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OSI model

dilkie — Fri, 13 Feb 2009 14:38:55 +0000

OSI model

From Wikipedia, the free encyclopedia

OSI Model
7	Application Layer
6	Presentation Layer
5	Session Layer
4	Transport Layer
3	Network Layer
2	Data Link Layer LLC sublayer MAC sublayer
1	Physical Layer

The Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative.^[1] In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

OSI model

dilkie — Fri, 13 Feb 2009 14:35:04 +0000

OSI model

From Wikipedia, the free encyclopedia

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OSI Model
7	Application Layer
6	Presentation Layer
5	Session Layer
4	Transport Layer
3	Network Layer
2	Data Link Layer LLC sublayer MAC sublayer
1	Physical Layer

“The OSI Reference Model — Understanding Layers” in Webopedia’s “Did You Know…?” section.
“Understanding The Data Link Layer” in Webopedia’s “Did You Know…?” section.

OSI model

dilkie — Fri, 13 Feb 2009 14:26:25 +0000

The 7 Layers of the OSI Model
The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Application (Layer 7)	This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.
Presentation (Layer 6)	This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Session (Layer 5)	This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
Transport (Layer 4)	This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
Network (Layer 3)	This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
Data Link (Layer 2)	At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Physical (Layer 1)	This layer conveys the bit stream – electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

RECOMMENDED READING:

“The OSI Reference Model — Understanding Layers” in Webopedia’s “Did You Know…?” section.
“Understanding The Data Link Layer” in Webopedia’s “Did You Know…?” section.

Image courtesy of The Abdus Salam International Centre for Theoretical Physics.

Last updated: March 03, 2008
Home > Quick Reference

Webopedia: he OSI Reference Model — Understanding Layers
It is time to take a trip up the OSI Reference Model, and learn what this mysterious thing is all about.

Webopedia: Understanding The Data Link Layer
What’s more important than IP and routing? Well, Layer 2 is much more important when it’s broken. Many people don’t have the Spanning Tree Protocol (STP) knowledge necessary to implement a layer 2 network that’s resilient.

ServerWatch: The OSI Model – Part 1
For many MCSE’s out there, the OSI Model is something that consisted of a few questions on the soon to be retired Networking Essentials 70-058 Microsoft exam. Amazingly, with the new Windows 2000 track, MCSE’s won’t be required to have any knowledge of the OSI Model.

ServerWatch: The OSI Model – Part 2
In Part One of this article, I covered the basics of the OSI model and gave a few tips for memorizing the seven layers of the stack. In this part, I’m going to cover each of the seven layers of the OSI model individually and present more detail to how the OSI model is utilized by networking.

Hello world!

dilkie — Sun, 08 Feb 2009 05:23:31 +0000

What is net work?
Types of net work
Net work topologies
Net work Components
Advantages & Disadvantages
OSI model

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