Omnya Ashraf.Network (chp1 OSI MODEL.).pptx
- 1. chp1:OSI MODEL OSI model Data encapsulation and decapsulation within the OSI model context OSI MODEL The Open Systems Interconnection (OSI) was created in 1984 by ISO, is a reference framework that explains the process of transmitting data between computers. It is divided into seven layers that work together to carry out specialised network functions as shown in Figure
- 3. The seven layers of the OSI reference model are described in more detail 1. Physical Layer The physical layer is responsible for the physical cable or wireless connection between network nodes. It defines the connector, the electrical cable or wireless technology connecting the devices, and is responsible for transmission of the raw data, which is simply a series of 0s and 1s, while taking care of bit rate control
- 4. 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, star, 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.
- 5. 2.Data Link Layer – Layer 2 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. 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.
- 6. 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.
- 7. 3.Network Layer – Layer 3 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 & receiver’s IP addresses are placed in the header by the network layer.
- 8. 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 & 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 the Network layer is referred to as Packet. Network layer is implemented by networking devices such as routers and switches.
- 9. 4.Transport Layer – Layer 4 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.
- 10. Functions of the Transport Layer . • 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. • 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
- 11. 5.Session Layer – Layer 5 This layer is responsible for the establishment of connection, maintenance of sessions, and authentication, and also ensures security.
- 12. 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.
- 13. 6.Presentation Layer – Layer 6 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.
- 14. 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. Note: Device or Protocol Use: JPEG, MPEG, GIF.
- 15. 7.Application Layer – Layer 7 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. Example: Application – Browsers, Skype Messenger, etc. Note: The application Layer is also called Desktop Layer. Device or Protocol Use : SMTP.
- 16. Functions of the Application Layer The main functions of the application layer : 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. Note: The OSI model acts as a reference model and is not implemented on the Internet because of its late invention. The current model being used is the TCP/IP model.
- 17. 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.
- 18. Disadvantages of OSI Model Complexity: The OSI Model has seven layers, which can be complicated and hard to understand for beginners. Not Practical: In real-life networking, most systems use a simpler model called the Internet protocol suite (TCP/IP), so the OSI Model isn’t always directly applicable. Slow Adoption: When it was introduced, the OSI Model was not quickly adopted by the industry, which preferred the simpler and already-established TCP/IP model. Overhead: 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. Theoretical: The OSI Model is more of a theoretical framework, meaning it’s great for understanding concepts but not always practical for implementa
- 19. Data encapsulation and decapsulation within the OSI model context Data encapsulation and de-encapsulation are fundamental concepts in computer networking and communication protocols. These processes are essential for transferring data across networks efficiently and securely. Encapsulation is the process of adding additional information when data is traveling in an OSI or TCP/IP model. The information has been added on the sender’s side, from the Application layer to the Physical layer. OSI Model and Working of Encapsulation No additional information will be added to the user’s data in the Application layer in the TCP/IP model or the Application, Presentation, or Session layers in the OSI model. The Session layer sends data to the Transport layer. In the Transport layer, the data is broken up into different pieces. It adds the header in each of the broken data, which contains information like source port, destination port, sequence number, etc. Now, everything is combined into a new form. The encapsulated data in the Transport layer is called Segments or Datagrams. If the transmission uses TCP, then it is called Segments, or UDP is called Datagrams.
- 20. De-encapsulation is the exact reverse process of encapsulation. The additional information added on the sender’s side(during encapsulation) gets removed when it travels on the receiver’s side from the Physical layer to the Application layer. OSI Model and Working of De-encapsulation • The Physical layer gets the bits and de-encapsulates them into frames and sends them to the upper layer. • The Data Link layer receives the frames and checks MAC address whether it is matching or not. If everything matched and also no error is found. Then layer 2 header and layer 2 trailer are removed. It de-encapsulates the data and packet sent to upper layer. • The Network layer receives the packet from Data Link layer. It checks for IP addresses. If it matched then, the header in layer 3 is removed. Now the de-encapsulated data packet is delivered to the Transport layer. • Now, the Transport layer gets the data segments/ datagrams from the Network layer and removes the layer 4 header. • After traveling through Session, Presentation, and Application layer, the de-encapsulated data is sent to the receiver. • That is how the de-encapsulation takes place.
- 21. Note: One important piece of information to keep in mind is that data flows 2 ways in the OSI model, DOWN (data encapsulation) and UP (data decapsulation). The picture below is an example of a simple data transfer between 2 computers and shows how the data is encapsulated and decapsulated: