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What is OSI Model? – Layers of OSI Model

The OSI (Open Systems Interconnection) Model is a set of rules that explains how different computer systems communicate over a network. OSI Model was developed by the International Organization for Standardization (ISO) . The OSI Model consists of 7 layers and each layer has specific functions and responsibilities.

This layered approach makes it easier for different devices and technologies to work together. OSI Model provides a clear structure for data transmission and managing network issues. The OSI Model is widely used as a reference to understand how network systems function.

In this article, we will discuss the OSI Model and each layer of the OSI Model in detail. We will also discuss the flow of data in the OSI Model and how the OSI Model is different from the TCP/IP Model.

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Layers of the OSI Model

There are 7 layers in the OSI Model and each layer has its specific role in handling data. All the layers are mentioned below:

Physical Layer

• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer

Application Layer

Layer 1 – Physical Layer

The lowest layer of the OSI reference model is the Physical Layer . It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. Physical Layer is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together. Common physical layer devices are Hub , Repeater , Modem , and Cables .

Functions of the Physical Layer

• Bit Synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level.
• Bit Rate Control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
• Physical Topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus topology , star topology , or mesh topology .
• Transmission Mode: Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex .

Layer 2 – Data Link Layer (DLL)

The data link layer is responsible for the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address . Packet in the Data Link layer is referred to as Frame. Switches and Bridges are common Data Link Layer devices.

The Data Link Layer is divided into two sublayers:

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

The packet received from the Network layer is further divided into frames depending on the frame size of the NIC(Network Interface Card) . DLL also encapsulates Sender and Receiver’s MAC address in the header.

The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.

Functions of the Data Link Layer

• Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
• Physical Addressing: After creating frames, the Data link layer adds physical addresses ( MAC addresses ) of the sender and/or receiver in the header of each frame.
• Error Control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
• Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment.
• Access Control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.

Layer 3 – Network Layer

The network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender and receiver’s IP address are placed in the header by the network layer. Segment in the Network layer is referred to as Packet. Network layer is implemented by networking devices such as routers and switches .

Functions of the Network Layer

• Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.
• Logical Addressing: To identify each device inter-network uniquely, the network layer defines an addressing scheme. The sender and receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally.

Layer 4 – Transport Layer

The transport layer provides services to the application layer and takes services from the network layer. The data in the transport layer is referred to as Segments . It is responsible for the end-to-end delivery of the complete message. The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found. Protocols used in Transport Layer are TCP , UDP   NetBIOS , PPTP .

At the sender’s side , the transport layer receives the formatted data from the upper layers, performs Segmentation , and also implements Flow and error control to ensure proper data transmission. It also adds Source and Destination port number in its header and forwards the segmented data to the Network Layer.

• Generally, this destination port number is configured, either by default or manually. For example, when a web application requests a web server, it typically uses port number 80, because this is the default port assigned to web applications. Many applications have default ports assigned.

At the Receiver’s side, Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data.

Functions of the Transport Layer

• Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message.
• Service Point Addressing: To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address. Thus by specifying this address, the transport layer makes sure that the message is delivered to the correct process.

Services Provided by Transport Layer

• Connection-Oriented Service
• Connectionless Service

Layer 5 – Session Layer

Session Layer in the OSI Model is responsible for the establishment of connections, management of connections, terminations of sessions between two devices. It also provides authentication and security. Protocols used in the Session Layer are NetBIOS, PPTP.

Functions of the Session Layer

• Session Establishment, Maintenance, and Termination: The layer allows the two processes to establish, use, and terminate a connection.
• Synchronization: This layer allows a process to add checkpoints that are considered synchronization points in the data. These synchronization points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
• Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

Let us consider a scenario where a user wants to send a message through some Messenger application running in their browser. The “ Messenger ” here acts as the application layer which provides the user with an interface to create the data. This message or so-called Data is compressed, optionally encrypted (if the data is sensitive), and converted into bits (0’s and 1’s) so that it can be transmitted.

Communication in Session Layer

Layer 6 – Presentation Layer

The presentation layer is also called the Translation layer . The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. Protocols used in the Presentation Layer are JPEG , MPEG , GIF , TLS/SSL , etc.

