OSI Model

The OSI model was created by the ISO. It's based on the concept of splitting up a communication system into 7 layers, each one stacked upon the last.

Layer 7: Application

  • The only layer that directly interacts with data from the user.
  • Client software applications (browsers, email clients) are NOT part of the application layer.
  • The application layer is responsible for protocols and data manipulation, including HTTP and SMTP.

Layer 6: Presentation

  • Prepares data so that it can be used by the application layer.
  • Responsible for translation, encryption, and compression of data.
  • If two communicating devices are using different encoding methods, layer 6 translates incoming data into a syntax that the receiving device can understand.
  • Over an encrypted connection. layer 6 is responsible for adding encryption on the sender's end and decrypting on the receiver's end.
  • Also will compress data received from layer 7 before sending it to layer 5.

Layer 5: Session

  • Responsible for opening and closing communication between two devices.
  • Time between open and close is called the session.
  • Also synchronizes data transfers with checkpoints, so that if a 100MB is being transferred and certain packets are lost, the network has a way to resume the transfer at the last checkpoint.

Layer 4: Transport

  • Responsible for end-to-end communication between two devices.
  • Takes data from the session layer and breaks it up into chunks called segments before sending it to layer 3.
  • The transport layer on the receiving device is responsible for reassembling segments.
  • Responsible for flow control and error control.
    • Flow control is ensuring that a sender with a fast connection doesn't overwhelm a receiver with a slow connection.

Layer 3: Network

  • Facilitates the data transfer between two different networks. If devices are on the same network, then this layer is unnecessary.
  • Network layer takes segments and breaks them up further, into packets, on the sender's device, then reassembles them on the receiving device.
  • This layer is also responsible for finding the best physical path for data to reach its destination (aka routing).
  • Similar to network layer, except it facilitates data transfers between two devices on the SAME network.
  • Takes packets from the network layer and breaks them into even smaller pieces called frames.
  • Also responsible for flow control and intra-network communication. (Transport layer only does flow control for inter-network comunications.)

Layer 1: Physical

  • Includes the physical equipment involved in the data transfer, i.e., cables and switches.
  • Frames are chopped up to its smallest unit - a bit stream. The physical layer of both devices must agree on a signal convention so that the bits can be distinguished on both devices (i.e., a 1 is a 1 and a 0 is a 0).