Computer Network Fundamentals

Chapter 2. ISO/OSI Model

Why ISO/OSI Model?

1. Application Layer: responsible for sending data
   • It interacts with the user takes input from him and passes to the next 6 layers.
   • It receives the reply and displays it to the user.
   • It establishes end-to-end connections with the application layer on the other end and either sender or receiver as it's a logical layer between them.

1. Presentation Layer: responsible for coding data

   • Its task is to find mutually unified format of data that can be transmitted over the internet with different apps
   • perform encryption + compression

2. Session Layer:

   • Establishes connection and terminates connection
   • Always ensures data is correctly sent between devices
   • Manages and controls and session is active
   • Terminates session if data transfer is ended
   • Reestablishes the connection loop if fails

3. Transport Layer:

   • performs data segmentation and when received reassemble the data in the correct order and without duplication
   • Sequencing identifiers avoid losses and duplication in pieces of data.

   • If the order is not correct, the receiver can reorder them.

   • Connection oriented: we wouldn't send any data before ensuring that the connection loop exists between the sender and receiver. we ensure data is sent completely and without loss i.e.(email).

   • Connection-less oriented: (best-effort delivery) Data being sent can be disordered, repeated, or incomplete => not important The most important thing is to transfer the data in the least possible time.

4. Network Layer:
   receives data in segments form and converts it to => packets then,
   add 2 addresses source + destination
   (should be unique & changeable according to geolocation) logical address = IP address

5. Datalink Layer:
   logical link control (LLC)

   1. Converts the data (from packets => frames)
   2. Reliability (keep frames with data to be delivered to the receiver & if not resend)
      Frame check sequence (FCS) each frame has a specific cell containing
      a number at the time of sending it & LLC layer in the receiver should check the frame
      number and confirms if not, data is corrupted and requires
      the sender to resend this frame again from the sender
   3. Flow control
      1. Adjusting the speed of sending and receiving frames according to the state of the sender and receiver devices
      2. Media Access Control (MAC) which can connect to the physical layer and adds
         (sender & receiver physical address (should be unique) like logical address on the packets)

NOTE: physical address i.e.(MAC address = hardware address)
the physical address allows the frame to move from the device to the nearest switch and then, from the switch to the router.

1. Physical Layer:
   1) Responsible for placing data on the transmission media wired OR wireless
   2) Converts data from ones & zeros to electric form if cables were => copper
   3) Converts data from ones & zeros to pulses of light form if cables were => fiper optic
   4) Converts data from ones & zeros to electromagnetic if WIFI

   NOTE: The only layer which connects a real connection, not a virtual one.