Main Body:
Both TCP/IP and OSI relate to the networking system divided into layers. Each layer performs a certain function, interacting directly only with the layer beneath or above it.
The laying of both models is the following:
Both models adhere to the same principle. Data is being passed from the application layer dawn the stack when it is being sent over the network (Physical Layer). When data is received on the other end, it is passed up the stack. As the data goes down the stack, each layer adds control information, or header, to the front of the data. That addition of extra control information is called encapsulation. When data is received on the remote end, and as it goes up the stack, each layer strips off the corresponding header and then passes the rest to the layer above.
Layer-by-Layer Comparison:
Physical layer in the OSI model defines the physical and electrical characteristics of the hardware. It transforms binary signals into corresponding signals for the particular transmission media (electromagnetic – wires, flashes of light – fibre optics, etc.). Network Interface Card and modem run at this level. Data Link layer controls the physical layer and provides error-free data transmission from one node to another by framing a binary stream into data frames and retransmission of every frame until acknowledgement from the other end is received.
Network Access layer in TCP/IP replaces Data Link and Physical layers by treating them as one. This layer as well frames the data and sends it over the network.
Both Network layer in OSI and Internet layer in TCP/IP manage connections across the network, routing and addressing. OSI provides both connectionless and connection-oriented networking. That is connection-oriented protocol exchanges control information with the receiving end (handshaking) to verify the readiness to receive data, and connectionless protocol does not handshake with the remote system. So in TCP/IP, the Internet layer is exclusively connectionless.
The Transport layer provides a reliable connection between source and destination on top of the Network layer, and ensures the receiver gets data exactly as it was sent.
The OSI Transport layer breaks data into packets that are sent and reassembled at the receiving end. It also provides retransmission of data if packet fails to arrive, and an acknowledgement of successful transmission from remote party is not received. The Session layer provides an ongoing exchange of data (session) between cooperating applications as long as session lasts. This layer manages initiation of session, message exchange, and closing of the connection.
The TCP/IP Transport layer uses two transport protocols: TCP and UDP (User Datagram Protocol). UDP provides low-overhead data transfer, but does not verify successful and correct data delivery. TCP provides reliable data transfer by means of PAR mechanism (Positive Acknowledgement with Re-transmission), where data is sent again until an acknowledgement from remote end is received. In TCP/IP most of the Session layer functions are provided by Transport layer where the protocols communicate via ports and sockets. Port is a virtual outlet that can be opened on a network device. Port numbers with data’s source and destination addresses determine a socket.
The OSI Presentation layer provides data presentation and type conversion routines. This layer is responsible for packing and unpacking of the application data, encryption/decryption and graphics expansion. The Application layer provides interfaces for the applications protocols, such as telnet, ftp, and mail.
Application layer of TCP/IP combines the functions of both the Application and Presentation layer of the OSI.
Conclusion:
Because the OSI model was developed before the protocols were invented, it is not biased towards any of them. That makes it a general model. In contrast, the TCP/IP was just a description of already existing protocols. That makes it an applied suite for internetworking. It is relatively simple and reliable and available on virtually every hardware and operating system.
Bibliography:
- Heywood, D, Scrimger, R. “Networking with Microsoft TCP/IP. Certified Administrator’s Resource Edition”, New Riders Publishing, 1997
- Parker, T et al. “TCP/IP Unleashed”, Macmillan Computer Publishing, 1996
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Hunt, C. “TCP/IP Network Administration” (2nd Edition), O’Reilly, 1997
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Connected: An Internet Encyclopaedia web page, 2002, http://freesoft.org/CIE/index.htm
