(Lifted from: http://www.gslis.utexas.edu/~l38613dw/readings/NotesOnInterconnection.html

The 5-Layer Model (the TCP Model)

The 5-layer model serves primarily the protocols known as Transmission Control Protocol (TCP) and Internet Protocol (IP), or jointly, TCP/IP. The User Datagram Protocol (UDP) is also served by this model. The 5-layer model was developed along with these protocols, antedating the 7-layer model, and is sometimes called the TCP Model.

The 7-Layer Model (the OSI Model)

What is OSI?

When people talk about OSI, they are usually referring to what is formally called the Basic Reference Model (BRM) for Open Systems Interconnection (OSI). The BRM is currently maintained by the International Organization for Standards (ISO), although much of the initial drafting of the model and its early promulgation was done by the International Consultative Committee on Telegraphy and Telephony (CCITT), now known as the Telecommunications Standardization Sector of the International Telecommunications Union ( ITU-TSS or ITU-T). The working groups concerned with OSI are part of the formal international structure for developing and maintaining standards that is headed by the ISO, whereas the working groups concerned with TCP/IP standards, the Internet Engineering Task Force (IETF) and the Internet Architecture Board, are somewhat less formal.

The 7-Layer BRM for OSI

The BRM for OSI consists of 7 layers of protocols, i.e., of 7 different areas in which the protocols operate. In principle, the areas are distinct and of increasing generality; in practice, the boundaries between the layers are not always sharp. The model draws a clear distinction between a service, something that an application program or a higher-level protocol uses, and the protocols themselves, which are sets of rules for providing services.

	TCP/IP layer	TCP/IP Description	OSI Layer	OSI Description
7			Application	Deals with the interface between a user and the host computer: e.g., Microsoft Word translating a signal, initiated by the user's typing in a string of characters and then depressing the "Search" function key, into instructions to Windows (or System X) to try to find that string in a file.
6			Presentation	Deals with syntactic representation of data: e.g., agreement on character code (e.g., ASCII, extensions to ASCII, Unicode), data-compression and data-encryption methods, representations of graphics (e.g., files using the .PIC or .BMP formats)
5	Process & Applications	Provide applications services to users and programs	Session	Deals with creating and managing sessions when one application process requests access to another applications process (e.g., Microsoft Word importing a chart from Excel)
4	Transport	Handles data-consistency functions, i.e., provides a reliable byte stream between two nodes on a network. TCP and UDP work at this level.	Transport	Deals with data transfer between end systems; flow control for two computers (e.g., how Netscape on your PC talks with the UT Libraries Online Webpage)
3	Internet (sometimes called the Network Layer)	Provides network addressing and routing, and does so in such a way as also to provide a common address space across multiple lower-level protocols. This makes possible the interconnection of networks that characterizes the Internet. The IP protocol operates at this level.	Network	Deals with establishing paths for data between a pair of computers and handling any switching among alternative routes between the computers, as well as with definitions of how to break files (or messages) up into individual packets of data, in such a way that the packets can be transmitted and then reassembled.
2	Network (sometimes called the Data Link Layer)	This layer contains whatever IP will run over, e.g., Ethernet, token-ring, and Fiber Distributed Digital Interface (FDDI) networks. Individual network protocols, e.g., Ethernet, work at this level.	Data-Link	Deals with the transmission of data frames (e.g., packets) over a physical link between network entities, including the incorporation of error-correction coding into the data frames.
1	Physical	Not really part of the model, since TCP and IP, as protocols, deal with software rather than hardware. This layer is generally thought of as referring to all hardware under the Network Layer.	Physical	Deals with the physical (i.e., electrical and mechanical) aspects of transmitting data (e.g., voltage levels, pin-connector design, cable lengths, and grounding arrangements).

5Summary: A Comparison of the 5-Layer and 7-Layer Networking Models

In essence, we can say that there are two different, but related models, for the interconnection of computer networks. The first is the OSI/ISO networking model; this approach originated in Europe and Europe's political model of the state-owned and -operated Post, Telephone, and Telegraph (PTT). It has seven layers and is the result of international deliberation that formulated the model before there were protocols to support it. This approach, not unintentionally, favored incumbent businesses with large market share, emphasized existing proprietary software protocols (especially IBM products), and relied on telephony and national PTTs for its conceptual foundation. The OSI/ISO model so emphasized telephony that it hardly mentioned computing. Such standards fiats also tend to stifle innovation.

In contrast to this top-down approach is what some have termed the TCP approach; this approach grew mostly in America and is deeply influenced by military concerns. In this case, the protocols preceded the model and sprang from what networking researchers and users were actually doing. The weakness of such a bottom-up, �empirical� approach is that it engenders great uncertainty about directions of research, interoperability, investment options, governmental and private roles, and the like. Further, the TCP approach does little to distinguish the concepts of protocol, interface, and service. These weaknesses are partially based on the ARPANET assumptions that network users are technical experts with great programming sophistication.

So we might summarize by saying that the OSI model has been very useful for talking about computer networks because of its emphasis on layering; but its protocols are very weak. On the other hand, the TCP protocols are resounding successes, but the TCP conceptual model is weak.

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TCP/IP Vs OSI layers comparison	2 / 3

2005-05-17T08:29:14