Notes on the 5-Layer and 7-Layer Models of Interconnection
Copyright © 2000 by R. E. Wyllys and Philip Doty
This lesson discusses two widely used, but slightly different, models of how computers are interconnected and sets of standards for doing so. The two models are often briefly called the "5-layer" and "7-layer" models.
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 layers in the 5-layer model are:
It is easy to see that the 5-layer model was developed primarily empirically, as people gained experience with the actual problems of working with inter-computer connections and with the solutions to those problems.
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.
Here are the seven layers in the Basic Reference Model for Open Systems Interconnection:
Current State of the OSI Movement
The OSI movement was an important thrust in the world of computers and computing in the early 1990s. Its goal was, and is, to provide standards to which all computer hardware and software vendors will adhere, so that the present multiplicity of interconnection and interface practices could be reduced, thus reducing the costs of designing and producing both hardware and software. The U.S. Government, through the National Institute of Standards and Technology (NIST; formerly, the National Bureau of Standards), and many states, including Texas, have backed the OSI movement through contractual policies and legislation. However, in many areas, the OSI movement has failed to win support for its formal standards, de jure standards, which have given way to standards imposed in the marketplace, de facto standards.
The O'Reilly Dictionary of PC Hardware and Data Communications Terms sums up the OSI movement as:
Summary: 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.
Please be sure to revisit the earlier material about standards and standards setting.
© As of July 2000, the material displayed here is under copyright by the LIS 386.13 class team at the Graduate School of Library and Information Science at UT-Austin: Ronald Wyllys, Philip Doty, Quinn Stewart, Carlos Ovalle, Lori Eichelberger, Tony Cherian, and Don Drumtra.
Appropriate educational and other non-profit use of the material is encouraged, provided that this copyright notice is appended, full attribution is given, and no fees are charged for access to the material.
For-profit use is strictly forbidden.