OSI Reference Model with Function of Different Seven-layer

• OSI(OpenSystemsInterconnection) model was created by the International Organization for Standardization (ISO) in 1994. It was designed to be a reference model for describing the functions of a communication system.
• The OSI model describes how information makes its way from application programs through a network medium to another application program on another computer.
• The OSI model provides a framework for creating and implementing networking standards and devices and describes how network applications on different computers can communicate through the network media.
• The OSI Model is a model that illustrates how data communications should take place. It segregates the process into seven groups, called layers.
• The layered approach to network communications provides the following benefits:
  • reduced complexity.
  • improved teaching and learning
  • modular engineering
  • accelerated evolution
  • interoperable technology
  • standard interface

Layers of OSI reference model
Seven layers are
•Application Layer
• Presentation Layer
• Session Layer
• Transport layer
• Network Layer
• Data-link Layer
• Physical Layer
The acronym used to remember these layers is:
•All People Seem To Need Data Processing

1. Application layer
   • The application layer is responsible for application type functions. To allow users to access the network resources.
   • Provides a set of interfaces for sending and receiving applications to gain access to and use network services like message handling, database query processing, file access, transfer, etc.
   • Determines the identity and availability of communication parameters and determines if sufficient resources are available to start the programs.
   • Service provides to the presentation layer while sending and the user while receiving.
   • The interface takes place between the user and the presentation layer.

