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What Is The OSI Layer?

OSI layer is also known as OSI Model. The International Organization for Standardization devised the open systems interconnection (OSI) model, which allows different communication systems to communicate using standard protocols. In layman's terms, the OSI establishes a standard for computer systems to communicate with one another. In the year 1984. It is a 7 layer architecture with each layer having specific functionality to perform. All these 7 layers work collaboratively to transmit the data from one person to another across the globe.

The OSI Model can be thought of as a universal computer networking language. It's based on the idea of breaking down a communication system into seven abstract layers that are piled on top of each other.

Each layer of the OSI Model handles a specific job and communicates with the layers above and below itself. The application layer attacks target layer 7, while the protocol layer attacks target layers 3 and 4.

Why Does The OSI Model Matter Or Still Relevant Today?

1. The OSI enables you to spot vulnerabilities throughout your whole technology stack. The OSI model has been used for decades to help IT experts understand networking and troubleshoot issues that may arise at any stage in the networking process.

2. You can use the OSI model to keep a data-centric security posture. With the OSI model providing a framework for conducting an inventory of your organization's assets, it's also useful in helping you understand where the biggest data security risks lie within your organization.

3. The OSI model is critical for fostering a security-first approach to cloud adoption. Given that the OSI model can be indispensable for conducting an inventory of your security resources and assets, it's no surprise that keeping the OSI model in mind when migrating to the cloud can also be useful. This is because the OSI model will help you understand the specific types of data security risks that cloud adoption might bring to your organization. This allows you to be more strategic about the types of cloud systems you adopt.

4. The OSI Model is still very useful for debugging network problems, even if the modern Internet does not fully follow it (it instead follows the simpler Internet protocol suite). The OSI Model can assist break down a problem and finding the cause of what's down for thousands of users. A lot of unneeded work can be avoided if the problem can be focused down to one specific layer of the model.

What Are The Seven Layers Of the OSI Model?

The seven abstraction layers of the OSI model can be defined as follows, from bottom to top:

v Layer 1 – Physical Layer

v Layer 2 – Data Link Layer

v Layer 3 – Network Layer

v Layer 4 – Transport Layer

v Layer 5 – Session Layer

v Layer 6 – Presentation Layer

v Layer 7 – 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 which is a string of 1s and 0s. It 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.

The functions of the physical layer are:

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 the way in which the different, devices/nodes are arranged in a network i.e. bus, star, or mesh topology.

Transmission mode: Physical layer also defines the way in which the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full-duplex.

Note:
*Hub, Repeater, Modem, and Cables are Physical Layer devices.

**Network Layer, Data Link Layer, and Physical Layer are also knowns as Lower Layers or Hardware Layers.

👉LAYER 2 - Data Link Layer (DLL)

The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the SAME network. The data link layer takes packets from the network layer and breaks them into smaller pieces called frames. Like the network layer, the data link layer is also responsible for flow control and error control in intra-network communication (The transport layer only does flow control and error control for inter-network communications)

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 the packet arrives in a network, it is the responsibility of the DLL to transmit it to the Host using its MAC address. Data Link Layer is divided into two sublayers:

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

The functions of the Data Link layer are:

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 address) of the sender and/or receiver in the header of each frame.

Error Control: 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 devices have control over the channel at a given time.

Note:
*Packet in Data Link layer is referred to as Frame
**Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines.
***Switch & Bridge are Data Link layer devices.

👉LAYER 3 - Network Layer

The network layer is responsible for facilitating data transfer between two different networks. If the two devices communicating are on the same network, then the network layer is unnecessary. The network layer breaks up segments from the transport layer into smaller units, called packets, on the sender's devices, and reassembles these packets on the receiving device. The network layer also finds the best physical path for the data to reach its destination; this is known as routing.

The functions of the Network layer are:

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: In order to identify each device on internetwork 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.

Note:

*Segment in Network layer is referred to as Packet

👉LAYER 4 - Transport Layer

Layer 4 is responsible for end-to-end communication between the two devices. This includes taking data from the session layer and breaking it up into chunks called segments before sending it to layer 3. The transport layer on the receiving device is responsible for reassembling the segments into data the session layer can consume.