Functions of the Presentation Layer

• Translation: For example, ASCII to EBCDIC .
• Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
• Compression: Reduces the number of bits that need to be transmitted on the network.

Layer 7 – Application Layer

At the very top of the OSI Reference Model stack of layers, we find the Application layer which is implemented by the network applications. These applications produce the data to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. Protocols used in the Application layer are SMTP , FTP , DNS , etc.

Functions of the Application Layer

The main functions of the application layer are given below.

• Network Virtual Terminal(NVT): It allows a user to log on to a remote host.
• File Transfer Access and Management(FTAM): This application allows a user to access files in a remote host, retrieve files in a remote host, and manage or control files from a remote computer.
• Mail Services: Provide email service.
• Directory Services: This application provides distributed database sources and access for global information about various objects and services.

How Data Flows in the OSI Model ?

When we transfer information from one device to another, it travels through 7 layers of OSI model. First data travels down through 7 layers from the sender’s end and then climbs back 7 layers on the receiver’s end.

Data flows through the OSI model in a step-by-step process:

• Application Layer: Applications create the data.
• Presentation Layer: Data is formatted and encrypted.
• Session Layer: Connections are established and managed.
• Transport Layer: Data is broken into segments for reliable delivery.
• Network Layer : Segments are packaged into packets and routed.
• Data Link Layer: Packets are framed and sent to the next device.
• Physical Layer: Frames are converted into bits and transmitted physically.

Each layer adds specific information to ensure the data reaches its destination correctly, and these steps are reversed upon arrival.

We can understand how data flows through OSI Model with the help of an example mentioned below.

Let us suppose, Person A sends an e-mail to his friend Person B .

Step 1: Person A interacts with e-mail application like Gmail , outlook , etc. Writes his email to send. (This happens at Application Layer ).

Step 2: At Presentation Layer, Mail application prepares for data transmission like encrypting data and formatting it for transmission.

Step 3: At Session Layer, There is a connection established between the sender and receiver on the internet.

Step 4: At Transport Layer , Email data is broken into smaller segments. It adds sequence number and error-checking information to maintain the reliability of the information.

Step 5: At Network Layer, Addressing of packets is done in order to find the best route for transfer.

Step 6: At Data Link Layer, d ata packets are encapsulated into frames, then MAC address is added for local devices and then it checks for error using error detection.

Step 7: At Physical Layer, Frames are transmitted in the form of electrical/ optical signals over a physical network medium like ethernet cable or WiFi.

After the email reaches the receiver i.e. Person B , the process will reverse and decrypt the e-mail content. At last, the email will be shown on Person B email client.

Protocols Used in the OSI Layers

Why does the osi model matter.

The OSI Model matters because it provides the user a clear structure of “how the data moves in the network?”. As the OSI Model consists of 7 layers, each layer has its specific role, and due to which it helps in understanding, identifying and solving the complex network problems easily by focusing on one of the layers not the entire network.

As the modern Internet does not prefer the OSI Model, but still, the OSI Model is still very helpful for solving network problems. It helps people understanding network concepts very easily.

Difference Between OSI and TCP/IP Model

OSI vs TCP/IP

Advantages of OSI Model

The OSI Model defines the communication of a computing system into 7 different layers. Its advantages include:

• It divides network communication into 7 layers which makes it easier to understand and troubleshoot.
• It standardizes network communications, as each layer has fixed functions and protocols.
• Diagnosing network problems is easier with the OSI model.
• It is easier to improve with advancements as each layer can get updates separately.

Disadvantages of OSI Model

• The OSI Model has seven layers, which can be complicated and hard to understand for beginners.
• In real-life networking, most systems use a simpler model called the Internet protocol suite (TCP/IP), so the OSI Model is not always directly applicable.
• Each layer in the OSI Model adds its own set of rules and operations, which can make the process more time-consuming and less efficient.
• The OSI Model is more of a theoretical framework, meaning it’s great for understanding concepts but not always practical for implementation.

