The OSI Reference Model
Chapter 2 — The OSI Reference Model
Seven layers, one job each. The OSI model is the shared blueprint that lets a Windows laptop, a Linux server, and a phone on the other side of the planet actually understand each other. This is the study map for the seven layers, how the Transport layer keeps data honest, how routers and switches split up the work, and how a single user message gets wrapped — and unwrapped — on its way across the wire.
Internetworking Models
Early networks could only talk to machines from the same manufacturer — a complete DECnet shop, or a complete IBM shop, never both at once. In the late 1970s the International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) reference model to break that barrier and let different vendors' equipment interoperate.
A reference model is a conceptual blueprint for how communication should happen. It breaks the job into logical groupings called layers — a layered architecture. Communication processes that relate to each other are grouped, or bound, at a particular layer, so a developer working on one layer's protocol never has to worry about what's happening at any other layer.
Advantages of Reference Models
These benefits apply to any layered model — OSI, or the TCP/IP model that grew out of the Department of Defense.
The Seven Layers
The top three layers (Application, Presentation, Session) govern how applications talk to each other and to end users. The bottom four (Transport, Network, Data Link, Physical) govern how the actual data gets transmitted end to end. None of the upper layers knows anything about networks or addresses — that's entirely the bottom four's job.
| Layer | Name | Primary Function |
|---|---|---|
| 7 | Application | File, print, message, database, and application services |
| 6 | Presentation | Data encryption, compression, and translation services |
| 5 | Session | Dialog control |
| 4 | Transport | End-to-end connection |
| 3 | Network | Routing |
| 2 | Data Link | Framing |
| 1 | Physical | Physical topology |
Layer Explorer
Click through each layer for what the exam actually expects you to know about it.
Application, Presentation & Session
The upper three layers, flipped for the detail worth memorizing.
Transport Layer — Connection-Oriented Communication
Before a sending host's TCP process transmits a single segment, it contacts the destination's TCP process to build a virtual circuit. This is connection-oriented communication — the TCP three-way handshake.
A service is connection-oriented if it...
Sets up a virtual circuit ~ uses sequencing ~ uses acknowledgments ~ uses flow control
TCP vs. UDP
Set up a virtual circuit → you're using TCP (connection-oriented). Skip that setup → you're using UDP (connectionless).
Flow Control & Windowing
Flow control lets the receiver govern how much data the sender pushes at once, so a fast sender can't overflow a slow receiver's buffer. The number of unacknowledged bytes a sender is allowed to have in flight is the window size. Drag the slider to see how window size changes how many segments go out before an acknowledgment is required.
Acknowledgments & Reliable Delivery
TCP uses positive acknowledgment with retransmission: the receiver acknowledges what it gets, the sender starts a timer on everything it sends, and anything that doesn't get acknowledged before the timer expires gets sent again.
Network Layer
The Network layer handles logical addressing, tracks where devices live on the network, and figures out the best way to move data between them — including hosts that aren't locally attached. Routers are the layer 3 devices that do this work.
What's in a routing table
| Field | What it holds |
|---|---|
| Network Address | Protocol-specific destination network, tracked per routing protocol (IP, IPv6, etc.) |
| Interface | The exit interface a packet takes to reach that network |
| Metric | Distance to the remote network — hop count for RIP; bandwidth, delay, or tick count for others |
Data Packets
Carry user data through the internetwork. The protocols that support them are routed protocols — IP and IPv6.
Route-Update Packets
Update neighboring routers about connected networks. Sent by routing protocols — RIP, RIPv2, EIGRP, OSPF.
Data Link Layer
The Data Link layer delivers messages to the right device on a local network using hardware (MAC) addresses, and translates Network layer messages into bits the Physical layer can transmit. It formats data into frames, adding a header with source and destination hardware addresses. It's the only OSI layer with sublayers.
Project 802
The IEEE 802 subcommittees, formed in February 1980 (80 + 2 = "802"), standardized LAN and MAN technologies. Only a handful of these are still active — the rest have been disbanded or gone into hibernation.
Physical Layer
The bottom of the stack does two things: sends bits, and receives bits. It defines the electrical, mechanical, procedural, and functional requirements for activating and deactivating a physical link, and it's where the interface between DTE (data terminal equipment — the attached device) and DCE (data communication equipment — usually a modem or CSU/DSU) is defined.
DTE
Data terminal equipment. The device attached to the network — your computer.
DCE
Data communication (or circuit-terminating) equipment. Usually a modem or CSU/DSU, typically at the customer premises.
The Physical layer also defines the transmission media's layout — its topology: bus, star, ring, or mesh.
Encapsulation
As data travels down the stack from a transmitting host, each layer wraps it with its own control information — a protocol data unit (PDU) — usually in a header, sometimes a trailer. Step through the process below.
Modulation Techniques
Modulation varies a carrier signal's properties to encode information onto it — the step that turns bits into something that can actually be transmitted on the wire, fiber, or airwave.
Exam Essentials
Know the layers cold
Application/Presentation/Session handle user-to-application communication. Transport segments, sequences, and builds virtual circuits. Network handles logical addressing and routing. Data Link frames and places data on the medium. Physical encodes bits for transmission.
Know the Data Link sublayers
LLC multiplexes and identifies Network layer protocols. MAC handles physical addressing and decides when to place data on the network.
Written Lab
Fill in the OSI layers in order, from layer 7 down to layer 1.
Exam Check
Twenty review questions pulled straight from this chapter.