• Some protocols used in this layer are:
  • HTTP, HTTPS, DHCP, FTP, POP, POP3, SMTP, TFTP, DNS, TELNET etc
    • HTTP stands for hypertext Transfer Protocol. Port 80
    • It is a protocol used to access the data on the World Wide Web (www).
    • The HTTP protocol can be used to transfer the data in the form of plain text, hypertext, audio, video, and so on.
  • TELNET: port 23
    • Telnet stands for theTELecomunicationsNETwork. It helps in terminal emulation. It allows the Telnet clients to access the resources of the Telnet server
    • telnet [\RemoteServer]
  • FTP: file transfer protocol protocol:21
    • It is the protocol that actually lets us transfer files
  • NFS(Port 111 for TCP and UDP and 2049 (TCP and UDP) for the NFS server.)
    • It stands for a network file system. It allows remote hosts to mount file systems over a network and interact with those file systems as though they are mounted locally. This enables system administrators to consolidate resources onto centralized servers on the network
  • SMTP: port 25, The Simple Mail Transfer Protocol, which delivers electronic mail.
  • Domain Name Service (DNS): port 53
    • Also called name service, this application maps IP addresses to the names assigned to network devices.
• Presentation layer:
  • The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems
  • The presentation layer is responsible for
    • Coding and conversion of application layer data.
    • Character code translation from ASCII to EBCDIC.
    • data encryption and decryption,
    • data compression and decompression,
    • Allows to reduce the number of bits that needs to be transmitted on the network
    • data translation.
  • Ensures that information sent from the application layer of one system is readable by the application layer of another system.
• Session Layer:
  • The Session Layer allows users on different machines to establish active communication sessions between them(between end-user application processes).
  • Its main aim is to establish, maintain and synchronize the interaction between communicating systems/ two different applications. The session layer is responsible for session establishment, maintenance, and release of the established connection
  • This layer is responsible for inter-host communication.
  • The session layer offers provisions for efficient data transfer
  • Responsible for security and access control to session.
  • Protocols:
    • NFS
    • SMP
    • RPC
  • Function
  • Synchronization
  • It synchronizes communication. It adds synchronization points or checkpoints in data streams for long communications. This ensures that data streams up to the checkpoints are successfully received and acknowledged.
  • Dialog control
  • This layer allows two systems to start communication with each other in half-duplex or full-duplex.
  • Token management:
  • This layer prevents two parties from attempting the same critical operation at the same time
  • Establish, manage, and end-user connections.
  • The layer allows the two processes to establish, use and terminate a connection.
• Transport layer:
  • It provides logical communication between application processes running on different hosts within a layered architecture of protocols and other network components
  • It divides the message received from the session layer into segments and numbers them to make a sequence.
  • The transport layer is responsible for data segmentation and reliable data delivery.
  • Sometimes when data is passed from the transport layer to the network layer, the data passed may be too large for the network layer to handle. In such a case, the transport layer will segment or divide the data packet into two or more parts
  • When data segments are passed from the network layer to the transport layer, the transport layer will combine the segments into a single data packet.
  • Another function of the transport layer is reliable data delivery. This ensures data reception
  • Transmit data in two method
    • Guaranteed.
    • No guaranteed
  • 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 OSI model.”
  • Function and protocols
  • The process to process the delivery of a message.
  • It delivers a message from a specific process of one computer to a specific process in another computer
  • Error detection and recovery
  • It identifies errors like damaged packets, lost packets, and duplication of packets, and provides adequate error-correction techniques.
  • Segmentation and reassembly
  • It divides a message into smaller segments such that each segment contains a sequence number along with the port address.
  • It ensures that the segments arrive correctly at the receiver’s end and then reassembles them.
  • Flow control and error control
  • The transport layer is also responsible for the management of error correction, providing quality and reliability to the end user.
  • Protocols used are:
    • TCP
    • UDP
    • SPX (Sequenced Packet Exchange)
• Network layer
  • indifferent networks, It takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available.
  • When packets go from system to system, a route must be established so that the packets can get from the source to the destination.
  • Network layer protocols find these routes and maintain a table of possible routes for later usage.
  • A router is a network layer device.
  • The network layer is responsible for route discovery and logical addressing.
  • Routing: The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.
    • Receives a packet from the transport layer
    • Finds the reliable and most efficient shortest path
    • Forward the packet to that path.
  • Logical Addressing: In order to identify each device on internetwork 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.
  • DDP : Datagram Delivery Protocol
  • DVMRP: Distance Vector Multicast Routing Protocol
  • IGMP: Internet Group Management Protocol
  • IPsec: Internet Protocol Security
  • OSPF: Open the shortest path first
  • ARP: ARP stands for Address Resolution Protocol.
    • It is used to associate an IP address with the MAC address.
  • RIP: Routing Information Protocol
  • Ipv4: IPv4 is a 32-bit addressing scheme used as a TCP/IP host addressing mechanism. IP addressing enables every host on the TCP/IP network to be uniquely identifiable
  • IPv6: IPv6 is the latest version of the Internet Protocol, a network layer protocol that possesses addressing and control information for enabling packets to be routed in the network
  • ICMP: Internet Control Message Protocol
    • used by network devices to send error messages and operational information.
• Data Link Layer
  • The data link layer is responsible for data framing, error control, and physical addressing.
  • When data is received at this layer, the packet is “framed” for transmission.
  • A parity bit or a Cyclic Redundancy Check (CRC) field can be added for data error checking.
  • Each system, connected to the network has a unique physical address. This address is usually “burnt-in” into the computer’s network card. It is a physically established number and therefore considered the computer’s physical address.
  • Again, each system must have a unique physical address. Sometimes, this physical address is referred to as the Media Access Control (MAC) address.
  • Examples of DLL LAN are
    • Frame relay(operates also on a physical layer)
    • PPP(on the physical layer also)
    • X.25(also on physical and network layer).
  • Bridge and switch operate at this layer.
  • 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 addresses) 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 device has control over the channel at a given time.
  • Sequencing of frame
  • Two sublayers of the data link layer are
    • LLC(Logical Link Control) sublayer
    • MAC(Media access control) sublayer.
  • LLC
    • manages communication between devices over a single link of a network.
    • Enables multiple higher-layer protocols to share a single physical data link.
    • This sublayer multiplexes protocols running atop the data link layer, and optionally provides flow control, acknowledgment, and error recovery.
    • The LLC provides addressing and control of the data link.
    • It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the originator and recipient machines
  • MAC
    • Manages protocol access to the physical network medium
    • Determine hardware address.
    • this refers to the sublayer that determines who is allowed to access the media at any one time
    • The Media Access Control sublayer also determines where one frame of data ends and the next one starts
• Physical layer
  • The physical layer consists of the basic networking hardware transmission technologies of a network. This level establishes the relationship between a device and a physical transmission medium.
  • This layer is responsible for Encoding and Signaling.
  • The physical layer is responsible for electrical and mechanical specifications.
  • Physical layer protocols will transmit or receive the data across the media.
  • Services:
  • Bit-by-bit or symbol-by-symbol delivery
  • Modulation
  • Line coding
  • Bit synchronization
  • Start-stop signaling
  • Circuit switching
  • Multiplexing
  • Carrier sense and collision detection
  • Physical network topologies, like bus, ring, mesh, or star network