In conclusion, the OSI (Open Systems Interconnection) model helps us understand how data moves in networks. It consists of seven distinct layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer has specific responsibilities and interacts with the layers directly above and below it. Since it is a conceptual model, but the OSI framework is still widely used to troubleshoot and understand networking issues.

Frequently Asked Questions on OSI Model – FAQs

Can osi layers work independently.

No, OSI layers do not work independently. Each layer depends on the services provided by the layer below it and, in turn, provides services to the layer above it. This layered approach ensures that data is transmitted smoothly from the source to the destination.

How does the OSI Model help in troubleshooting network issues?

By breaking down communication into layers, the OSI Model helps network administrators isolate problems more easily.

What happens if a layer in the OSI Model fails?

If a particular OSI layer fails, data transmission may be disrupted or fail entirely. Network administrator will check layer by layer to identify and resolve the issue, make sure that each layer is functioning correctly or not.

How does DNS fit into the OSI Model?

The Domain Name System (DNS) operates at Layer 7 (Application Layer). It translates domain names into IP addresses, facilitating communication between users and services across the network.

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The Cisco Learning Network

Osi model reference chart, aug 19, 2021 knowledge, information.

Those who start towards the path of CCNA understand the OSI model as a core component that continues throughout the rest of the Cisco certification tracks.

The Open Systems Interconnection model (OSI model) is a seven layer conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system.

The physical layer , which is the bottom layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and it carries the signals for all of the higher layers.

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications.

The session layer allows session establishment between processes running on different stations.

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known by the application layer at the receiving station.

The application layer serves as the window for users and application processes to access network services.

The Cisco Learning Network has developed an OSI model chart to help aid in your learning. This chart organizes and describes the seven layers, providing examples in the form of protocols and devices, as well as in relation to the DOD TCP/IP Model.

Article Details

Great summarized chart.

david.wadsworth80

Nice chart, though I would update it to include the TCP/IP 5 Layer model as well. For anyone looking at the OSI model, the top 3 layers (layer 7, 6, 5) are mapped to the Application layer for the TCP/IP model, with the lower layers (Layer 4 - Layer 1) are mapped 1:1, with the Data Link Layer mapped as the Network Interface Layer.

Thank you for sharing

[email protected]

Instant Like! Never thought that in here i could find so much Information for the self educated People.

Matt Saunders

Glad you like!

jdewitt2011

Very good information and definitely added value that we can download the OSI Model Chart for studying purposes. Even though the Transport layer is missing.

Thank you for sharing all this information with us. 

You're welcome Nicole! 

I appreciate you letting me know that the transport layer has now been added to the summary page.

akhilkamalakaran

A good one, thanks for share

This looks awesome!

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  Layer 6 Presentation Layer

De/Encryption, Encoding, String representation

The presentation layer (data presentation layer, data provision level) sets the system-dependent representation of the data (for example, ASCII, EBCDIC) into an independent form, enabling the syntactically correct data exchange between different systems. Also, functions such as data compression and encryption are guaranteed that data to be sent by the application layer of a system that can be read by the application layer of another system to the layer 6. The presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable for both systems data format, the ASN.1 (Abstract Syntax Notation One) used.

OSI Layer 6 - Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer. In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format. Wikipedia

• Data conversion
• Character code translation
• Compression
• Encryption and Decryption

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

Sase specific application service element, layer 7   application layer, layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

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The Presentation Layer of OSI Model

The presentation layer (Layer 6) ensures that the message is presented to the upper layer in a standardized format. It deals with the syntax and the semantics of the messages.

The main functions of the presentation layer are as follows −

• It encodes the messages from the user dependent format to the common format and vice versa, for communication among dissimilar systems.
• It is responsible for data encryption and decryption of sensitive data before they are transmitted over common channels.
• It is also responsible for data compression. Data compression is done at the source to reduce the number of bits to be transmitted. It reduces the storage space and increases the file transfer rate. It is particularly useful for transmission of large multimedia files.

