application Layer describes how real work actually gets done. This layer would implement file system operations.

These layers are also termed as stack. Each layer can talk to the layer directly above it and directly below it but not with any other layer. Hence Transport layer can talk to the session layer and the Network layer but not with any other layer. Two computers exchanging information between themselves always implement these layers and a physical layer of one host would talk to other hosts  physical layer. Say if data is coming from the Transport layer ( since a host may choose not to implement all the layers) of one host it would then be passed to the Network layer and then to Data link layer and finally the Physical layer would transmit the packets to the physical layer of the other computer where it would check if needs to pass the data to any other layer or not. If yes (as in this case) it would pass the data to Data Link layer which would pass it to the Network layer and finally the Network  layer would pass it over to the Transport layer.

TCP - IP

Transmission Control Protocol (TCP) and Internet Protocol (IP) is the protocol suite on which Internet depends heavily. In fact TCP/IP contains a set of protocols such as telnet, FTP or file transfer protocol etc.

IP

This is called Internet working Protocol or simply IP. This protocol provides functions to transmit data (Internet datagram) from source to destination on a network of interconnected computers.

TCP

This is called as Transmission Control Protocol or simply TCP. This is a connection oriented protocol which simply means that a connection similar to a telephone is established where you require, that the two phones should be on the network and when one person calls other she the receives the call and to and fro communication takes place without any time lapse. Whereas a connectionless protocol resembles to a letter delivered by post. Even if the addressed person is not available at the given address, the letter would be delivered, which can then be checked by the person to whom it was addressed. IP falls in the category of connectionless protocol.

UDP

User Datagram Protocol or UDP (often referred to as Unreliable Datagram Protocol). This is called the User Datagram Protocol precisely because it takes the IP layer, which provides a datagram protocol, and provides a programming interface that makes it suitable for use in general programming. UDP does not implement any kind of acknowledgement mechanism.

PPP and SLIP

Point to Point Protocol (PPP) and  Serial Line Internet working Protocol (SLIP) are used by routers to communicate over telephone lines. These protocols allow a user to establish connection to a remote computer using a telephone line.

SNMP

SNMP or Simple Network Management Protocol allows users to monitor and control network and devices over a TCP/IP network.

SMTP

SMTP or Simple Mail Transfer Protocol defines how a computer connected to a network  should send electronic mail.

POP3

Post Office Protocol version 3 or POP3 defines how a networked computer can act as a post office and queue the mails until a user who has an account with the post office, retrieves her mail.

FTP

File Transfer Protocol or FTP defines how networked computers should transfer files between themselves. Both ASCII and binary transfer modes can be used.