Dial-up Internet access is a form of Internet access that uses telephone lines. The user's computer or router uses an attached modem connected to a telephone line to dial into an Internet service provider's (ISP) node to establish a modem-to-modem link, which is then used to route Internet Protocol packets between the user's equipment and hosts.

The term was coined during the early days of computer telecommunications when modems were needed to connect dumb terminals or computers running terminal emulator software to mainframes, minicomputers, online services and bulletin board systems via a telephone line.

Availability

Dial-up connections to the Internet require no infrastructure other than the telephone network. As telephone access is widely available, dial-up remains useful to travelers. Dial-up is usually the only choice available for rural or remote areas where broadband installations are not prevalent due to low population and demand. Dial-up access may also be an alternative for users on limited budgets as it is offered for free by some ISPs, though broadband is increasingly available at lower prices in many countries due to market competition.

Dial-up requires time to establish a usable telephone connection (several seconds, depending on the location) and perform handshaking for protocol synchronization before data transfers can take place. In locales with telephone connection charges, each connection incurs an incremental cost. If calls are time-metered, 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. