TCP/IP Vs.OSI Reference Model

In this informative post we will discuss comparison between two main referencing models which uses the concept of protocol layering. These models are
  • Open System Interconnection Model (OSI)
  • Transport Control Protocol/ Internet Protocol (TCP/IP)
Both of these models are based on different layers. In OSI model these layers are
  • Application layer
  • Presentation layer
  • Session layer
  • Transport layer
  • Network layer
  • Data link layer
  • Physical layer
In TCP/IP model only 4 layers are used. Basically these 7 layers are merged to obtain 4 important layers.
  • Application layer
  • Transport layer
  • Internet layer
  • Subnet layer
The upper layer of OSI reference model include Application Layer, Presentation Layer and session layer. In TCP/IP only Application layer is considered as upper layer.
  • Data link/ Physical Vs. Subnet layer:- The main function of Data Link layer is to provides for the control of the physical layer. It also detects and possibly corrects errors which occurs. The main functioning of physical layer is to provide procedural, functional,electrical and mechanical means for activating a physical connection for bit transmission. 
  • Network Vs. Internet layer:- unlike all OSI layers, network layer provides both connection oriented and connection less services. In TCP/IP architecture, the internet layer is exclusively connections. 
  • Transport layer:- the main function of transport layer is to provide transparent transfer of data from a source end open system to a destination end open system. In OSI, it takes the information which is to be sent and breaks it into various packets which are sent and then reassembled in a complete message by the transport layer. In TCP/IP it defines two standard transport protocols i.e. UDP and TCP. Here UDP implements an unreliable data stream and TCP implements reliable data stream. 
  • Session layer:- In OSI model session layer permits both the parties to hold ongoing communications. This is called session across a network. But this functioning is not found in TCP/IP. In TCP/IP its characteristics are provided by the TCP protocol i.e. Transport layer.
  • Presentation layer:- Data format information for network communication is handled by Presentation layer. Basically this is done by converting data into a generic format which can be easily understand by both sides. In TCP/IP this functioning is provided by Application layer. Eg. Multipurpose Internet Mail Extension(MIME) and External Data Representation Standard(XRD).
  • Application layer:- It is the topmost layer of reference models. It provides a set of interface for applications to obtain access to networked services and access to the kinds of network services which support application directly. In OSI model FTAM, VT, MHS, DS and CMI works and in TCP/IP FTP,SMTP,TELNET, DNS and SNMP works. The notion of an application process is common to both of the approaches for constructing application entities in different.
It is also known as Horizontal Approach. According to this approach, OSi asserts that distributed applications operate over a strict hierarchy of layers and are constructed from a common tool kit of standardized application service elements. Also in OSI, each distributed application service selects functions from a large common tool box of application service element(ASEs)also these are complemented with application service elements which perform functions specific to provided end user services.

This approach is aslo known as Vertical approach. In TCP/IP each entity of application is composed of whatever ste of function it needs beyond end to end transport to support a distributed communications service. Mostly these application processes builds on what it assumes and needs only an underlying transport mechanism is provided.


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