In this article, we discuss what happens when information traverses the Internet.
| Prerequisites: | You need to know what a network is. |
|---|---|
| Objective: | Learn how computer networks send data across the Internet. |
To pass through a network, data must go through many processes to make sure it arrives accurately. The seven layer schema of the OSI model (Open Systems Interconnection) provides a helpful way to conceptualize the journey of data across the network:
- The user sees only the first layer, the application layer. This layer converts a message's data into bits and attaches a header identifying the sending and receiving computers.
- The presentation layer translates the data into an encoding that the receiving computer will recognize (often ASCII, a way of encoding text as bits). This layer also compresses and encrypts the data, and adds another header to specify which conventions have been used for encoding, compression, and encryption.
- The session layer opens communications. It sets boundaries (called brackets) for the beginning and end of the message and establishes whether the message will be sent half duplex (the computers take turns sending and receiving) or full duplex (both computers send and receive at the same time). The details of these decisions are placed in a session header.
- The transport layer protects the data being sent. It subdivides the data into segments and creates checksum tests - mathematical sums based on the contents of data - that can be used later to check if the data was scrambled. It also makes backup copies of the data. This layer attaches a header which identifies each segment's checksum and it's position in the message.
- The "network layer" selects a route for the message. It forms segments into packets, counts them, and adds a header containing the sequence of packets and the address of the receiving computer.
- The "data-link layer" supervises the transmission. It confirms the checksum, and then addresses and duplicates the packets. This layer keeps a copy of each packet until it receives a confirmation from the next point along the route that the packet has reached un-damaged.
- The "physical layer" encodes the packets into the medium that will carry them - such as an analog signal, if the message is going across a telephone line - and sends the packets along that medium.
At the receiving node, the seven layered process that sent the message is reversed. The "physical layer" reconverts the message into bits. The data-link layer recalculates the checksum, confirms arrival, and logs in the packets. The network layer recounts incoming packets for security. The transport layer recalculates the checksum and reassembles the message segments. The session layer holds the parts of the message until it is complete and sends it to the next layer. The presentation layer decrypts, expands, and translates the message. The application layer identifies the recipient, converts the bits into readable characters, and directs the data to the correct application.