What is OSI model ?

What is OSI model ?

• Open Systems Interconnect Model.
•  It is defined by ISO (International Organization for standardization).
• Idea behind to develop OSI model is to avoid propriety systems and allow interoperability between two between vendors.
• Useful tool for network troubleshooting.
• It is made up of seven layers.

1. Physical Layer 
2. Data link Layer 
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

    Here 1 2 3 4 are called Lower Layer and 5 6 7 are called Upper Layers.

OSI mnemonics 7 to 1     All     People     Seem     To     Need     Data     Processing

OSI mnemonics 1 to 7     Please     Do     Not     Throw     Sausage     Pizza     Away

1.       Physical Layer 

 

       Defines physical media properties

•  Physical data rates.
• Physical connectors.
• Cable distances.
• Electrical functions.
• Optical wavelengths.
• Wireless frequencies.

      Typical layer 1 devices are CAT 5 cables, Fiber Optics, Repeaters, and Hubs.

2.       Data Link Layer

• It defines data format for transmission for example Ethernet, frame relay, PPP.
• It controls an access to physical media like Ethernet, csma/cd
• It also helps for Defining physical address with MAC address.
• Typically has error detection with Cyclical Redundancy Check (CRC).
• The devices like bridges, switches, wireless access points, Ethernet, ppp etc. are the devices comes under the data link layer.

3.       Network Layer

•  Network layer provides the connectivity and the path selection. In other word we can say Routing.
• It defines local addressing with IPv4 and IPv6 Addressing.
• As we seen above Routers, IPv4, IPv6 etc. are the devices and protocols of network layer.

4.       Transport Layer

• Transport layer helps to break up data between senders and receiver into smaller parts which is called segmentation.
• It creates end-to-end connectivity and can ensure reliable delivery with error detection and retransmission.
• TCP (Transmission control protocol) and UDP (User diagram protocol) protocols comes under transport layer.

5.       Session Layer

• It defines how connection can be established, maintained and terminated.
• It is responsible for synchronizing data exchange between computers, structuring communication sessions and other issues directly related to conservations between network computers.

6.       Presentation Layer

• It ensures that data can be understood between two end hosts by encryption and decryption, data compression and decompression, character set conversion etc.

7.       Application Layer

• It enable the user to access or login with provided credentials with its interfaces and support services such as Email, remote file access and transfer and share database management and other types of distributed information services.
• HTTP, FTP, SMTP, NFS, NVT are the examples of application layer protocols.