OSI reference model

The ISO/OSI reference model defines network infrastructure in terms of seven discreet layers, each describing a certain logical fimction in the transfer of data in a network. Not all of these seven layers need necessarily be present in any given network. The layers are described in Table 37.1. 

The OSI reference model defines an architecture of seven layers, with each layer responsible for communicating with the same protocol layer running in the opposite computer and also providing services to the layer above it. The layers are described in Table 11.1. 

The GAMP software categories can be correlated with the OSI reference model as follows ... 

Network architectures and protocols are described in reference models. In the following subsections, a few very important reference models will be discussed the OSI reference model, the TCP/IP reference model, and the B-ISDN ATM reference model. 

One of the first standards for computer communications was proposed and developed by the International Standards Organization (ISO) in the early 1980s. This network architecture model, the open systems interconnection (OSI) reference model shown in Fig. 1, describes a network through seven layers. On any of these layers, one or more protocols can implement the functions specified for the layer. Some protocol specifications based on this... 

Many control vendors claim very different products to be open systems. This requires a clear definition of open control systems. In the ISA dictionary open system is defined as one that complies with the requirements of the OSI reference model in its communication with other open system. Intuitively open can be defined as a combination of data connectivity and hardware and software portability. 

OSI. 1994. X.200 Information technology - Open System Interconnection - Basic Reference Model The basic model. 

This network became known as ARPANET. In the I980 s the International Organization for Standardization (ISO) implemented its Open Systems Interconnection—Reference Model (OSI—RM) to facilitate the interoperability of different hardware components. The creation in the I990 s of new software and programming languages, such as browsers and HTML, made the explosive growth of Internet activity and Web sites possible. 

Network adapter cards, also known as network interface cards are the physical link between a client or server PC and the shared media of the network. Providing this interface between the network and the PC or workstation requires that the network adapter card have the ability to adhere to the access methodology (CSMA/CD or token passing) of the network architecture to which it is attached. These software rules, implemented by the network adapter card which control the access to the shared network media, are known as media access control (MAC) protocols and are represented on the MAC sublayer of the data link layer (layer 2) of the OSI 7-layer reference model. 

The OSI model was hrst introduced in Sec. 19.7.1 as a common framework or reference model within which protocols and networking systems could be developed with some assurance of interoperability. In this section, the OSI model provides a most effective framework in which to distinguish between the operational characteristics of the previously mentioned internetworking devices. Figure 19.62 depicts the relationship between each type of internetworking device and its related OSI layer. Each device as weU as the related OSI model layer will be explained in detail. 

While the OSI model was carefully designed, standardized, and then implemented, the Transmission Control Protocol (TCP)/Internet Protocol (IP) architecture was implemented first in the early 1980 s and modeled afterwards. Well accepted in the university circles first, and later in the industry, TCP/IP, also called the Internet architecture, is nowadays one of the major architectures. However, the TCP/IP reference model is not general and consistent, but rather implementation bound, and it does not have a clear concept of services and protocols. The IP protocol serves as a joining point for many different networks, providing them with a method for communication. 

As depicted in Fig. 3, the B-ISDN ATM reference model has different layers and a different structure as compared to the OSI model. While the OSI model is two dimensional, the ATM model is three dimensional. ATM s physical layer corresponds to both the physical and data link layer of the OSI model. The physical layer deals with the physical transmission of the bit stream and therefore depends on the physical medium used. Copper cable or fiber optics can be used for ATM. Above the physical layer is the ATM layer that deals with flow control, virtual circuit management, and cell header generation. The ATM layer is functionally equivalent to the OSI data link and network layers. On top of the ATM layer sits the ATM adaptation layer (AAL) that supports the different ATM services. AAL lays somewhere between the transport and session layers in the OSI model and provides assembly and reassembly of packets that are larger than a cell. Four different services are currently defined for ATM, resulting in four different AAL classes. AALl supports circuit... 

This paper evaluates existing NAC implementations from a security perspective. It lists vulnerabilities of NAC architectures and standards, and proposes solutions to improve NAC services. Moreover, it demonstrates an attack to defeat a commonly used NAC standard. References to computer network protocols and layers in the paper are based on OSI Model (OSI 1994). Concepts associated to NAC are not explained however, references are provided for the readers. 

Testing a local area network (LAN)—whether copper, fiber optic, or wireless—presents special challenges to the design engines or maintenance technician. The protocol analyzer is commonly used to service LANs and other communications systems. The protocol analyzer performs data monitoring, terminal simulation, and bit error rate tests (BERTs). Sophisticated analyzers provide high-level decide capability. This ability refers to the open system interconnection (OSI) network seven layer model. These functions enable the engineer to observe activity of a communications Hnk and to exercise the facility to verily proper operation. Simulation capability is usually available to emulate almost any data terminal or data communications equipment. 

---