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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:

  Layer Name Function
  5 Process & Applications Provide applications services to users and programs
  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.
  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.
  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.
  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.

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:

Layer Name Function
  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 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 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 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 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 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).

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:

A suite of protocols and standards sponsored by the ISO for data communications between otherwise incompatible computer systems.

Unfortunately (for the many people and companies that spent so much time and money on the effort), the TCP/IP suite of protocols has eclipsed OSI, and you don't hear much about OSI anymore (except for a few applications, such as the X.500 directory service).

When work began (in the late 1970s) on providing a standard method for communications between different hardware platforms, TCP/IP was not considered an option for serious commercial applications, since TCP/IP:

Required you to run UNIX (which, at the time, was not used for commercial applications and had only a command-line user interface)

Had poor security and management features

Had too small an address size

Therefore the ISO promoted development of OSI (how palindromic).

Although all major (and many minor) computer vendors now have OSI products, the OSI protocols were never widely implemented, and TCP/IP has become the first choice for multivendor networking, because of its:

Lower-cost and more-efficient implementation (less CPU time required, smaller programs)

Availability for most operating systems

Fast standardization and development cycle (usually using the Internet to facilitate communications) when a new requirement is identified

Familiarity among college graduates (universities use TCP/IP, so once out of school, a graduate's first choice when designing a system is to use TCP/IP)

Easier-to-access (and zero-cost) documentation and standards (they are all available on the Internet)

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.

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Last updated 5 February 2001 by Don Drumtra