Jan 03

Importance of The OSI in Understanding Networking

OSI in Networking

Open Systems Interconnection (OSI) is a model which illustrates how an application communicates over a network. OSI is a conceptual framework which has been developed for understanding relationships between layers and the main purpose of the OSI reference model is to guide developers and vendors so that the software applications and digital communication products can interoperate which in result facilitate a clear framework that describes how a networking or a telecommunications system function.

7 Layers of OSI model

Application Layer This layer is responsible for the user interaction. Here the communication partners are identified, quality of service is identified, authentication and privacy of users are considered any constraints on data syntax is distinguished.

Ex:- HTTP-80, SMTP-25, Telnet-23, FTP-21, TFTP-69, SNMP-161, NNTP-119, HTTPS-443,

POP3-110, DNS-53

Presentation Layer This layer is responsible for data representation, code formatting, data format compressing, translation & encryption.


Session Layer Session layer establishes, maintains, and manages the communication session between computers.

Keep the data streams separate.

Ex :-

·      NFS – Network File System

·      SQL – Structured Query Language

·      RPC – Remote Procedure Call

Transport Layer Maintains flow control which prevents buffer overflow and data loss. Reliable or un reliable transmission of segments using port numbers.

Reliable data transmission provides for:

·      Acknowledgement of received segments.

·      Retransmission of segments not acknowledged.

·      Proper re-sequencing of segments.

·      Data is managed by Flow Control so no data is lost.

v  TCP – Reliable with establishment of virtual circuit and numbering and acknowledgement of

Packets and establishes the connection before sending data.

v  UDP – Unreliable as packets are sent without acknowledgement.

Network Layer This layer provides switching and routing technologies, provides logical addressing and creating logical paths, known as virtual circuits, for transmitting data (packet) from node to node. Functions of this layer can be identified as routing and forwarding, error handling, internetworking, congestion control and packet sequencing.

Ex :-

·      IP ( Internet protocols )

·      IPX (internet packet Exchange)

Device:- Router

Data Link Layer MAC (Media Access Control) Layer

MAC sub layer manages how a computer gains access to data and permission to transmit it on a network.

Bit -> bytes -> Frames

Ex :- Switch , Bridge

·         HDLC – High Level Data Link Control – Implemented by the International Standards Organization

(ISO) – 802.2

·         PPP – Used to support several protocols over a slow serial interface – 802.5

·         Frame Relay – Higher speed than X.25 packet switching network – 802.11

Physical Layer This layer defines the physical and electrical specifications for the networking media that carry the data bits across a network. (bit-level transmission between network nodes)

Ex :- EIA/TAI-232 ,RS 232 ,V.35

Devices :- Hub, repeater, and patch panel


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