The OSI Model

Understanding the OSI Model

What is the OSI model?

• Open Systems Interconnection Reference Model

It’s a guide (thus the term “model”)

• Don’t get wrapped up in the details

This is not the OSI protocol suite

• Most of the OSI protocols didn’t catch on

↻ There are unique protocols at every layer

You’ll refer to this model for the rest of your career

• Often

↻ [A]ll [P]eople [S]eem [T]o [N]eed [D]ata [P]rocessing

Protocol Data Unit: The name given to data at a specific layer of the OSI Model i.e., Bits, Frames etc.

• To remember PDUs of first 4 layers, the mnemonic is [B]acon [F]rying [P]rodues [S]alivation

Layer 1 — Physical Layer

The physics of the network

• Signaling, cabling, connectors
• This layer isn’t about protocols
• Data is referred to as Bits at layer 1
• This layer doesn’t process any data, dealing only with physical transmission of signals i.e., Networking Cables, Hubs, Repeaters are layer 1 devices.

“You have a physical layer problem.”

• Fix your cabling, punch-downs, etc.
• Fun loopback tests, test/replace cables, swap adapter cards

Layer 2 — Data Link Layer

The basic network “language”

• The foundation of communication at the data link layer

Decisions are made based on MAC Addresses at layer 2.

• A 48-bit address “burned-in” to a network interface card (NIC) by its manufacturer.
• The device at this layer is Ethernet Switch
• The data is referred to as Frames
• Switches, Bridges, Network adapters (NICs) are layer 2 devices. NICs also operate layer 1.

Data Link Control (DLC) protocols

• MAC (Media Access Control) address on Ethernet
• ARP (Address Resolution Protocol) operates at layer 2, which used to resolve IP addresses (layer 3) to MAC addresses (layer 2)

The “switching” layer

Layer 3 — Network Layer

The “routing” layer

• The forwarding decisions based on Internet Protocol (IP) Address
• ICMP (Internet Control Message Protocol) operates at layer 3. It doesn’t transfer data, rather used for error reporting and diagnostics (ping and traceroute), and network control messages.
  • It is encapsulated directly within IP packets and doesn’t use transport layer protocols like TCP or UDP.
• The PDU is Packets

Fragments frames to traverse different networks

Layer 4 — Transport Layer

The “post office” layer, it concerns with network connections

• Parcels and letters
• The PDU at layer 4 is called Segments and Datagram

There are two types of protocols at play at layer 4:

1. Connection-oriented TCP, Reliable, PDU for TCP is segments
2. Connectionless UDP, Unreliable, PDU for UDP is datagram

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)

Layer 5 — Session Layer

Communication management between devices

• Start, stop, restart
• SIP (session initiation protocol) for VoIP

Control protocols, tunneling protocols

Layer 6 — Presentation Layer

• The layer we see
• Displaying an image or ASCII
• Character encoding
• Application encryption
• Often combined with the Application Layer

Layer 7 — Application Layer

• The Protocols that give us network functionality, not the graphics display
• HTTP/s, FTP, DNS, POP3, SMTP
• It enables direct interaction between the end-user and the network

Real-World to OSI Model

OSI in the real world:

Follow the conversation:

TCP/IP Model

• A model version in which Physical and Data Link Layers become Network Access layer.
  • Some variant of this model, Network Access Layer may be called Network Interface layer or Link Layer.
• Network layer becomes Internet layer
• Session, Presentation, Applications layers become single Application layer.

Another variant, in which Physical and Data Link Layers kept intact.

Another variant, may have Data Link as Network Interface Layer: