OSI Layers(Open Systems Interconnection) and associated Protocol Data Unit(PDU)

• OSI Model is a 7 abstract layer table that’s responsible for explaining the flow of data within a network.
• Experienced individuals will often call out an issue by the culprit layer number. example: “This is a level 3 issue.”
• Protocol data unit(PDU) is a name given to the data when passing through each OSI layer. Bits, Frames, etc

Physical - Layer 1 - Bits (PDU)

• Bit-by-bit delivery, modulation, start-stop signals, physical network.

• Anything to do with physical transmission of data.

  • From transmission mode(full, half duplex) to pinout of a ethernet connector.

• Some related devices:

  • Ethernet hub, repeater, and physical transmission medium(wire, plug, outlet).

Data link - Layer 2 - Frames (PDU)

• Encapsulation, frame sync, LLC, MAC

• Provides a link between two devices, via a physical layer, and provides flow control.

• It also detects and corrects errors on layer 1.

• Defines two sublayers:

  1. Media Access Control(MAC) - controls device access and grants permissions for data transit.
  2. Logical link control(LLC) - flow control, handles network layer protocols and error checking/frame sync.

• Some related devices:

  • Switch, bridge, NIC

Network - Layer 3 - Packets (PDU)

• Controls addressing, routing, and traffic control.

• This layer routes packets between two network nodes by providing cross-network addressing, routing and updates both routing tables & fragmented packets.

Transport - Layer 4 - Segments & Datagrams (PDU)

• Reliability, flow control, connection-oriented communications, and multiplexing.

• Layer 4 is responsible for segmenting data and relieving congestion by determining the protocol to be used(TCP and UDP, or others), size of window(packet size(MTU)), and order of transmission.

Session - Layer 5 - Data (PDU)

• Session management(build and control sessions) between two hosts.

• Authentication, Authorization, Session restoration.

• This layer opens, manages, and closes sessions between end-user application processes.

• API, sockets, WinSock

Presentation - Layer 6 - Data (PDU)

• Translation > data de/compression > data de/encryption.

• This layer converts data(MOV, JPEG, ASCII, etc), to and from the form that the application layer accepts, so it can be sent over the network.

Application - Layer 7 - Data (PDU)

• Communication protocol and interface methods, used by host processes, to enable network communications.
• In other words, applications handling data through specific protocols which then is sent through layer 6.
  • This would include HTTP(S), IMAP, POP3, SMTP, FTP, SSH, etc
• Also provides service advertisement on the network.