Foundations of Networking

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Understanding the OSI Reference Model

It is important for you to understand the basic concepts of the OSI reference model because it is the underpinning of every intranet and network. This section will introduce the reader to the OSI reference model s history, purpose, basic terminology, as well as concepts associated with the OSI reference model. A thorough discussion of the OSI reference model is outside the scope of this book. For complete and exhaustive coverage of the OSI reference model, the following important ISO standards and specifications for the OSI protocol are recommended:

  Physical layer
  CCITT X.21. 15-pin physical connection specification
  CCITT X.21 BIS-25 pin connection similar to EIA RS-232-C
  Data Link layer
  ISO 4335/7809. High-level data link control specification (HDLC)
  ISO 8802.2. Local area logical link control (LLC)
  ISO 8802.3. (IEEE 802.3) Ethernet standard
  ISO 8802.4. (IEEE 802.4) Token Bus standard
  ISO 8802.5. (IEEE 802.5) Token Ring standard
  ISO 802.3u. Fast Ethernet standard
  ISO 802.3z. Gigabit Ethernet standard
  ISO 802.10. VLAN standard
  Network layer
  ISO 8473. Network layer protocol and addressing specification for connectionless network service
  ISO 8208. Network layer protocol specification for connection-oriented service based on CCITT X.25
  CCITT X.25. Specifications for connecting data terminal equipment to packet-switched networks
  CCITT X.21. Specifications for accessing circuit-switched networks
  Transport layer
  ISO 8072. OSI Transport layer service definitions
  ISO 8073. OSI Transport layer protocol specifications
  Session layer
  ISO 8326. OSI Session layer service definitions, including transport classes 0, 1, 2, 3, and 4
  ISO 8327. OSI Session layer protocol specifications
  Presentation layer
  ISO 8822/23/24. Presentation layer specification
  ISO 8649/8650. Common application and service elements (CASE) specifications and protocols
  Application layer
  X.400. OSI Application layer specification for electronic message handling (electronic mail)
  FTAM. OSI Application layer specification for file transfer and access method
  VTP. OSI Application layer specification for virtual terminal protocol, specifying common characteristics for terminals
  JTM. Job transfer and manipulation standard

Some other good references include the ISO Web page ( and the Institute of Electrical and Electronics Engineers (IEEE) Web page (

The Consultative Committee for International Telephone and Telegraph (CCITT) is responsible for wide-area aspects of national and international communications and publishing recommendations.

In addition, because the OSI reference model has become the standard upon which protocols and applications are based throughout the networking community, knowledge about its features and functionality will always be of use to you. The sections that follow will answer a few basic questions concerning the OSI reference model.

What Is the OSI Reference Model?

OSI stands for Open Systems Interconnection, where open systems refers to the specifications surrounding its structure as well as its non-proprietary public availability. Anyone can build the software and hardware needed to communicate within the OSI structure.

The work on OSI reference model was initiated in the late 1970s, and came to maturity in the late 1980s and early 1990s. The International Organization of Standardization (ISO) was the primary architect of the model in place today.

Why Was the OSI Reference Model Needed?

Before the development of the OSI reference model, the rapid growth of applications and hardware resulted in a multitude of vendor-specific models. In terms of future network growth and design, this rapid growth caused a great deal of concern among network engineers because they had to ensure the systems under their control could to interact with every standard. This concern encouraged the International Organization of Standardization (ISO) to initiate the development of the OSI reference model.

Characteristics of the OSI Layers

To provide the reader with some examples of how the layers are spanned by a routing protocol, please refer to Figure 1-3. You might also want to contact Network General, as their Protocol chart shows how almost every single protocol spans the seven layers of the OSI reference model (see below).

Figure 1-3  How a protocol spans the OSI reference model.

Figure 1-3 provides a very good illustration to help the reader understand how the seven layers are grouped together in the model, as previously discussed. For a larger picture of how protocols are laid in the OSI reference model, go to the following locations and request a copy of their applicable posters:

Wandell & Golterman offer free OSI, ATM, ISDN, and Fiber Optics posters at
Network Associates offers a Guide to Communications Protocols at

Figure 1-4 shows the division between the upper and lower OSI layers.

Figure 1-4  OSI layer groupings.

A cute little ditty to help you remember all seven OSI Layers and their order is as follows:

All Application
People Presentation
Seem Session
To Transport
Need Network
Data Data Link
Processing Physical

Understanding the Seven Layers of the OSI Reference Model

The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers. The upper layers of the model are typically concerned only with applications, and the lower layers primarily handle data transportation.

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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