ISO Model – Computer Networking

The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers.

In the OSI model, control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. It is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology.

The OSI model takes the task of internetworking and divides that up into what is referred to as a vertical stack that consists of the following 7 layers:

Physical (Layer 1)

Physical layer is the only layer of OSI network model which actually deals with the physical connectivity of two different stations. This layer defines the hardware equipment, cabling, wiring, frequencies, pulses used to represent binary signals etc. Physical layer provides its services to Data-link layer.

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

Data Link (Layer 2)

At OSI Model, Layer 2, data packets are encoded and decoded into bits. The data link layer or layer 2 is the second layer of the seven-layer OSI model of computer networking course. This layer is the protocol layer that transfers data between adjacent network nodes in a wide area network (WAN) or between nodes on the same local area network (LAN) segment.

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

Network (Layer 3)

The main aim of this layer is to deliver packets from source to destination across multiple links (networks). If two computers (system) are connected on the same link then there is no need for a network layer. It routes the signal through different channels to the other end and acts as a network controller.

It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.

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

Transport (Layer 4)

The Transport layer ensures the reliable arrival of messages and provides error checking mechanisms and data flow controls. The Transport layer provides services for both “connection-mode” transmissions and for “connectionless-mode” transmissions. For connection-mode transmissions, a transmission may be sent or arrive in the form of packets that need to be reconstructed into a complete message at the other end.

Layer 4 Transport examples include SPX, TCP, and UDP.

Session (Layer 5)

Session layer manages and synchronize the conversation between two different applications. Transfer of data from one destination to another session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

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

Presentation (Layer 6)

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.

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

Application (Layer 7)

The application layer is a layer in the Open Systems Interconnection (OSI) seven-layer model and in the TCP/IP protocol suite. It consists of protocols that focus on process-to-process communication across an IP network and provides a firm communication interface and end-user services.

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

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