The idea
Imagine writing the software for a web browser, and having to handle everything yourself: the voltage levels on the copper, deciding which of the machines on your local wire may transmit right now, working out a route across the world, retransmitting whatever got lost, and rendering the page.
Now imagine the network switches to fibre. You rewrite the browser.
That is the problem layering solves. The task is too big to hold at once, so it is cut into smaller functions, related functions are grouped into a layer, and each layer is given a standard interface to the ones above and below it. Then a layer’s internals are nobody else’s business. Swap copper for fibre and only the bottom layer changes; the browser never notices.
The lecture puts it as five points: the task is too complex whole, so divide it into simpler functions, group functions into layers, have each layer provide services to the layer above and use services from the layer below, and keep a standard interface between them.
The OSI model
The International Standards Organization built the Open Systems Interconnection model. ISO is the organisation; OSI is the model — the two get swapped constantly and it is an easy mark to lose.
An open system is one that lets any two different systems communicate regardless of their underlying architecture. That is the goal the model exists to serve, and the benefits follow from it: standardised interfaces make modular engineering possible, interoperability becomes achievable, complexity drops, and the thing becomes possible to teach.
Aside
The lecture slide ends with a line worth keeping: it is your slave, not your master. OSI is a way of organising your thinking about networks. Real protocols do not always respect its boundaries — ARP, which you meet in the next topic, sits awkwardly between two layers and is none the worse for it.
How it works
The seven layers
Bottom to top, with the job each one actually does.
1 — Physical. Transmits individual bits from one node to the next. It owns the physical characteristics of the interface and medium, how a bit is represented (the encoding), bit synchronisation (clocks), line configuration (point-to-point or multipoint), the physical topology, and the transmission mode (simplex, half duplex, full duplex). Wired or wireless.
2 — Data link. Transmits frames from one node to the next — hop-to-hop delivery — and makes the physical layer appear error-free to everything above. Framing, physical (MAC) addressing, flow control, error control, access control.
3 — Network. Responsible for source-to-destination delivery of a packet across multiple networks — end-to-end. Logical addressing, and routing between networks.
4 — Transport. Process-to-process delivery of a message. Service point (port) addressing, segmentation and reassembly, connection control (connection-oriented or connectionless), end-to-end flow control, and process-to-process error control.
5 — Session. Dialogue control — half or full duplex — and synchronisation.
6 — Presentation. Translation, compression and encryption. Syntax and semantics.
7 — Application. Services to the user: network virtual terminal (remote
desktop), file transfer access and management (FTAM), mail services
(X.400), directory services (X.500).
Where marks get lost
Three layers all describe “delivery” and they are constantly confused:
- Data link — hop-to-hop. One node to the next adjacent one.
- Network — source-to-destination (end-to-end). Across many networks.
- Transport — process-to-process. Between the actual programs at each end.
When a question asks which layer is responsible for delivery, it is testing exactly this distinction. Answer with the qualifier, never just “delivery”.
The summary table
The lecture gives this as a single table, and it is the most compressed form of the whole model. Worth being able to reproduce.
| Layer | Function | Examples |
|---|---|---|
| Application | Communication services to applications; data formats; encryption | Telnet, HTTP, FTP, web browsers |
| Presentation | Syntax and semantics; data formats | JPEG, ASCII, binary, encryption |
| Session | Start, control and end conversations; manage bidirectional messages | NFS, SQL |
| Transport | End-to-end delivery of the entire message; flow control, error control, sequencing, segmentation and reassembly | TCP, UDP, SPX |
| Network | End-to-end delivery of packets; addressing, routing | IP, IPX |
| Data link | Getting data across one particular link correctly; framing, physical addressing, flow control, error control, access control | Ethernet, token ring, FDDI, Frame Relay, ATM |
| Physical | Physical characteristics of the medium — connectors, pins, data rate, synchronisation, coding, modulation | EIA/TIA-232, EIA/TIA-449, V.24, RJ45 |
Check yourself
Cover the middle column and name each layer’s function from the layer name alone. Then cover the left column and name the layer from the examples. The second direction is harder and is the one exam questions usually take.
TCP/IP — the stack that actually runs
OSI is the vocabulary. TCP/IP is what your machine is running right now.
It was developed before the OSI model, which is why the layers do not line up exactly. It has five: physical, data link, network, transport, application. TCP/IP is a hierarchical protocol made of interactive modules — and, unlike OSI’s clean story, those modules are not necessarily independent.
| OSI (7) | TCP/IP (5) | What runs there |
|---|---|---|
| Application Presentation Session | Application | HTTP, FTP, Telnet, DNS |
| Transport | Transport | TCP, UDP |
| Network | Network | IP, ICMP, ARP |
| Data link | Data link | Ethernet, PPP, WiFi |
| Physical | Physical | Cable, fibre, radio |
Who writes the standards
Four bodies, and the exam wants you to know which does what.
- IETF — Internet Engineering Task Force. The standards body for the Internet. Its standards are called RFCs, Requests for Comments.
- ISO — International Standards Organization. Worldwide agreement on international standards across many fields, not just networking. Produced OSI.
- ITU-T — International Telecommunications Union, Telecommunication Standardization Sector. Related to the United Nations; develops telecommunications standards.
- ANSI — American National Standards Institute. The US representative to both ISO and the ITU-T. That relationship is the examinable part.
In the exam
- Why layering? Give the complexity argument and the standard-interface point. Do not say it makes things faster — it generally does not.
- Name the seven layers in order. Bottom-up or top-down; be able to do both. Mnemonics are fine, but you also need each layer’s job in a sentence.
- The hop-to-hop / end-to-end / process-to-process trio. Data link, network, transport. Asked constantly, in both directions.
- ISO is the body, OSI is the model. A free mark, frequently dropped.
- The OSI-to-TCP/IP mapping. The collapse of session, presentation and application into one is the whole content of the question.
- IETF standards are RFCs. One line, easy mark.