• Related Articles
• The Transport Layer of OSI Model
• The Session Layer of OSI Model
• The Data Link Layer of OSI Model
• Explain the functions of Presentation Layer.
• The OSI Reference Model
• What is Presentation Layer?
• What is a presentation layer?
• Advantages and Disadvantages of the OSI Model
• Computer Networks – Layers of OSI Model
• What is the OSI Reference Model?
• OSI Model in Computer Networking
• Why Does the OSI Reference Model Matter?
• What are the critiques of OSI Model and Protocols?
• Explain the advantages and disadvantages of the OSI reference Model
• OSI vs. TCP/IP Reference Model

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COMPUTER NETWORK BASICS

• Introduction To Computer Networks
• Uses of Computer Networks
• Line Configuration
• Types of Network Topology
• Transmission Modes
• Transmission Mediums
• Bounded/Guided Transmission Media
• UnBounded/UnGuided Transmission Media
• Types of Communication Networks
• Connection Oriented and Connectionless Services
• Network Layer
• Quality of Service(QoS)
• IGMP Protocol
• Reference Models

Physical Layer

• Digital Transmission
• Multiplexing
• Circuit-Switched
• Message-Switched Networks
• Packet Switching

Data link layer

• Error Correction
• Data Link Control
• Flow and Error
• Simplest Protocol
• Stop-and-Wait Protocol
• Go-Back-N Automatic Repeat
• Sliding Window Protocol
• HDLC Protocol
• Point-to-Point Protocol
• Multiple Access in DL
• Channelization Protocols
• Gigabit Ethernet
• Random Access Protocol
• Controlled Access Protocols
• Carrier Sense Multiple Access

Transport layer

• Transport Layer
• Telnet vs SSH
• UDP Protocol
• TCP - Protocol

ISO/OSI REFERENCE MODEL

• Introduction to Reference Models
• OSI Model: Physical Layer
• OSI Model: Datalink Layer
• OSI Model: Network Layer
• OSI Model: Transport Layer
• OSI Model: Session Layer
• OSI Model: Presentation Layer
• OSI Model: Application Layer

TCP/IP REFERENCE MODELCOMPUTER NETWORKS

• The TCP/IP Reference Model
• Difference between OSI and TCP/IP Model
• Key Terms - Computer Network

Session layer

• Session Layer

Computer Networks

• Components of Computer Networks
• Features of Computer Network
• Protocols and Standards
• Connection Oriented and Connectionless
• OSI Vs TCP/IP

Presentation layer

• Presentation Layer

Application layer

• HTTP Protocol
• FTP Protocol
• SMTP Protocol
• POP Protocol
• SNMP Protocol
• Electronic Mail
• MIME Protocol
• World Wide Web
• DNS Protocol

Presentation Layer - OSI Reference Model

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.

In order to make it possible for computers with different data representations to communicate, the data structures to be exchanged can be defined in an abstract way. The presentation layer manages these abstract data structures and allows higher-level data structures(eg: banking records), to be defined and exchanged.

Functions of Presentation Layer

• Translation: Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
• Encryption: It carries out encryption at the transmitter and decryption at the receiver.
• Compression: It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.

Design Issues with Presentation Layer

• To manage and maintain the Syntax and Semantics of the information transmitted.
• Encoding data in a standard agreed upon way. Eg: String, double, date, etc.
• Perform Standard Encoding on wire.
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OSI MODEL. Rahul Bandhe. Introduction.

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• network layer
• presentation layer
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• session layer dialog initiation

Presentation Transcript

OSI MODEL Rahul Bandhe

Introduction • Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

OSI Model's 7 Layers APPLICATION Application to Application APPLICATION HIGHER LAYER Application to Application PRESENTATION PRESENTATION SESSION Application to Application SESSION TRANSPORT TRANSPORT Process to Process MIDDLE LAYER NETWORK NETWORK Source to Destination Router Source to Destination Hop to Hop DATA LINK DATA LINK Switch LOWER LAYER Hop to Hop Hub and Repeater PHYSICAL PHYSICAL Physical Medium

