In this article, you will understand the process of sending a piece of information across the Internet.
| Prerequisites: | You need to know what a network is. |
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| Objective: | You will understand how computer networks send data across the Internet. |
- For data to pass through a network, it must pass through seven layers, all designed to make sure the data gets through the network accurately. The first layer, the application layer, is the only part of the process a user sees. This layer converts a message's data into bits and attaches a header identifying the sending and receiving computers.
- 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 presentation layer translates the data into a language that the receiving computer can understand (often ASCII, a way of encoding text as bits). This layer also compresses and encrypts the data. It adds another header specifying the language as well as 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, each computer taking turns sending and receiving, or full duplex, both computers sending and receiving at the same time. The details of these decisions are placed in the session header.
- 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.
