A Basic Overview

1. Physical Layer

The Physical Layer ensures the transmission of binary data via a medium. It defines cabling, or other methods of data carriage, connectors, electrical characteristics of the communication channel and the transmitted signal. It also defines the protocol to establish and terminate a connection between two 'directly' connected nodes.

The NIC converts the binary data into electrical voltages (high and low). Devices that operate on the Physical Layer include network interface cards, hubs, switches, bridges and routers etc. However, some of these devices do work on other layers as well.

2. Data Link Layer

The Data Link Layer provides a reliable link between two directly connected nodes. It does this by detecting and possibly correcting any errors that may occur in the Physical Layer. It also ensures that all of the data from the sending computer has been received, thereby, providing flow, error control, and synchronisation of the Physical Layer.

Error detection prevents two computers from accessing the same media simultaneously i.e. collision detection. The Point-to-Point Protocol (PPP) is an example of the Data Link Layer in the TCP/IP stack.

3. Network Layer

The Network Layer manages the transfer of packets that are to be forwarded on to different networks, between nodes on the same network. Network Addressing and determining the best path are two operations carried out on this layer. Data is routed through a network and can also be routed through interconnected networks. Splitting the data up for transmission and re-assembling upon reception. The IP part of TCP/IP also operates at this level.

4. Transport Layer

The Transport Layer manages any end-to-end messages delivered on the network. It provides a reliable and sequential packet delivery through error recovery and flow control mechanisms. It can also keep track of the segments and provides the ability to resend any failed segments and acknowledge the successful delivery before sending the next packet of data.

5. Session Layer

The Session Layer manages the communication between applications after a connection is made. It sets up the session, manages information exchanges, and then breaks it down when the session ends.

The Session Inter-Host Communication sets up, maintains, and closes down a session. This operation provides an open communication path with another system.

6. Presentation Layer

The Presentation Layer formats the data for exchange between the Application and Session layers. It masks the differences of data formats between dissimilar systems. It also encodes and decodes data, encrypts and decrypts data, compresses and decompresses data.

Presentation Data Representation and Interpretation Translation may need to occur between two systems using different presentation standards e.g. different character sets or character codes. The Presentation layers allow data to be interpreted using a set of translations and can also add data encryption for security purposes.

7. Application Layer

Finally, the Application Layer provides the user interface for communication. These functions typically include; email, file transfer, file management, message handling and database query functions.

Application Network Services to Application Programs provides network services to application programs such as electronic mail and file transfer. This includes Telnet, SSH, FTP, TFTP, etc.

	Created by Liam-Beckwith almost 8 years ago

This set of notes will discuss the OSI Model and look each of the seven layers. It will also look at why the OSI Model with introduced as a way to set standards.

The OSI Model