Host and Media Layer

Data, Protocol & Activities OSI Layers TCP/IP Suit Activities Application Application Telnet, FTP, SMTP, HTTP, DNS, SNMP, Specific address etc… To allow access to network resources Presentation Presentation To Translate, encrypt, and compress data Session Session To establish, manage, and terminate session Transport Transport SCTP, TCP, UDP, Sockets and Ports address To Provide reliable process-to-process Message delivery and error recovery Network Network IP, ARP/RARP, ICMP, IGMP, Logical address To move packets from source to destination; to provide internetworking Data Link Data Link IEEE 802 Standards, TR, FDDI, PPP, Physical address To organize bits into frames; to provide Hop-to-hop delivery Physical Physical Medium, Coax, Fiber, 10base, Wireless To Transmit bits over a medium; to provide Mechanical and electrical specifications

Physical Layer From data link layer To data link layer • One of the major function of the physical layer is to move data in the form of electromagnetic signals across a transmission medium. • Its responsible for movements of individual bits from one hop (Node) to next. • Both data and the signals can be either analog or digital. • Transmission media work by conducting energy along a physical path which can be wired or wireless • Concerned: • Physical characteristics of interface and medium (Transmission medium) • Representation of bits (stream of bits (0s or 1s) with no interpretation and encoded into signals) • Data rate (duration of a bit, which is how long it last) • Synchronization of bits (sender and receivers clock must be synchronized) • Line configuration (Point-to-Point, Point-to-Multipoint) • Physical topology • Transmission mode (Simplex, half duplex, full duplex) Physical layer Physical layer 110 10101000000010111 110 10101000000010111 Transmission medium

Data Link Layer (Host to Host) From network layer To network layer • Data link layer is responsible for moving frames from one hop (Node) to the next. • Concerned: • Framing (stream of bits into manageable data units) • Physical addressing (MAC Address) • Flow Control (mechanism for overwhelming the receiver) • Error Control (trailer, retransmission) • Access Control (defining master device in the same link) Data link layer Data link layer H2 Data T2 H2 Data T2 To physical layer From physical layer

Network Layer (Source to Destination) To transport layer From transport layer • The network layer is responsible for the delivery of individual packets from the source host to the destination host. • Concerned: • Logical addressing (IP Address) • Routing (Source to destination transmission between networks) Network layer Network layer H3 Data Packet H3 Data Packet To data link layer From data link layer

Transport Layer (Process to Process) From session layer From session layer • The transport layer is responsible for the delivery of a message from one process to another • Concerned: • Service-point addressing (Port address) • Segmentation and reassembly (Sequence number) • Connection control (Connectionless or connection oriented) • Flow control (end to end) • Error Control (Process to Process) Segments Segments Transport layer Transport layer H4 Data H4 Data H4 Data H4 Data H4 Data H4 Data To network layer From network layer

Session Layer (Dialog initiation) From Presentation layer To Presentation layer • The session layer is responsible for dialog control and synchronization • Concerned: • Dialog Control (Half Duplex/Full duplex) • Synchronization (Synchronization points, process inline within same page) H5 Data Data Data H5 Data Data Data Session layer Session layer Syn Syn Syn Syn Syn Syn To transport layer From transport layer

Presentation Layer (dependency) To application layer From application layer • The presentation layer is responsible for translation, compression and encryption • Concerned: • Translation (interoperability between different encoding system) • Encryption (Privacy schemes) • Compression (data compression) presentation layer H6 Data H6 Data presentation layer To session layer From session layer

Application Layer (user level service) USER (Human or Program) USER (Human or Program) • The application layer is responsible for providing services to the user. • Concerned: • Network virtual terminal (Software) • File transfer, access and management • Mail services • Directory services (access to distributed database sources for global information about various objects and services) X.500 FTAM X.400 X.500 FTAM X.400 Application layer Application layer H7 Data Message H7 Data Message To presentation layer From presentation layer

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OSI Seven Layers Model Explained with Examples

This tutorial explains the OSI reference model. Learn the seven layers of the OSI model and the functions of each layer in detail through examples.

The OSI (Open System Interconnection) reference model is a comprehensive set of standards and rules for hardware manufacturers and software developers. By following these standards, they can build networking components and software applications that work in any environment. It was published in 1984 by ISO (International Organization for Standardization).

