Open Systems Interconnection Reference Model OSI

1. Open Systems Interconnection Reference Model (OSI) After this presentation you should: Know the history of the OSI Reference Model Understand the role of the OSI Reference Model Know the different layers of the model and what they are used for Understand that other models exist

2. OSI Reference Model Definition: A model that network professionals can use to develop and administer networking systems. Purpose: To allow heterogenious applications, data, and hardware to be able to function as networks.

3. OSI Reference Model Describes: A general architecture for data network design and interconnection. Intention: To provide functional guideline for permitting the standardization of network protocols.

4. OSI Reference Model History: Not initially accepted Manufactures had their own networking standards Allowed others to enter then world of networking Specified network standards for compatibility Became the international standard for networking The OSI model was not accepted initially. Manufacturers such as IBM and Digital had their own network models that were developed from the ground up. These companies had the resources to develop the networking applications right down to sending and receiving data. As the OSI model gained acceptance, other manufacturers could enter the world of computer networking by producing network components that would be compatible with the rest of the network. These new computer companies could follow the requirements set out by the OSI model and they would be ensured that their products would fit international networks. Once the OSI model became the established standard for international networking the large companies such as IBM and Digital had to follow suit if they were to remain competitive. People were no longer willing to purchase their networks "in a can" from one manufacturer but rather, they wanted some assurance that their systems could be modified with interchangeable components. The open system network design was born.The OSI model was not accepted initially. Manufacturers such as IBM and Digital had their own network models that were developed from the ground up. These companies had the resources to develop the networking applications right down to sending and receiving data. As the OSI model gained acceptance, other manufacturers could enter the world of computer networking by producing network components that would be compatible with the rest of the network. These new computer companies could follow the requirements set out by the OSI model and they would be ensured that their products would fit international networks. Once the OSI model became the established standard for international networking the large companies such as IBM and Digital had to follow suit if they were to remain competitive. People were no longer willing to purchase their networks "in a can" from one manufacturer but rather, they wanted some assurance that their systems could be modified with interchangeable components. The open system network design was born.

5. OSI Reference Model History: Based on a proposal developed by the International Standards Organization (ISO) It deals with connecting open systems - systems that are open for communication with other systems Adopted in 1983 Open Systems was born The OSI Reference Model is based on a proposal developed by the International Standards Organization (ISO). The model is called ISO OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems - that is, systems that are open for communication with other systems or not dependent on any one vendor. MULTIVENDOR. The OSI Reference Model is based on a proposal developed by the International Standards Organization (ISO). The model is called ISO OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems - that is, systems that are open for communication with other systems or not dependent on any one vendor. MULTIVENDOR.

6. OSI Reference Model Allows the physical transmission of bit stream voltages and currents, frames data, inserts addresses, takes care of error control between nodes on the network, establishes paths between source and destination, makes sure reliable delivery of the frames is made between computers and the process-to-process communications is performed. In addition, it takes care of end-user services, data compression and encryption. There is no part of the data communications process that is not covered in at least one part of the OSI reference model. There is no part of the data communications process that is not covered in at least one part of the OSI reference model.

7. Layering The OSI model is defined by layers. The principles that were applied to arrive at the seven layers are as follows: 1. A layer should be created where a different level of abstraction is needed. 2. Each layer should perform a well defined function. Modern computer networks are designed in a highly structured way. To reduce their design complexity, most networks are organized as a series of layers, each one built upon the previous layer. Modern computer networks are designed in a highly structured way. To reduce their design complexity, most networks are organized as a series of layers, each one built upon the previous layer.

8. Layering - cont’d 3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols. 4. The layer boundaries should be chosen to minimize the information flow across the interfaces. 5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy. The main idea in OSI is that the process of communication between two end users in a telecommunication network can be divided into layers, with each layer adding its own set of special, related functions. Every user is at a computer equipped with these seven layers of function. So, in a given message between users, there will be a flow of data through each layer at one end down through the layers in that computer and, at the other end, when the message arrives, another flow of data up through the layers in the receiving computer and ultimately to the end user. The actual programming and hardware that furnishes these seven layers of function is usually a combination of the computer operating system, applications (such as your Web browser), TCP/IP or alternative transport and network protocols, and the software and hardware that enable you to put a signal on one of the lines attached to your computer. NO MATTER which model you are using all of the functions are performed. They are just defined differently. The main idea in OSI is that the process of communication between two end users in a telecommunication network can be divided into layers, with each layer adding its own set of special, related functions. Every user is at a computer equipped with these seven layers of function. So, in a given message between users, there will be a flow of data through each layer at one end down through the layers in that computer and, at the other end, when the message arrives, another flow of data up through the layers in the receiving computer and ultimately to the end user. The actual programming and hardware that furnishes these seven layers of function is usually a combination of the computer operating system, applications (such as your Web browser), TCP/IP or alternative transport and network protocols, and the software and hardware that enable you to put a signal on one of the lines attached to your computer. NO MATTER which model you are using all of the functions are performed. They are just defined differently.

9. Layering - cont’d Basic Ideas Protocols running between entities at layer N implement functions which are provided as services to layer N+1 Communication at layer N is accomplished by making use of the services provided at layer N-1 By building upon services offered by lower layers, the higher layers can provide increasing levels of power and functionality If the interfaces between layers are maintained, then a layer can be replaced, if a more advanced technology appears, without affecting the other layers. If the interfaces between layers are maintained, then a layer can be replaced, if a more advanced technology appears, without affecting the other layers.

10. OSI Reference Model Most commonly used model for defining network layers Originally developed for mainframe networks Defines 7 layers Physical * Data Link * Network * Transport Session * Presentation * Application Layers The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. Control information typically takes one of two forms: headers and trailers. Headers are placed in front of the data that has been passed down from upper layers.Trailers are appended to data that has been passed down from upper layers. An OSI layer is not required to attach a header or trailer to data from upper layers. Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit. At the network layer, an information unit, for example, consists of a Layer 3 header and data. At the data link layer, however, all the information passed down by the network layer (the Layer 3 header and the data) is treated as data.The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. Control information typically takes one of two forms: headers and trailers. Headers are placed in front of the data that has been passed down from upper layers.Trailers are appended to data that has been passed down from upper layers. An OSI layer is not required to attach a header or trailer to data from upper layers. Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit. At the network layer, an information unit, for example, consists of a Layer 3 header and data. At the data link layer, however, all the information passed down by the network layer (the Layer 3 header and the data) is treated as data.

11. OSI Reference Model