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Open Systems Interconnection(OSI) Model

Posted On 2007-05-02 by FortyPoundHead
Keywords: 640-607:Cisco Certified Network Associate (CCNA) Exam
Tags: Cisco Networking Tutorial 
Views: 2135


The Open Systems Interconnection (OSI) model began as a reference model, but has since been implemented. It was created by the International Organization for Standardization (ISO) to provide a logical framework for how data communication processes should interact across networks. Standards were created for the computer industry allowing different networks to work together efficiently.

There are 7 layers in the OSI model. Each layer is responsible for a particular aspect of data communication. For example, one layer may be responsible for establishing connections between devices, while another layer may be responsible for error checking during transfer.

The layers of the OSI model are divided into two groups: the upper layer and lower layer. The upper layers focus on user applications and how files are represented on the computers prior to transport. For the most part, network engineers are more concerned with the lower layers. It's the lower layers that concentrate on how the communication across a network actually occurs.

Application Layer
Provides network services to user applications. It is responsible for exchanging information between programs running on the machine, such as an e-mail program, and other services running on a network, such as a print server or another computers'' application.

Presentation Layer
Concerned with how data is converted and formatted for data transfer. Examples of format conversions include ASCII text for documents and .gif and JPG for images. This layer performs code conversion, data translation, compression and encryption.

Session Layer
Determines how two devices establish, maintain and manage a connection - how they talk to each other. These connections are called sessions.

Transport Layer
Responsible for breaking the data into segments, establishing an end-to-end logical connection between machines, and providing for error handling.

Network Layer
Responsible for determining addressing on the network, determining the routes that information will take on its journey, and managing network traffic congestion. Data at this level is packaged into packets.

Data Link Layer
Provides the link for how data, packaged into frames is communicated through hardware to be transported across a medium. It communicates with network cards, manages physical layer communications between connecting systems and handles error notification.

Physical Layer
Specifies how data is processed into bits and physically transferred over medium, such as cables. It''s responsible for activating and maintaining the physical link between systems.

When a host transmits data across a network to another device, the data is encapsulated with protocol information at each layer of OSI model. Each layer communicates only with its peer layer on the receiving device.

When you send e-mail, the information in the message is converted to usable data in the top three layers (Application, Presentation, Session) and passed to the Transport Layer. At the Transport Layer, protocol information is attached as a header to the data, ensuring that the computer at the other end will be able to communicate. The data and header is then packaged into a segment. The segment is transferred to the Network Layer where an additional header containing source and destination logical address is attached. The segment and header is packaged into a packet and transferred to the Data Link Layer. At the Data Link Layer, both a header and trailer with additional information is attached and packaged as a frame. The frame provides information that allows connection to the next network device on the link. The frame is converted into a series of 0's and 1's (bits) for transmission across the network medium at the Physical Layer.

When the remote device receives a sequence of bits, it passes them to the data link layer for frame manipulation. This process is referred to as de-encapsulation. When the data link layer receives the frame, It reads the control information provided by the peer source device. It strips the control information from the frame. It passes the frame up to the next layer, following the instructions that appeared in the control portion of the frame. Each subsequent layer will perform this same de-encapsulation process.


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