Online and offline

In computer technology and telecommunications, online indicates a state of connectivity, and offline indicates a disconnected state. In modern terminology this usually refers to an Internet connection, but (especially when expressed 'on line' or 'on the line') could refer to any piece of equipment or functional unit that is connected to a larger system. Being online means that the equipment or subsystem is connected, or that it is ready for use. 'Online' has come to describe activities performed on and data available on the Internet, for example: 'online identity', 'online predator', 'online gambling', 'online shopping', 'online banking', and 'online learning'. Similar meaning is also given by the prefixes 'cyber' and 'e', as in the words 'cyberspace', 'cybercrime', 'email', and 'ecommerce'. In contrast, 'offline' can refer to either computing activities performed while disconnected from the Internet, or alternatives to Internet activities (such as shopping in brick-and-mortar stores). The term 'offline' is sometimes used interchangeably with the acronym 'IRL', meaning 'in real life'. During the 19th century, the term 'on line' was commonly used in both the railroad and telegraph industries. For railroads, a signal box would send messages down the line (track), via a telegraph line (cable), indicating the track's status: 'Train on line' or 'Line clear'. Telegraph linemen would refer to sending current through a line as 'direct on line' or 'battery on line'; or they may refer to a problem with the circuit as being 'on line', as opposed to the power source or end-point equipment. One example of a common use of these concepts with email is a mail user agent (MUA) that can be instructed to be in either online or offline states. One such MUA is Microsoft Outlook. When online it will attempt to connect to mail servers (to check for new mail at regular intervals, for example), and when offline it will not attempt to make any such connection. The online or offline state of the MUA does not necessarily reflect the connection status between the computer on which it is running and the internet. That is, the computer itself may be online—connected to Internet via a cable modem or other means—while Outlook is kept offline by the user, so that it makes no attempt to send or to receive messages. Similarly, a computer may be configured to employ a dial-up connection on demand (as when an application such as Outlook attempts to make connection to a server), but the user may not wish for Outlook to trigger that call whenever it is configured to check for mail. Another example of the use of these concepts is digital audio technology. A tape recorder, digital audio editor, or other device that is online is one whose clock is under the control of the clock of a synchronization master device. When the sync master commences playback, the online device automatically synchronizes itself to the master and commences playing from the same point in the recording. A device that is offline uses no external clock reference and relies upon its own internal clock. When a large number of devices are connected to a sync master it is often convenient, if one wants to hear just the output of one single device, to take it offline because, if the device is played back online, all synchronized devices have to locate the playback point and wait for each other device to be in synchronization. (For related discussion, see MIDI timecode, word sync, and recording system synchronization.) A third example of a common use of these concepts is a web browser that can be instructed to be in either online or offline states. The browser attempts to fetch pages from servers while only in the online state. In the offline state, users can perform offline browsing, where pages can be browsed using local copies of those pages that have previously been downloaded while in the online state. This can be useful when the computer is offline and connection to the Internet is impossible or undesirable. The pages are downloaded either implicitly into the web browser's own cache as a result of prior online browsing by the user or explicitly by a browser configured to keep local copies of certain web pages, which are updated when the browser is in the online state, either by checking that the local copies are up-to-date at regular intervals or by checking that the local copies are up-to-date whenever the browser is switched to the online state. One such web browser capable of being explicitly configured to download pages for offline browsing is Internet Explorer. When pages are added to the Favourites list, they can be marked to be 'available for offline browsing'. Internet Explorer will download to local copies both the marked page and, optionally, all of the pages that it links to. In Internet Explorer version 6, the level of direct and indirect links, the maximum amount of local disc space allowed to be consumed, and the schedule on which local copies are checked to see whether they are up-to-date, are configurable for each individual Favourites entry. For communities that lack adequate Internet connectivity—such as developing countries, rural areas, and prisons—offline information stores such as the eGranary Digital Library (a collection of approximately thirty million educational resources from more than two thousand web sites and hundreds of CD-ROMs) provide offline access to information. Numerous organizations have developed, or are developing, flash memory chips with collections of educational materials for offline use in smartphones, tablets, and laptops. Likewise, offline storage is computer data storage that is not 'available for immediate use on demand by the system without human intervention.' Additionally, an otherwise online system that is powered down may be considered offline.