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The 7 OSI model layers Explained

The open system interconnection OSI model also known as the DOD model was first created by the International Standards Organization (iso) to address many of the difficulties in telecommunications. This is a conceptual model of telecommunications networking in which the entire process is divided into 7 layers. It synchronizes the entire process and helps to understand the interoperation of diverse communication systems under standard protocols. The seven layers we are talking about are the abstraction layer and each layer is either based on its position One or two layers are connected. Interlinked layers help each other to advance the communication process, as well as the OSI model of how data or information will be sent or received in a network.

The OSI model is also known as the DOD model.

OSI is reference model for how applications can communicate over a network, And Department of Defense (DOD) is a condensed version of the OSI model. Instead of 7 layers, the DOD model is composed of four layers

OSI model layers

In the OSI model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

OSI layers

Physical layer

The first layer of the OSI model is the physical layer PHYSICAL LAYER. Data transfer in this layer includes physical devices, such as cables and LAN cards. It is also the layer where data is converted into a bit stream, which is a string of 1s and 0s. The physical layer of both devices should also agree on a Signal Convention so that 1s can be separated from 0s on both devices.

Physical Layer Property

Representation of Bits: This layer has a stream of bits in its data. The bits must be encoded in Signal for transmission. This defines the type of encoding ie how 0s and 1s are converted into signals.

Data Rate: This layer defines the transmission of data which is bits per second.

Bit synchronization: Physical layer performs synchronization of bits by providing a clock. It controls both clock sender and receiver and thus synchronization at the bit level is known.

Physical Topology: Physical layer specifie the way in which various, devices/nodes are arranged in network, bus, star or mesh topology.

Transmission Mode: Physical layer also defines the way in which data flows between two connected devices. Different Transmission modes are possible: Simplex, Half Duplex and Full Duplex.

Hub, Repeater, Modem and cable are physical layer devices.

Network layer, data link layer and physical layer are lower layer or hardware level.

Data link layer

The data link layer is the 2nd layer in the OSI model. The data link layer is responsible for the delivery of the message from node to node (meaning computer or any device that can connect to the network). The main function of this layer is to ensure that the data transfer above the physical layer is from one node to another without error. When a packet arrives in a network, it is the responsibility of the DLL (data link layer) to pass it to the host using its MAC address.

The Data Link Layer is divided into two Sub-Layers:

  • Logical Link Control (LLC)
  • Media Access Control (MAC)

Packets received from the network layer are divided into frames based on the frame size of the NIC (Network Interface Card). The DLL encapsulates the MAC address of the sender and receiver in the header.

An ARP (Address Resolution Protocol) on the Reciver’s MAC Address Wire is requested by asking “Who has that IP address?” And the Destination host will reply with its MAC address.

Data Link Layer Property

Framing: Framing is the function of the data link layer. This provides a way to send a set of bits to a sender that are Meaningful to the receiver. This can be accomplished by accomplishing special bit patterns at the beginning and end of the frame.

Physical Addressing: After creating a frame, the data link layer adds the physical address (MAC address) of the Sender / or receiver to each frame’s header.

Error Control: Data Link Layer provides the mechanism of error control in which it detects damage or lost frame and re-transmit.

Flow Control: The data rate must be constant on both sides otherwise the data may be corrupted, the flow control coordinates the amount of coordinates that can be sent before receiving the acknowledgment.

Access Control: When a single communication channel is shared by multiple devices, the MAC Sub-Layer of the Data Link Layer helps determine which device will have control over the channel at any given time.

Packets are referred to as frames in the data link layer.

The data link layer is controlled by the NIC (network interface card) and device drivers of the host machines.

Switch and bridge are data link layer devices.

Network Layer

The network layer is the 3rd layer in the OSI model. The network layer controls the operation of the subnet. The main purpose of this layer is to transport multiple networks (networks) from the source to the destination. If two computers (systems) are connected on the same link, there is no need for a network layer. It routes the signal through various channels to the other end and acts as a network controller.

It splits outgoing messages into packets and collects incoming packets into messages for higher levels. In broadcast networks, the routing problem is simple, so the network layer is often thin or non-existent.

Network Layer Functions

  • It translates the Logical Network (IP Address) address into Physical Address (MAC Address). Related to circuit, message or packet switching.
  • Routers and gateways operate in the network layer. Mechanism is provided by the network layer to carry the packet to the final destination.
  • Connection services are provided including network layer flow control, network layer error control and packet sequince control.
  • Breaks large packets into smaller packets.

