Packet

Every idea in Topic 2 sits somewhere on one journey. Data has to travel; it is divided into packets so that many users can share the same network; routers choose a route for each packet independently; the method of transmission (how the bits travel and in which directions) is chosen to suit the situation; interference on the way can corrupt the data; an error-detection method reveals that corruption; automatic repeat query asks for the damaged data again; and encryption keeps the contents private from anyone who intercepts them on the way. Learn the chapter as that single chain and every individual method has an obvious place to hang.

Before data is transmitted across a network it is broken down into packets. Each packet carries a part of the original data, not the whole of it, together with information about where it is going and where it belongs in the sequence. Splitting the data this way means many users can share the same network instead of one transfer occupying the link from start to finish; it means each packet can be routed independently; and it means a single damaged part can be dealt with on its own. At the far end the receiving system reassembles the parts into the original data.

A packet of data contains a packet header, a payload and a trailer. The packet header includes the destination address, the packet number and the originator's address. The payload is the part of the data being transmitted that this packet is carrying. The trailer is the information at the end of the packet; depending on the protocol in use it may contain error-checking information and a marker showing that the end of the packet has been reached.

Packet switching is a method of transmitting data in which the data is broken down into packets and each packet is sent across the network independently. A router controls the route a packet takes, so each packet could take a different route from the same sender to the same receiver. Because the routes can differ in length and congestion, packets may arrive out of order. Once the last packet has arrived, the packets are reordered using the packet numbers in their headers, and the original data is reconstructed.

These four tools let you build a packet, put out-of-order packets back together, watch packets take different routes, and choose a method of transmission for a scenario. Every tool works with the mouse and with the keyboard, states every assumption it makes, and reports invalid input rather than guessing. Nothing you type leaves this page, and no tool needs an internet connection once the page has loaded.