Foundations — OSI model — 7 layers, responsibilities, PDU names

How to read this page

The parent note throws around words like header, frame, MAC address, port, and a scary-looking acronym PDU. If any of those are fuzzy, the whole 7-layer story collapses. Below, each term is built in strict order: earlier ones are the bricks for later ones. Nothing is used before it is defined.

0. The very bottom: a bit and a byte

The picture: eight boxes in a row, each holding a $0$ or $1$.

Why the topic needs it: the parent's smallest data unit (at Layer 1) is literally called a Bit, and every larger data-bundle — Frame, Packet, Segment (all defined below) — is just a longer row of bytes. If you don't see data as "a stream of bytes," nothing above makes sense.

1. Data / payload — the thing you actually want to send

The picture: a plain rectangle labelled "the message." Hold onto this rectangle — every step from here on wraps it.

Why the topic needs it: in the parent's data-unit table, Layers 7-6-5 all call their unit Data — because at those layers nothing has been wrapped yet; it's still pure payload.

2. Header and trailer — the labels stuck onto data

Why the topic needs it: the parent's whole "why does the data-unit name change?" story is headers (and one trailer). Each layer prepends its header → the bundle grows → we give the bigger bundle a new name.

3. Encapsulation — the wrapping process itself

Why the topic needs it: this is the engine behind everything else on the page. Headers and trailers are the parts; encapsulation is the assembly line that adds them in order. See Encapsulation and De-encapsulation for the full down-and-up journey.

4. Protocol — an agreed set of rules

The picture: two stick figures, a speech bubble between them labelled "same rulebook."

Why the topic needs it: the parent lists HTTP, FTP, DNS, TCP, UDP — these are all protocols. Each layer has its own rulebook, and a protocol is simply that rulebook made concrete.

5. PDU — the name of the bundle at each layer

Each layer's PDU has its own name. Here is what each one means and where it lives:

The parent's chain, built from the tools above:

$\underset{\text{payload}}{\underbrace{\text{Data}}}\stackrel{+h_{\text{L4}}}{\to }\text{Segment}\stackrel{+h_{\text{L3}}}{\to }\text{Packet}\stackrel{+h_{\text{L2}}}{\to }\text{Frame}\to \text{Bits}$

Why the topic needs it: "PDU names" is literally half the parent's title. Now you can see why: Data → Segment → Packet → Frame → Bits is one payload gaining one wrapper (via encapsulation) at a time.

6. Three kinds of address

To deliver anything, you need to say where. The parent uses three different address types, each answering a different "where."

Why the topic needs it: the parent's "Address used" column is these three. Getting them straight is what separates a switch (MAC) from a router (IP).

7. Layer and stack — the organizing idea

Why the topic needs it: this is the parent's core "layered architecture." The TCP-IP Model — 4 vs 5 layers is the same stack idea with fewer slices; TCP vs UDP is a choice made inside one layer (Layer 4).

Prerequisite map

Equipment checklist

Test yourself — you are ready for the parent note only if each reveal feels obvious.