OSI Model – The 7 Layers in Networking

OSI Model – The 7 Layers in Networking

Layer 1: Physical Layer

It defines all the electrical and physical specifications for devices which basically includes the following –

  • Layout of pins,
  • Voltages cable specifications
  • Hubs,
  • Repeaters,
  • Network adapters
  • Host Bus Adapters

The physical layer performs the following actions –

A) Establishment and termination of a connection to a communications medium.

B) Participation in the process where the communication resources are shared among multiple users.

C) Modulation of digital data in user equipment and the corresponding signals are transmitted over a communications channel.

The Parallel SCSI buses various physical-layers Ethernet standards operate in this layer. Are also in this layer.

Layer 2: Data Link Layer

It provides functional and procedural means to transfer data between network entities and to detect and also correct errors that are followed.Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks.

n IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this can be split into a MAC layer and IEEE 802.2 LLC layer. It arranges bits from physical layer into logical chunks of data, known as frames. This is the layer at which the bridges and switches operate.

Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding. Other protocols may be imposed on the data frames to create tunnels and logically separated layer 2 forwarding domains.

Layer 3: Network Layer

This layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform segmentation/desegmentation, and report delivery errors. Routers operate at this layer sending data throughout the extended network and making the Internet possible (there also exist layer 3 (or IP) switches). This is a logical addressing scheme values are chosen by the network engineer. The addressing scheme is hierarchical. The best known example of a layer 3 protocol is the Internet Protocol.

Layer 4: Transport Layer

The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable data transfer. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control.

Some protocols are state and connection oriented. This means that the transport layer can keep track of the packets and retransmit those that fail. The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets.

Layer 5: Session Layer

The layer controls the dialogues (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures.

The OSI model made this layer responsible for “graceful close” of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocols suite.

Layer 6: Presentation Layer

This layer transforms data to provide a standard interface for the Application layer. MIME encoding, data compression, data encryption and similar manipulation of the presentation is done at this layer to present the data as a service or protocol developer sees fit.

Examples: converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of, e.g., XML.

Layer 7: Application Layer

The Application layer provides a means for the user to access information on the network through an application. This layer is the main interface for the user(s) to interact with the application and therefore the network.

Some examples of application layer protocols include Telnet, File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and Hypertext Transfer Protocol (HTTP).

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