20-08-2012, 01:33 PM
TCP/IP vs.OSI
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OSI REFERENCE MODEL:
The OSI model is not a networking standard; rather it is a framework into which the various networking standards can fit. The OSI model specifies what aspects of a network’s operation can be addressed by various network standards. So, in a sense, the OSI model is sort of a standard of standards.
OSI LAYER FUNCTIONS:
• Physical Layer: The physical layer is concerned with sending raw bits between the source and destination nodes, which, in this case are adjacent nodes. The Physical Layer provides an electrical, mechanical, and procedural interface to the transmission medium.
• Data Link Layer: The data link layer is responsible for transmitting a group of bits between the adjacent nodes.
The Data-Link layer contains two sub layers that are described in the IEEE-802 LAN standards:
• Media Access Control (MAC)
• Logical Link Control (LLC)
TCP/IP Protocol Suite:
The TCP/IP suite is based on a four-layer model of networking that is similar to the seven-layer OSI model. The figure shows how the TCP/IP model matches up with the OSI model and where some of the key TCP/IP protocols fit into the model. As we can see, the lowest layer of the model, the Network Interface layer, corresponds to the OSI model’s Physical and Data Link layers. TCP/IP can run over a wide variety of Network Interface layer protocols, including Ethernet, as well as other protocols, such as Token Ring and FDDI (an older standard for fiber-optic networks).
Network (Internet) Layer: The Internet layer is very important from the context of communication over an internet or the Internet. This layer is concerned with the format of datagrams, as defined in the Internet Protocol (IP), and also about the mechanism of forwarding datagrams from the source computer to the final destination via one or more routers. Thus, this layer is also responsible for actual routing of datagrams. This layer makes internetworking possible, and thus creates an illusion of a virtual network. It routes and forwards a datagram to the next hop, but is not responsible for the accurate and timely delivery of all the datagrams to the destination in a proper sequence.
Transport Layer: There are two main protocols in this layer-Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).TCP ensure that the communication between the sender and the receiver is reliable, error-free and in sequence. TCP is responsible for checking the CRC, detecting any errors, reporting them, and acknowledging the correct delivery of datagrams. Finally, it also sequences all the datagrams that are received correctly, to form the original message. Unlike TCP, UDP does not offer reliability. Whenever we are not bothered about slight variations as much as about speed (i.e., in case of voice and video), we can use UDP.
A Comparison of the OSI and TCP/IP Reference Models:
The OSI and TCP/IP reference models have much in common. Both are based on the concept of a stack of independent protocols. Also, the functionality of the layers is roughly similar. For example, in both models the layers up through and including the transport layer are there to provide an end-to-end, network-independent transport service to processes wishing to communicate. These layers form the transport provider. Again in both models, the layers above transport are application-oriented users of the transport service.