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OSI (Open System Interconnection) Model

  • it defines and used to understand how data is transferred from 1 computer to another in a computer network
  • in its basic form, 2 computers are connected via a LAN (Local Area Network) cable and connector (RJ45)
    • shares data with the help of NIC (Network Interface Controller) card
    • this forms the computer network

Problem​

  • if 1 computer uses Windows OS, and another uses Mac OS, how would they communicate?

Solution​

  • OSI Model was introduced by ISO (International Organization for Standardization) in 1984

OSI Model contains 7 layers​

  • each layer is a package of protocols alt text

Application layer (7)​

  • this layer is used by network applications
    • network application means computer apps that uses internet
      • computer apps e.g.: chrome, firefox, outlook, skype, etc.
        • these apps are a network app that runs in your PC and does not reside in the application layer
          • but uses application layer protocols to use the internet

application layer protocols​

  • examples
    • HTTP (Hypertext Transfer Protocol)
    • HTTPS (Hypertext Transfer Protocol Secure)
    • FTP (File Transfer Protocol)
    • NFS (Network File System)
    • FMTP (Flight Message Transfer Protocol)
    • DHCP (Dynamic Host Configuration Protocol)
    • SNMP (Simple Network Management Protocol)
    • Telnet
    • SMTP (Simple Mail Transfer Protocol)
    • POP3 (Post Office Protocol version 3)
    • IRC (Internet Relay Chat)
    • NNTP (Network News Transfer Protocol)
  • the application layer protocols forms the basis for various network services
    • File Transfer: uses FTP
    • Web Surfing: HTTP/S
    • Emails: SMTP
    • Virtual Terminals: Telnet

Summary​

  • application layer provides services for network applications with the help of protocols to perform user activities

Presentation layer (6)​

  1. Translation
    • receives data in the form of alphanumeric characters from Application layer
    • Presentatin layer then translate the data into binary code format for the machine to understand
      • e.g.: convert ASCII (American Standard Code for Information Interchange) to EBCDIC (Extended Binary Coded Decimal Interchange Code)
  2. Data Compression
    • before translated data is transmitted, presentation layer will reduce the number of bits that were used to represent the original data
    • can be in 2 type of formats
      1. Lossy
      2. Lossless
    • it reduces the amount of space used to store the original file
      • e.g.: a 5 MB file into a 2 MB file
    • when space is reduced, the amount of time required to transmit also reduces, thus allow data transmission to be faster
      • useful for realtime audio and video streaming
  3. Encryption / Decryption
    • use to maintain the integrity of data before transmission
    • it helps to enhance the security of sensitive data
      • data gets encrypted on the sender side, and gets decrypted on the receiver side
    • examples:
      • SSL (Secure Sockets Layer) is used for encryption and decryption

Session layer (5)​

  • it helps in setting up and managing connections enabling sending and receiving of data
    • followed by termination of connections or sessions
  • it has its own helpers called APIs (Application Programming Interface)
    • examples:
      • NetBIOS (Network Basic Input/Output System)
        • it is an API example that allows applications on different computers to communicate with each other
  1. Authentication
    • done before a session or connection is established with the server
    • it is a process of verifying who you are
      • for this, server uses username and password
      • when matched with the server, a connection or session is established between the server and your computer
  2. Authorization
    • after authentication, authorization is checked
    • it is a process used by the server to determine if you have permission to access a file
  3. Session Management
    • session layer keeps track of the files being downloaded
      • webpage example
        • it contains text and image files, and are stored as separate files on the web server
        • when you request a website on the web browser, it opens a separate session to the web server to download each of the text and image files separately
        • files are received at the computer in the form of Data Packets
        • Session layer keeps tracks of which data packets belongs to which file and where it goes to (web browser for this case)
  • the web browser performs all functions of Application, Presentation, and Session layer

Transport layer (4)​

  • controls the reliability of communication

Segmentation​

  • data received from the Session layer is divided into small data units called Segments
    • each Segment or data unit contains
      • a source and destination Port number
        • helps to direct the segment to the correct computer application
      • a sequence number
        • helps to reassemble segments in the correct order to form correct message at the receiver

Flow Control​

  • controls the amount of data being transmitted
  • example: a mobile app connected to a server
    • server can transmit a max data of 100 Mbps
    • mobile can process data with a max of 10 Mbps
    • when mobile is downloading a file from a server
      • if server sends data at 50 Mbps
        • it is a a rate the mobile can't process
        • mobile uses the Transport layer to tell the server to slow the data transmission rate to 10 Mbps so that no data gets lost
      • if server sends data at 5 Mbps
        • mobile uses the Transport layer to tell the server to increase the data trasmission rate to 10 Mbps to maintain system performance

