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Comparison between OSI and TCP/IP model.

TCP/IP and OSI models are the two very popular models in the field of inter-networking.
In this article, we give a brief comparison between these two protocols.
A point to be noted is that the adoption of either protocol does not conflict with the other one because the development of both these protocols took place concurrently. There are some ways in which TCP/IP has made contributions to the OSI model and vice versa.

However, there are a number of significant differences that arise as a result of the following requirements of the TCP/ IP protocol:

  • Dynamic routing
  • Common applications
  • Universal connectivity
  • Wireless protocols at the networking level
  • Packet switching

Differences between OSI and TCP/IP at different layers

– There are a number of differences between the architecture of the two models and all these differences can be related to the layers above the layer 4 i.e., the transport layer and layer 3 i.e., the network layer.
– The first major difference is that the OSI model has session layer as well as the presentation layer whereas the TCP/ IP combines them in to one.
– TCP/ IP requires a connection-less protocol which causes it to combine the physical layer and the data link layer of the OSI model in to a single layer at the networking level.

Physical Layer:
– This layer may either involve Ethernet, some time slot of the PDH signal, SDH – DCC etc.
– Both the TCP/ IP protocols and OSI protocols have been build up on the similar physical layer standards and therefore when it comes to this aspect there exists no difference between the TCP/ IP and OSI models.

Data Link Layer:
– Providing error free data transmission is the purpose of the data link layer, be it on the clear links or the noisy links.
– For achieving this, framing of the data and frame re-transmission is necessary via flow control mechanisms.
– Error detection codes are used as a means for the error detection.
– In inter-networking, there is nothing as such an actual data link layer rather there is a subnet protocol with which it shares a number of similarities.
– This protocol is constituted of the IMP – IMP protocol that helps in providing a connection between the IMPs that is quite reliable.
– For the networks such as LAN that are based up on Ethernet, the LLC (logical link control) protocol finds use equal use in the TCP/ IP and OSI networks.

Network Layer:
– Providing routing capabilities between the two systems on same or different network is the purpose of this network layer.
– The connection less network service protocols ES – IS are used by the OSI model for establishing communication between an end system and an intermediate system and for communicating among the intermediate systems it uses IS – IS.
– On the other hand, a message is divided in to data grams by the TCP.
– Each of these data grams has a header as well as a text part.
– Apart from this, the address of the source and destination is also contained in these data grams.
– These data grams are routed by the IP through the network using protocols such as the RIP (route information protocol), OSPF (open shortest path first) etc. for the purpose of path calculation.
– It is possible that the data grams might get lost through the network or might be received in a wrong order.

Transport Layer:
– The purpose of this layer is to set up a reliable end – to end connection up on the network layer from source to destination.
– This layer forms an integral part of both the models.
– Another difference between the two is that TCP uses only one transport protocol data unit whereas the OSI uses nine of these types.
– This means that the architecture of TCP is quite simple when compared to OSI but this leads to an increase in the size of the TCP header (20 bytes. In OSI 5 bytes).

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