Transport Layer

Transport layers (layer 4) work transparently within the top layer to deliver and recive data without error. The send side application breaks the messages into segments (packets) and sends them to the network layer (layer 3). The Reciver side then resends the segments in the messages and sends them to the application layer (layer 7).

Transport Layer Functions

Connection-Oriented Communication: At the end-point of network communication, devices establish a handshake protocol such as TCP to ensure a connection before exchanging data.
A weakness of this method is that for each deliver message, an acknowledgment is required, which adds significantly more network load than self-error-correcting packets.

The defective byte stream or datagram is repeatedly caused to slow down the speed of the requested network when it is sent.

Same Order Delivery: This ensures that packets are always delivered in strick sequence by specifying a number.

Although the network layer is responsible, the transport layer can correct any discrepancie by rearranging it due to packet drops or device interruptions.

Data Integrity: Using checksum, data integrity can be ensured in all distribution layers. These checksums guarantee that the transmitted data is the same as received and is not corrupted.
Missing or corrupted data can be resend by requesting re-request from other layers.

Flow Control: Devices at each end of a network connection often have no way of knowing each other’s capabilities in terms of data throughput.

Data can be sent faster than the speed at which the receiving device is able to buffer or process it. When this happens, buffer overruns can cause a complete communication breakdown.
Conversely, if the receiving devices are not receiving data rapidly, it causes buffer underruns, which may cause unnecessary reductions in network performance.
Flow control ensures that data is sent at a rate that is acceptable to both parties by managing the data flow.

Traffic Control: Network bandwidth and Proccesing speed in digital communication are subject to restictions, which can mean large amounts of data congestion on the network.

This network congestion can affect almost every part of the network. The transport layer can identify overload nodes and signs of low flow rates and take appropriate steps to address these issues.

Multiplexing: Transmission of multiple packet sources from unrelated application or other sources (multiplexing) into a network requires some very dedicated control mechanisms, which are found in the Trasport layer.

This multiplexing allows the use of multiple applications simultaneously on a network such as when different Internet browsers are opened on the same computer. In the OSI model, multiplexing is handled in the service layer.

Byte orientation: Some applications prefer to receive byte streams instead of packets; The Transport Layer allows the transmission of a byte-oriented data stream if necessary.

The data in the transport layer is called a segment.
The transport layer is operated by the operating system. It is a part of the OS and communicates with the application layer by making system calls.

The transport layer is called the Heart of OSI model.

Session Layer

In OSI Model, the session layer is the 5th layer. This layer is responsible for establishing connection, maintenance of session, Authentication and security.

Functions of session layer:

Session creation, maintenane and termination: This layer allows two processes to establish, use and terminate a connection.

Syncronization: This layer allows adding processes that are considered synchronization points in the data. These synchronization points help identify the error so that the data is properly synchronized again, and the ends of the message are not cut prematurely and data loss is avoided.

Dialogue Controller: The session layer allows two processes to initiate communication with each other in half duplex or full duplex.

All 3 layers below (including session layer) in TCP / IP model Integrated as single layer as “application layer”. These 3 layers are implemented by the network application itself. They are also known as upper layers or software layers.

Presentation Layer

The presentation layer is also called the translation layer. The data at the application layer is extracted from here and manipulated according to the format required for broadcasting over the network.

The functions of the presentation layer are:

Translation: for example, ASCII to EBCDIC.

Encryption / Decryption: Data encryption translates data into another form or code. Encrypted data is known as cipher text and decrypted data is known as plain text. A critical value is used to encrypt as well as to decrypt the data.

compression: reduces the number of bits that need to be transmitted over the network.

Application Layer

At the top of the OSI reference model stack, we find the application layer that is implemented by the network application. These applications produce data, which has to be transferred over the network. This layer also acts as a window for application services to access the network and display information received to the user.

Ex: Application – Browser, Skype Messenger etc.
The application layer is called the desktop layer.

Application layer functions

Mail Services: This layer provides the basis for email forwarding and storage.

Network Virtual Terminal: This allows a user to log on to a remote host. The application creates software emulation of a terminal on the remote host. The user’s computer talks to the software terminal which in turn interacts with the host and vice versa. The remote host then assumes that it is communicating with one of its own terminals and allows the user to log on.

Directory Services: This layer provides access to global information about various services.

File Transfer and Management (FTAM): It is a standard mechanism for accessing and managing files. Users can access and manage files on the remote computer. They can also recover files from remote computers.

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