Error Control​

  • Automatic Repeat Request
    • this is used when a data unit did not arrive at the destination
    • it helps to retransmit the lost or corrupted data unit from the server to the app
  • Checksum
    • it is added to each data unit by the Transport layer to find out the received corrupted or missing segment

transport layer performs 2 types of services​

  1. Connection-oriented Transmission
    • done via TCP (Transmission Control Protocol)
      • more reliable than UDP
        • because it provides feedback
          • you will know of the data delivery status
          • data can be retransmitted if lost or corrupted
      • used when whole data delivery is a must
        • www (world wide web), email, FTP
  2. Connectionless Transmission
    • done via UDP (User Datagram Protocol)
      • UDP (User Datagram Protocol)
        • faster than TCP
          • because it provides no feedback
            • you do not know of the data delivery status
        • used when receiving of whole data is not necessary
          • e.g.: online streaming movies, songs, games, voice over ip, TFTP (Trivial File Transfer Protocol), DNS (Domain Name System)

Network layer (3)​

  • Transport Layer sends data segments to the network layer
  • Network Layer works for the transmission of the recieved data segments from one computer to another located in different networks
  • Data Units in Network Layer is called Data Packets
  • The Function of Network Layer are:
  1. Logical Addressing
    • IPv4 & IPv6 + Mask
      • every computer in a network has a unique IP address
      • Network layer assigns sender and receiver IP address to each segment to form an IP Packet
        • IP address are assigned to ensure each data packet can reached the correct destination
      • Mask tells that the first 3 combination of an IP address represent the which network
        • the last combination of the IP address represents which host or computer
  2. Routing
    • it is a method of moving data packet from source to destination
    • it is based on the logical addressing format (IP addresses and Mask), routing decisions are made in the computer network
  3. Path Determination
    • a computer can be connected to an internet server or a computer in a number of ways
      • choosing the best possible path for delivery from source to destination is called path determination
    • examples of protocols used to determine the best possible path for delivery
      • OSPF (Open Shortest Path First)
        • it is a link-state routing protocol
      • BGP (Border Gateway Protocol)
        • it is a standardized exterior gateway protocol
      • IS-IS (Intermediate System to Intermediate System)
        • it is a routing protocol
  • Data link layer receives data packets from network layer
    • Data packets contains IP addresses of sender and receiver
      • There are two types of addressing:
        1. Logical Addressing
          • done at the Network layer
          • where senders and receivers IP addresses are assigned to each segments to form a data packets
        2. Physical Addressing
          • done at the Data Link layer
          • head is added infront of the IP packet, while the tail is added behind
          • where MAC addresses of sender and receiver are assigned to each data packets to form a frame
          • MAC Address is 12 digit Alpha-Character Number Data unit in data link layer is called Frame.
            • it is embedded into the network card by the manufacturer
  • Data Unit in Data Link layer is referred to as frame
    • it is embedded as a software in NIC (Network Interface Card) of the computer
      • it provide means to transfer data from 1 computer to another via a local media
        • such as copper wire, optical fiber, and air
        • media refers to the physical links between 2 computer or networks

performs 2 basic function​

  1. allows upper layer of OSI Model to access the media
    • via techniques such as framing
      • IP packet gets encapsulated into a frame before sending, upon receiving it gets decapsulated
        • this process repeats itself until the data reaches its final destination
  2. controls how data is placed and received from the media
    • via techniques such as media access control, and error detection
      • multiple devices could be connected via a common media
        • if 2 or more devices sends data at the same time
          • there will be a possibility of collision of the 2 messages resulting in a useless message that no recipient can understand
            • use CSMA (Carrier-Sense Multiple Access) to avoid such a situation
              • it is a (MAC) Media Access Control protocol
              • keeps an eye on when the shared media is free so that device can transmit data to the receiver
              • this helps to control data transmission
            • each frame contains bits which are used to detect errors in the receiver frame
              • errors occur due to certain limitations of the media used for transporting data

Physical layer (1)​

  • Till now, Data from Application Layer has been segmented by Transport Layer - placed into Packets by Network Layer
  • and Framed by Data Link Layer
    • which is a sequence of binary 0's and 1's
  • physical layer converts this binary bits into signals and transmit over local media
  • It can be an electrical signal in the case of Copper cable or LAN cable, Light signal in the case of Optical Fiber, and Radio Signal in the case of AIR
    • So signal generated by physical layer depends on the type of media used to connect two devices
  • At the receiver, Physical Layer receives signal and converts to bits and pass it to Data Link Layer as a Frame
  • Frame is further decapsulated as data moves through higher layers
  • Finally,data is moved to Application Layer
  • Application Layer Protocol makes the sender's message visible in the application in the receiver's computer screen
  • In this way, OSI model is helping to transfer data between different hosts
    • So, these Seven Layers of OSI model are lying behind the smooth functioning of Internet