It provides a framework for creating and implementing networking standards, devices, and internetworking schemes. It explains the networking from a modular perspective, making it easier to understand and troubleshoot.

Seven layers of the OSI Model

The OSI model has seven different layers, which are divided into two groups. The following table lists all the layers with their names and numbers.

Let’s understand each layer in detail.

This tutorial is the second part of the article " Networking reference models explained in detail with examples. ". Other parts of this article are the following.

This tutorial is the first part of the article. It summarizes why the OSI model was created and what advantages it has.

This tutorial is the third part of the article. It compares the OSI reference model with the TCP/IP model and lists the similarities and differences between both.

This tutorial is the fourth part of the article. It explains the five layers of the TCP/IP model in detail.

This tutorial is the fifth part of the article. It explains how data is encapsulated and de-encapsulated when it passes through the layers.

The Application Layer

This is the last and topmost layer of the OSI model. This layer provides an interface between the local system and the application program running on the network. If an application wants to use the resources available on the remote system, it interacts with this layer. Then, this layer provides the protocols and services that the application needs to access those resources.

There are two types of application programs: Network-aware and Network-unaware . An application program is considered a Network-aware application if it can make any type of network request. If an application program cannot make any type of network request, it is considered a Network-unaware program.

Network-aware programs are further divided into two types.

Programs that are mainly built to work on a local system. This type of program occasionally accesses the network for particular reasons such as updates, documentation, and troubleshooting. MS-Word, Adobe-Photoshop, and VLC Player are examples of this type of program.

Programs that are mainly built to work with a remote system. This type of program provides a platform to access resources available on a remote system. This type of program only works if the system is connected to the network. SSH, FTP, and TFTP are examples of this type of program.

The Application layer describes only the programs which fall in the second type. But it doesn’t mean that the first type of programs can’t take the advantage of the Application layer. It simply means that they are not documented in the Application layer. But if required, they can also connect to the network through the Application layer.

The Top layer of the OSI model is the application layer. It provides the protocols and services that are required by the network-aware applications to connect to the network. FTP, TFTP, POP3, SMTP, and HTTP are examples of standards and protocols used in this layer.

The Presentation Layer

The sixth layer of the OSI model is the Presentation layer. Applications running on the local system may or may not understand the format that is used to transmit the data over the network. The presentation layer works as a translator. When receiving data from the Application layer, it converts that data in such a format that can be sent over the network. When receiving data from the Session layer, it reconverts the data in such a format that the application, which will use it, can understand.

Conversion, compression, and encryption are the main functions that the Presentation layer performs on the sending computer while on the receiving computer these functions are reconversion, decompression, and decryption. ASCII, BMP, GIF, JPEG, WAV, AVI, and MPEG are examples of standards and protocols that work in this layer.

The Session Layer

The session layer is the fifth layer of the OSI model. It is responsible for setting up, managing, and dismantling sessions between presentation layer entities and providing dialogs between computers.

When an application makes a network request, this layer checks whether the requested resource is available on the local system or on a remote system. If the requested resource is available on a remote system, it tests whether a network connection to access that resource is available or not. If a network connection is not available, it sends an error message back to the application informing that the connection is not available.

If a network connection is available, it establishes a session with the remote system. For each request, it uses a separate session. This allows multiple applications to send or receive data simultaneously. When data transmission is completed, it terminates the session.

The session layer is responsible for establishing, managing, and terminating communications between two computers. RPCs and NFS are examples of the session layer.

The Transport Layer

The transport layer is the fourth layer of the OSI model. It provides the following functionalities: -

Segmentation

On the sending computer, it breaks the data stream into smaller pieces. Each piece is known as a segment and the process of breaking the data stream into smaller pieces is known as the segmentation . On the receiving computer, it joins all segments to recreate the original data stream.

Data transportation

This layer establishes a logical connection between the sending system and receiving system and uses that connection to provide end-to-end data transportation. This process uses two protocols: TCP and UDP.

The TCP protocol is used for reliable data transportation. TCP is a connection-oriented protocol. UDP protocol is used for unreliable data transportation. UDP is a connection-less protocol.

