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OSI Reference Model


Application Layer

This Application Layer interface directly performs application services for the application processes; it also issues requests to the Presentation Layer.This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation Layer

The presentation layer does the Encryption for the data. 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, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session Layer

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination. The protocols used in this layer NFS [ Network File System ], SQL, RPC.

Transport Layer

It ensures the data transfer completion that is it provides the transparent transfer of data between systems and hosts, and it is responsible for end-to-end error recovery and flow control. Ensures proper sequencing and without loss and duplication. Acknowledges successful receipt of data. The protocols used in this layer TCP, UDP, SPX, NetBEUI.

Network Layer

This layer provides Switching and Routing technologies.It address Logical addressing – That is thro protocol ARP & RARP software addresses to hardware addresses are resolved. Protocol IP and IPX routing the packets [ messages ] between hosts and networks; It determines the best route. Pass the  information to the correct address. It sends messages and reports errors regarding packet delivery The protocol ICMP does this job. The protocols used in this layer  IP, IPX, RIP, ICMP, ARP, RARP.Devices used in this layer : Routers.

Datalink Layer

At this layer, data packets are encoded and decoded into bits.Packets placed into frames at this layer. CRC is added at this Layer (Error detection). If CRC fails at the receiving computer, this layer will request re-transmission. MAC addresses are resolved at this Layer. Sends data from network layer to physical layer. The protocols used in this layer HDLC [High-level Data Link Control ], SLIP, PPP.Devices used in this layer : Bridges, Switches.

It has 2 Sub Layers

  1. LLC (Logical Link Control) – It establishes and maintains links between communicating devices. Also responsible for frame error correction and hardware addresses
       2.  MAC (Media Access Control) – It controls how devices share a media channel. Either through CONTENTION or  TOKEN PASSING.
Physical Layer

At this layer, data packets are converted into bits.It establish the physical connectivity between the devices. Data (BITS) is sent across physical media like wires and hubs.Devices used in this layer : Hubs, Switches, Repeaters, Transmitters.

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