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Dial-up access is a form of Internet access via telephone line.^[1] The client uses a modem connected to a computer and a telephone line to dial into an Internet service provider's (ISP) node to establish a modem-to-modem link, which is then routed to the Internet.

Dial-up requires no additional infrastructure on top of the telephone network. As telephone points are available throughout the world, dial-up remains useful to travelers. Dial-up is usually the only choice available for most rural or remote areas where getting a broadband connection is impossible due to low population and demand. Sometimes dial-up access may also be an alternative to people who have limited budgets as it is offered for free by some, though broadband is now increasingly available at lower prices in countries such as the United States, Canada, Hong Kong, Australia and the United Kingdom due to market competition.

Dial-up requires time to establish a telephone connection (several seconds, depending on the location) and perform handshaking before data transfers can take place. In locales with telephone connection charges, each connection incurs an incremental cost. If calls are time-charged, the duration of the connection incurs costs.

Dial-up access is a transient connection, because either the user or the ISP terminates the connection. Internet service providers will often set a limit on connection durations to prevent hogging of access, and will disconnect the user — requiring reconnection and the costs and delays associated with it.

In recent years, the availability of dialup access numbers has been shrinking. Most notably in part due to consolidations within the primary U.S. modem network operators such as MCI, Qwest, Sprint, and Level3, who have refocused their attention to broadband Internet access services.

Modern dial-up modems typically have a maximum theoretical speed of 56 kbit/s (using the V.92 protocol), although in most cases only up to 53 kbit/s is possible due to overhead and, in the United States, FCC regulation. These speeds are currently considered the maximum possible; in many cases transfer speeds will be lower, averaging anywhere between 33-43 kbit/s. Factors such as phone line noise and conditions, as well as the quality of the modem itself, play a large part in determining connection speeds. Some connections may be as low as 26.4 kbit/s depending on the various factors listed. Dial-up connections usually have high latency that can be as high as 400 ms or even more, which can make online gaming or video conferencing difficult, if not impossible. Some games, such as Star Wars: Galaxies, The Sims Online, Warcraft 3, Guild Wars and Unreal Tournament are capable of running on 56 K dial-up. Gamers with dial-up connections are often disconnected from game servers due to the "lag", or high latency, of the connection. Many computer games released in 2005 (such as Battlefield 2 or Star Wars: Battlefront 2) are not compatible for online play with dial-up modems. These first person shooter style games are the most sensitive to latency, making playing them impractical on dial-up.

Since approximately the year 2000, broadband Internet access (cable and DSL) has been replacing dial-up access in many parts of the world. The reason for this is mostly due to broadband connections featuring speeds which far exceed the capacity of dial-up, many of which provide speeds greater than 20 Mbit/s, as well as reducing prices under dial-up prices offered by companies such as Verizon. An increasing amount of Internet content such as Adobe Flash, online gaming and streaming media require large amounts of bandwidth.

However, many areas still remain without high speed Internet despite the eagerness of potential customers. This can be attributed to population, location, or sometimes ISPs' lack of interest due to little chance of profitability and high costs to build an infrastructure where none exists.

What is often advertised as "high-speed dial-up Internet" or "accelerated dial-up" by service providers such as MSN Earthlink, People PC, NetZero in the United States and Gonuts4free in Spain, is a form of dial-up access that uses the newer modem standard V.92 to shorten the log-on (or handshake) process, and then once a connection has been established the provider will selectively compress, filter, and cache data being sent to the user with the overall effect of increasing the speed of browsing most standard web pages.

The term high speed is misleading as these processes do not increase the overall throughput of the line, only making more efficient use of the bandwidth that is already there. Certain applications cannot be accelerated, such as HTTPS, streaming media, or file transfers. The compression of certain files such as pictures can have a negative effect on the browsing experience of the user, due to the lower quality that it imposes. However, the user can manually choose to view the images in true quality whenever he or she chooses to do so.

• High-speed dial-up at HowStuffWorks
• Dial-up ISP Directory Worldwide ISPs
• Dial-up ISP Directory for the United States of America