The OSI Model A Framework for Communications David A. Abarca July 19, 2005

Open System Interconnection (OSI) Model: –Was developed in 1984 by the International Organization for Standardization (ISO), a group that provides a worldwide common language for many technology industries, –Is only a framework for communications between computers, not a method of communications. Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary

Open System Interconnection (OSI) Model –The Model defines the separate functions necessary to process data communications between devices. –The Model uses 7 separate layers, one layer to represent each function. –The layers are stacked on top of each other and pass data from the top of the “stack” to the bottom. –When the data reaches the bottom of the “stack”, the data is transferred across a cable, or other media, and then back up the “stack” to the other device. Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary

Each layer performs a specific network communication function which allows devices, such as computers, to communicate with other devices on a network. Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Communications from Top to Bottom Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary All People Seem ToTo Need Processing Data

The 7 layers are divided into two groups: –The Upper layers Are implemented by software. Address the user’s ability to interact with other applications. –The Lower layers Are implemented by both software and hardware. Handle the transport of data over the network including the physical medium, or cable (or wireless medium). Communications from Top to Bottom Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary

HDataT H TH T H Information (data) is packaged by adding a header and, in some cases a trailer, then the package is handed off to the next layer where the process is repeated. Communications from Top to Bottom Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary User AUser B Data After the data is transmitted and received, the process is reversed and the headers and trailers are stripped off the lower layer data package and handed up to the next layer where the process is repeated. T HDataT HA Layer Data T HDataT HP Layer DataT HS Layer Data T HT Layer DataT HDataT HN Layer DataT DL Layer Data

The role of the Application Layer is to provide services that allow software applications (Word, Excel, Outlook, etc.) to transfer data (files), , web pages, and other network software on the network. Application Layer Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary Interface with the user’s applications Uses protocols including TELNET, HTTP, FTP One Data File

The role of the Presentation Layer is to transform data into the form that the application layer can accept. This layer converts, compresses, formats and encrypts data to be sent across a network Presentation Layer Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary Provides Translation and Encryption Specs. defined include: GIF, TIFF, JPEG, and ASCII. One Data File

The role of the Session Layer is to establish, maintain and terminate connections, or “sessions”, between application. The Session Layer also reports error in data transmission to the other upper layers. Session Layer Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary Provides logon and password services and provides error checking One Data File

The role of the Transport Layer is to convert the data from the upper layers into segments. The Transport Layer is responsible for delivering ALL the segments to ensure the complete message is sent. Error checking is a major responsibility of this layer and helps to ensure complete data transfer. Transport Layer Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary Reliability and Error Checking Protocols: TCP, UDP, NETBEUI, SPX. Segments

The role of the Network Layer is to convert the segments from the Transport Layer into datagrams without regard to their contents. Creation of logical paths, routing and delivery across a network using Layer 3 Switches and Routers are also functions of this layer. Network Layer Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary Software Addressing, IP, IPX, AppleTalk, ICMP, X.25 SegmentsDatagrams

The role of the Data Link Layer is to encode and decode the data packets from the Network Layer into bits, and is responsible for reassembling bits taken of the wire. This layer makes sure they are in the correct order and requests retransmission of the data in case an error occurs. Data Link Layer Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary Hardware Addressing NICs, Bridges, Hubs DatagramsData Packets

The role of the Physical Layer is to transport the bits of data (1s and 0s) across the media used for a particular network as electrical or optical impulses, or as radio signals. Physical Layer Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary Wire, Wireless Fiber-Optics Data Packets 1s and 0s

The OSI Model provides the framework for communications by using the concept of 7 stacked layers to handle the necessary network communication functions to accomplish the task of transferring data across a network. Summary Background Top to Bottom The Layers Application Presentation Session Transport Network Data Link Physical Summary User AUser B Questions ?