What Is the OSI Model?
OSI stands for Open Systems Interconnection. It describes seven layers that computer systems use to communicate over a network. It was the first standard model for network communications, adopted by all major computer and telecommunication companies in the early 1980s.
The modern Internet is not based on OSI model, but on the simpler TCP/IP model. However, the OSI 7-layer model is still widely used, as it helps visualize and communicate how networks operate, and helps isolate and troubleshoot networking problems.
OSI model was introduced in 1983 by representatives of the major computer and telecom companies, and was adopted by ISO as an international standard in 1984.
OSI Model Explained: The OSI model 7 Layers
The OSI model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software.
In the OSI model, the communications between a computing system are split into 7 different abstraction layers.
These 7 layers of OSI model are:
- Physical Layer
- Data Link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
1. Physical Layer
This layer includes the physical equipment involved in the data transfer, such as the cables and switches.
This is also the layer where the data gets converted into a bit stream, which is a string of 1s and 0s.
The physical layer of both devices must also agree on a signal convention so that the 1s can be distinguished from the 0s on both devices.
The functions of the physical layer in OSI model 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 bit level.
- Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second.
- Physical topology: 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.
* Hub, Repeater, Modem, Cables are Physical Layer devices. * Network Layer, Data Link Layer and Physical Layer are also known as Lower Layers or Hardware Layers.
2. Data Link Layer
At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. It also corrects errors that may have occurred at the physical layer.
Data Link Layer is divided into two sub layers :
- Logical Link Control (LLC)
- Media Access Control (MAC)
The packet received from Network layer is further divided into frames depending on the frame size of NIC(Network Interface Card). Data Link Layer 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.
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, Data link layer adds physical addresses (MAC address) of 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 re-transmits 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 that amount of data that can be sent before receiving acknowledgement.
- Access control: When a single communication channel is shared by multiple devices, MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time.
* Packet in Data Link layer is referred 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.
3. Network Layer
The network layer is responsible for receiving frames from the data link layer, and delivering them to their intended destinations based on the addresses contained inside the frame. The network layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to quite literally route information where it needs to go between networks.
The functions of the Network layer are :
- Routing: The network layer protocols determine which route is suitable from source to destination. This function of network layer is known as routing.
- Logical Addressing: In order to identify each device on internetwork uniquely, network layer defines an addressing scheme. The sender & receiver’s IP address are placed in the header by network layer. Such an address distinguishes each device uniquely and universally.
* Segment in Network layer is referred as Packet. * Network layer is implemented by networking devices such as routers.
4. Transport Layer
The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. 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 acknowledgement of the successful data transmission and re-transmits the data if an error is found. One of the most common examples of the transport layer is TCP or the Transmission Control Protocol.
- At sender’s side:
- Transport layer receives the formatted data from the upper layers
- Performs Segmentation
- Implements Flow & Error control to ensure proper data transmission
- Adds Source and Destination port number in its header and forwards the segmented data to the Network Layer.
The sender need to know the port number associated with the receiver’s application. Generally, this destination port number is configured, either by default or manually. For example, when a web application makes a request to a web server, it typically uses port number 80, because this is the default port assigned to web applications. Many applications have default port assigned.
- At receiver’s side:
- Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application.
- Performs sequencing and reassembling of the segmented data.
The functions of the transport layer are :
- Segmentation and Reassembly: This layer accepts the message from the (session) layer , breaks the message into smaller units . Each of the segment produced 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 correct process, transport layer header includes a type of address called service point address or port address. Thus by specifying this address, transport layer makes sure that the message is delivered to the correct process.
The services provided by the transport layer :
- Connection Oriented Service: It is a three-phase process which includes
- Connection Establishment
- Data Transfer
- Termination / disconnection
In this type of transmission, the receiving device sends an acknowledgement, back to the source after a packet or group of packet is received. This type of transmission is reliable and secure
- Connection less service: It is a one-phase process and includes Data Transfer. In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connection-less Service.
* Data in the Transport Layer is called as 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 OSI model.
5. Session Layer
When two devices, computers or servers need to “speak” with one another, a session needs to be created, and this is done at the Session Layer. Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.
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 as synchronization points into the data. These synchronization point 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.
* All the below 3 layers(including Session Layer) are integrated as a single layer in the TCP/IP model as “Application Layer”. * Implementation of these 3 layers is done by the network application itself. These are also known as Upper Layers or Software Layers.
6. Presentation Layer
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.
The functions of the presentation layer are :
- 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 cipher text 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.
7. Application Layer
To further our bean dip analogy, the Application Layer is the one at the top -it’s what most users see.
In the OSI model, this is the layer that is the “closest to the end user”.
It receives information directly from users and displays incoming data it to the user. Oddly enough, applications themselves do not reside at the application layer. Instead the layer facilitates communication through lower layers in order to establish connections with applications at the other end. Web browsers (Google Chrome, Firefox, Safari, etc.) TelNet, and FTP, are examples of communications that rely on Layer 7.
The functions of the Application layer are :
- Network Virtual Terminal
- FTAM-File transfer access and management
- Mail Services
- Directory Services
* Application Layer is also called as Desktop Layer.
OSI model acts as a reference model and is not implemented in the Internet because of its late invention.
Current model being used is the TCP/IP model.
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