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In 1994 the International Organization for Standards (ISO) developed a network model that could communicate and work together. That model is known as the Open System Interconnection Model (OSI). The model was developed to allow vendors of proprietary networks to develop networks that could be compatible with other vendor's networks. Purchasers of Networks could therefore integrate networks from different vendors in the same environment. The OSI model provides a set of standards that allow compatibility between network technologies developed by different vendors.
The OSI Model is based on Layers. Each Layer has specific duties that it performs on the data as it passes through the network. Each layer performs a separate network function. Cisco identifies several advantages of this approach.
| Layer 7 | The Application Layer |
| Layer 6 | The Presentation Layer |
| Layer 5 | The Session Layer |
| Layer 4 | The Transport Layer |
| Layer 3 | The Network Layer |
| Layer 2 | The Data Link Layer |
| Layer 1 | The Physical Layer |
This layer interacts with the end user when the end user is performing some network function such as transferring files, sending email, starting a remote session. The Application Layers purpose is to interface with the communication component of an application. This level provides a service to the application. Unlike other layers, the Application Layer does not provide a service to other layers. Cisco has identified the following responsibilities of this level.
The presentation layer is responsible for the formatting and representation of data. This layer has the ability to translate between different data formats. For example, this layer would be responsible for translating between ASCII and EBCDIC character representation schemes when moving data from a Windows Operating System to an IBM Mainframe. Windows utilizes the ASCII character representation scheme while an IBM Mainframes utilizes EBCDIC. Cisco identifies three function provided by this layer.
The Session Layer establishes, manages, and terminates sessions between applications. The session layer utilizes protocols to regulate communications between applications. Protocols are just a set of rules that govern communications. Protocols used by this layer include: Network File System (NFS), Structured Query Language (SQL), Remote Procedure Call (RPC). The protocols regulate and control requests and responses between the applications.
The transport layer's functions are to transport and regulate the flow of information from source to destination. The transport layer ensures that information received is reliable and accurate. The Internet Protocols implemented at this layer are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). The main difference between the protocols is that TCP is connection oriented and reliable while UDP is connectionless and unreliable.
The function of the network layer is to navigate the data trough the network by finding the best path from source to destination. The Internet Protocol (IP) is a Layer 3 protocol. The network addressing scheme is used by devices to determine the destination of data as it moves through the network. IP addresses are an example of a hierarchal network addressing scheme that can be used to route, find a path, data. NetBui is and example of a protocol hat does not have a network layer and therefore is found only on small networks. A router is an example of a network layer device that uses the logical addressing scheme implemented by a layer 3 protocol
The purpose of the data link Layer is to provide reliable transit of data across a physical link. The data link layer is concerned with physical addressing ( Media Access Control (MAC) address of the NIC Card), network topology, error notification, etc. Layer 2 technologies include Ethernet, Fiber Distributed Data Interface (FDDI), and Token Ring. Layer 2 devices include network interface card (NIC), bridges, and switches,
The purpose of the physical layer is to transmit data by defining the electrical specification between the source and destination. The media found at this level are twisted pair cable, unshielded twisted pair, coaxial cable, and fiver-optic cable. Layer 1 devices include repeaters and hubs.