Basic Networking (part-2) – OSI Model

The OSI Model :

OSI stands for Open System Interconnection .This is a standard model for networking protocols, developed by ISO (Industrial standard Organization) in 1984. OSI is standard for worldwide communications that defines a networking framework for implementing protocols in seven layers . Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
Understanding how the OSI Model works is not only useful for taking certification exams, but also for real life scenarios The short form used to memorize the layer names of the OSI Model is “All People Seem To Need Data Processing”. The lower two layers are normally put into practice with hardware and software. The remaining five layers are only implemented with software.

source: wikimedia.org

Layer 1 – Physical
Physical layer defines the cable or physical medium, e.g., electrical impulse, light or radio signals. It is responsible for bit-level transmission between nodes. It provides the sending and receiving of data through physical carrier, including defining cables, cards.
Layer 2 – Data Link
The Data Link layer of the OSI model defines the communication between adjacent nodes. At this layer, data packets are encoded and decoded into bits. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. Logical Link Control (LLC), Media Access Control (MAC), Data Framing, Addressing, Error Detection and Handling are the key tasks in Data link layer. Switches, bridges, Network interface boards are the devices normally associated with data link layer.Layer 3 – Network
Network layer describes the methods or paths for moving data among multiple networks called as routing. Network layer routes the data using switching, network layer addressing and routing algorithms .Routers operates the network layer.
Layer 4 – Transport
The transport layer is responsible for transferring data between end users. Transport layer provides reliable transfers to the upper layers by using segmentation/desegmentation, flow control and error control.
Layer 5 – Session
The session layer controls the connection between nodes. It establishes, manages and terminates the connections between the local and remote application.
Layer 6 – Presentation
The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network. The Presentation Layer is responsible for defining the syntax which two network hosts use to communicate. Encryption and compression should be Presentation Layer functions, although they are frequently provided on other layers. HTTP, SMTP, and SNMP. Works in presentation layer.
Layer 7 – Application
Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications