The OSI Reference Model and Its 7 Layers

The OSI reference model (OSI基本参照モデル) is an international framework defined by ISO that sorts the functions needed for computer-to-computer communication into seven layers.

The rules for the steps and formats of communication are called communication protocols (通信プロトコル), and each layer runs on protocols suited to its role. Each layer uses the functions of the layer directly below and serves the layer directly above, so swapping the method used in one layer has little effect on the others. Layers are counted from the bottom as Layer 1, Layer 2 and so on; lower numbers sit closer to signal transmission, and higher numbers sit closer to the user's applications.

AnalogyIt is like splitting parcel delivery among a packer, a label writer, and a driver. As long as each handoff follows the agreed rules, the packing stays the same even if the trucks are replaced by trains.

How to rememberOSI stands for Open Systems Interconnection. To recall the seven layers from the top down, use the phrase "All People Seem To Need Data Processing".

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The physical layer (物理層) is Layer 1 of the OSI reference model; it turns bits, the 0s and 1s, into electrical signals, light, or radio waves and sends them over the transmission medium.

It covers the physical connection standards, such as the shape of cables and connectors and the voltage and timing of signals. It does not care what the bit stream means. A repeater, which boosts a weakened signal so it can travel farther, is an example of a device working at this layer.

AnalogyIt is like paving a road so cars can drive on it. Which car carries what cargo is none of its concern.

How to rememberAs the name suggests, this bottom layer is about things you can touch, like cables and connectors.

The data link layer (データリンク層) is Layer 2; it reliably passes data between devices that are directly connected, such as by a cable.

A device connected to a network is called a node (ノード), and this layer groups bits into units called frames (フレーム) and sends them between neighboring nodes. It identifies the destination by the MAC address (MACアドレス) assigned to each device, and it also detects errors that occur during transmission. A switch (switching hub, スイッチングハブ), which reads the destination MAC address of a frame to pick the port to send it out of, is a device at this layer.

AnalogyIt is like a clerk in one building who checks the room number and hands papers to the room next door.

How to rememberA link is one ring in a chain. Picture it covering just one ring: the stretch between two neighbors.

The network layer (ネットワーク層) is Layer 3; it sends data across multiple networks by choosing a route that reaches a distant destination.

Choosing the path to the destination is called routing (ルーティング), and data travels in units called packets (パケット). The destination is identified by an IP address (IPアドレス). A router, which links networks together, is the device that decides where to forward each packet at this layer.

AnalogyIt is like a sorting center that reads the address and decides which relay stations a parcel must pass through.

How to rememberThis is the layer that connects networks to one another. The "route" in routing means the path to follow.

The transport layer (トランスポート層) is Layer 4; it manages data transfer between the sending and receiving devices and, when needed, makes up for lost or out-of-order data to ensure reliability.

Its best-known protocol, TCP, keeps end-to-end transfer in order by resending data lost on the way and putting received data back in sequence. A port number (ポート番号) tells which application inside a device the communication belongs to.

AnalogyIt is like an inspector at the receiving end who checks that every box of a split shipment has arrived and asks for any missing box to be sent again.

How to rememberTransport means carrying goods. Link it to a shipping company that carries freight all the way from sender to recipient.

The session layer (セション層) is Layer 5; it manages the whole exchange from the start of communication to its end, called a session (セション).

It sets up a session with the other party, places sync points along the way as checkpoints, and finally carries out the closing steps. Coordinating how the exchange proceeds, such as whose turn it is to send, is also this layer's job.

AnalogyIt is like a phone call: you say hello, confirm each point is settled, and say goodbye before hanging up.

How to rememberA session is one sitting of a meeting. Link it to the chair who runs the meeting from opening to close.

The presentation layer (プレゼンテーション層) is Layer 6; it aligns the representation format of data, such as character codes and image formats, so both sender and receiver can understand it.

It converts data into a common format so it reads correctly even when sender and receiver represent it differently. Data compression and encryption are also given as examples of its role.

AnalogyIt is like an interpreter who keeps the content of a talk unchanged but puts it into a language the listener understands.

How to rememberPresentation means how something is shown. Link it to a layer that tidies the form, not the content.

The application layer (アプリケーション層) is the topmost Layer 7; it provides communication services such as email and file transfer to the applications people use.

Each purpose has its own protocol; for instance, viewing web pages uses HTTP, and sending email uses SMTP. It is the layer closest to the user, and data created here is passed down through the lower layers in turn before reaching the other side.

AnalogyIt is like the front counter of a shop that deals with customers directly. Orders still get delivered even if the counter staff know nothing about the shipping system behind them.

How to rememberAn application is the app you use every day. Remember this as the top layer, the one nearest the app.

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