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.

The functions of the transport layer are:
Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the message into smaller units. Each of the segments produces has a header associated with it. The transport layer at the destination station reassembles the message.

Service Point Addressing: In order 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.

Note:
*Data in the Transport Layer is called Segments.
**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.

***Transport Layer is called as Heart of the OSI model.

👉LAYER 5 - Session Layer

This is the layer responsible for opening and closing communication between the two devices. The time between when the communication is opened and closed is known as the session. The session layer ensures that the session stays open long enough to transfer all the data being exchanged, and then promptly closes the session in order to avoid wasting resources. This layer also is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.

The functions of the session layer are:

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 which 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.

Note:
*All the below 3 layers (including Session Layer) are integrated as a single layer in the TCP/IP model as "Application Laye".

**Implementation of these 3 layers is done by the network application itself. These are also known as Upper Layers or Software Layers.

👉LAYER 6 - Presentation Layer

The presentation layer is also known as the Translation layer. The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. This layer is primarily responsible for preparing data so that it can be used by the application layer; in other words, layer 6 makes the data presentable for applications to consume. The presentation layer is responsible for the translation, encryption, and compression of data.

The functions of the presentation layer are:

Translation: such as ASCII to EBCDIC.

Compression: Reduces the number of bits that need to be transmitted on the network.

Encryption/Decryption: Data encryption translates the data into another form or code. The encryption 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.

👉LAYER 7 - Application Layer

The application layer is also known as the desktop 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, which has to be transferred over the network. This layer also serves as a window for the application services to access the network and display the received information to the user.
Ex: Application - Browsers, Skype Messenger, etc.

The function of the Application layer are:
Network Virtual Terminal
FTAM-File transfer access and management
Mail Services
Directory Services

How data flows through the OSI Model?

A Cobham's college student is trying to access the OnlineLearning System located in the Cobham's College network facilities. The student is using her modem to connect to the public network before able to connect to the Cobham's College facilities. From the Cobham's College network, the data then was forwarded to IT Center where the OnlineLearning System server located.

In order for human-readable information to be transferred over a network from one device to another, the data must travel down the seven layers of the OSI Model on the sending device and then travel up the seven layers on the receiving end.

Data flow from layer 7 down to layer 1 from the sender and then flows from layer 1 to layer 7 on the recipient device. The simplest example of communication flow through the OSI Model is an email application.

When a sender clicks "Send" on an email application, the message is sent to the presentation layer using a defined protocol (SMTP for outgoing email). The presentation layer compresses the data and sends the message to the session layer, which opens a session for communication between the sender's device and the outgoing server.

The message is sent to the transport layer where data is segmented, and then the network layer breaks the segments into packets. Then, the packets are sent from the network layer to the data link layer, where packets are further broken down into frames. The frames are sent to the physical layer where data is converted to bitstreams of ones and zeros and transferred across a medium such as wireless connections or cables.

When the message reaches the recipient, the process is reversed. Data is sent from the physical layer to the application layer, where data is converted from the bitstreams ones and zeros to the message available in the recipient's email client. When a message is sent back to the sender, the process is repeated, and communication flows down to layer 1 from layer 7 and back up the OSI Model when it reaches the recipient's device.

Based on the scenario: Explain how the data travels from the lecturer’s computer to reach the OnlineLearning System server which is in the Cobham College IT Center. The explanation should be related to the OSI Layer Model. Explain the possible network components involves throughout the communications. Illustrate the diagram to support your explanation.

APPLICATION LAYER

A Cobham College Lecturer type in the system website address and click the enter button on the browser, the link address send to the presentation layer using a defined protocol HTTP or HTTPS.

PRESENTATION LAYER

Receives link address from the application layer and converts the link address to machine-understandable binary formats such as 1s and 0s.
Reduce the number of bits that are used to represent the original data which is called Data Compression.
Encrypted the data before the transmission of data to the session layer.

SESSION LAYER

Setting up and managing connections that will enable sending data from the lecturer to OnlineLearning System
Server will authenticate the lecturer by require a user ID and Password, if both are correct a session is established student's computer and server.