OSI Model

The Open System Interconnection (OSI) model defines networking framework to implement protocols in seven layers. There is really nothing to the OSI model. In fact, it’s not even tangible. The OSI model doesn’t perform any functions in the networking process. It is a conceptual framework so we can better understand the complex interactions that are happening.

Who Developed the OSI Model?

The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model. It divides network communication into seven layers. In this model, layers 1-4 are considered the lower layers, and mostly concern themselves with moving data around. Layers 5-7, called the the upper layers, contain application-level data. Networks operate on one basic principle: “pass it on.” Each layer takes care of a very specific job, and then passes the data onto the next layer.

Why do we need the OSI Model?

OSI Model 7 Layers:

Application (Layer 7)

OSI Model, Layer 7, 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.

 Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP

PDU: User Data

 

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.

 Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.

PDU: Formatted Data

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.

 Layer 5 Session examples include NFS, NetBios names, RPC, SQL.

PDU: Formatted Data

Transport (Layer 4)

OSI Model, Layer 4, 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.

 Layer 4 Transport examples include SPX, TCP, UDP.

PDU: Segments Data

Network (Layer 3)

Layer 3 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.

 Layer 3 Network examples include AppleTalk DDP, IP, IPX.

PDU: Packets Data

Data Link (Layer 2)

At OSI Model, Layer 2, 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.

 Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.

PDU: Frames Data

Physical (Layer 1)

OSI Model, Layer 1 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.

 Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.

PDU: Bits Data

PDU:

PDU stands for Protocol Data Units. The term used to describe how data moves from one layer of the OSI model to another. In this reference, PDU is often used synonymously with packet.

Data Encapsulation:

Data Flow through a Network:

Type of Data Transmission:

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