The 7 Layers of the OSI Model

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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.

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Application51Testing软件测试网b^(iS;^)U8rD (Layer 7)	This layer supportsapplicationandend-user processes. Communication partners are identified, quality of service is identified, userauthenticationand privacy are considered, and any constraints on datasyntaxare identified. Everything at this layer is application-specific. This layer provides application services for file transfers,e-mail, and othernetworksoftwareservices.TelnetandFTPare applications that exist entirely in the application level.Tiered application architecturesare part of this layer.
Presentation51Testing软件测试网-i)s\JcY]"O (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 thesyntax layer.
Session ~MPugR l4R1F-C2Mc2t9f0(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 GfWH3}RfB0(Layer 4)	This layer providestransparenttransfer of data between end systems, or hosts, and is responsible for end-to-end error recovery andflow control. It ensures complete data transfer.
Network51Testing软件测试网y `!T^l0g-q&Ik (Layer 3)	This layer providesswitchingandroutingtechnologies, creating logical paths, known asvirtual circuits, for transmitting data fromnodeto node. Routing and forwarding are functions of this layer, as well as addressing,internetworking, error handling, congestion control andpacketsequencing.
Data Link g| S5x-c&Li0(Layer 2)	At this layer, data packets are encoded and decoded intobits. 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 sublayers: TheMedia Access Control(MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer 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 !^&Z k3bIcu0(Layer 1)	This layer conveys thebitstream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides thehardwaremeans of sending and receiving data on a carrier, including defining cables,cardsand physical aspects.Fast Ethernet,RS232, andATMare protocols with physical layer components.

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O#Ar3j&M!]N2Q0This graphic is taken fromThe Abdus Salam International Centre for Theoretical Physics.

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