The main difference between a connection-less and connection-oriented protocol is that a connection-oriented protocol provides reliable data delivery. For reliable data delivery, it uses several mechanisms such as the three-way handshake process, acknowledgments, sequencing, and flow control.

Multiplexing

Through the use of port numbers, this layer also provides connection multiplexing. Connection multiplexing allows multiple applications to send and receive data simultaneously.

The main functionalities of the Transport layer are segmentation, data transportation, and connection multiplexing. For data transportation, it uses TCP and UDP protocols. TCP is a connection-oriented protocol. It provides reliable data delivery.

The Network Layer

The third layer of the OSI model is the Network Layer. This layer takes the data segment from the Transport layer and adds a logical address to it. A logical address has two components; network partition and host partition. The Network partition is used to group networking components while the host partition is used to uniquely identify a system on the network. A logical address is known as the IP address. Once the logical address and other related information are added to the segment , it becomes the packet .

This layer decides whether the packet is intended for the local system or a remote system. It also specifies the standards and protocols which are used to move data packets over networks.

To move data packets between two different networks, a device known as the router is used. Routers use the logical address to make the routing decision. Routing is the process of forwarding data packets to their destination.

Defining logical addresses and finding the best path to reach the destination address are the main functions of this layer. Routers work in this layer. Routing also takes place in this layer. IP, IPX, and AppleTalk are examples of this layer.

The Data Link Layer

The Data Link Layer is the second layer of the OSI model. This layer defines how networking components access the media and what transmission methods they use. This layer has two sub-layers: MAC and LLC.

MAC (Media Access Control)

This sub-layer defines how the data packets are placed in media. It also provides physical addressing. The physical address is known as the MAC address. Unlike logical addresses that need to be configured, physical addresses are pre-configured in NIC. The MAC address is used to uniquely identify a host in the local network.

LLC (Logical Link Control)

This sub-layer identifies the network layer protocol. On the sending computer, it encapsulates the information of the Network Layer protocol in the LLC header from which the Data Link layer receives the data packet. On the receiving computer, it checks the LLC header to get the information about the network layer protocol. This way, a data packet is always delivered to the same network layer protocol from which it was sent.

Defining physical addresses, finding hosts in the local network, specifying standards and methods to access the media are the primary functions of this layer. Switching takes place in this layer. Switches and Bridges work in this layer. HDLC, PPP, and Frame Relay are examples of this layer.

The Physical Layer

The Physical Layer is the first layer of the OSI model. This layer specifies the standards for devices, media, and technologies that are used in moving the data across the network such as:-

• Type of cable used in connecting the devices
• Patterns of pins used in both sides of the cable
• Type of interface-card used in the networking device
• Type of connector used to connect the cable with the network interface
• Encoding of digital signals received from the Data Link layer based on the attached media type such as electrical for copper, light for fiber, or a radio wave for wireless.

On the sending computer, it converts digital signals received from the Data Link layer, into analog signals and loads them on the physical media. On the receiving computer, it picks analog signals from the media and converts them into digital signals, and transfers them to the Data Link layer for further processing.

The Physical Layer mainly defines standards for media and devices that are used to move data across the network. 10BaseT, 10Base100, CSU/DSU, DCE, and DTE are examples of the standards used in this layer.

That’s all for this tutorial. In the next part of this article, we will compare the OSI model with the TCP/IP model and explains the similarities and differences between both models. If you like this tutorial, please don’t forget to share it with friends.

By ComputerNetworkingNotes Updated on 2024-10-16

ComputerNetworkingNotes CCNA Study Guide OSI Seven Layers Model Explained with Examples

• EtherChannel Load Distribution Explained
• Link Aggregation Control Protocol (LACP) Explained
• Port Aggregation Protocol (PAgP) Explained
• EtherChannel Manual Configuration
• EtherChannel Basic Concepts Explained
• STP, RSTP, PVST, RPVST, and MSTP
• Similarities and Differences between STP and RSTP
• RSTP / RPVST Explained with Examples
• PVST/RPVST and EtherChannel Explained
• STP/RSTP Timers Explained

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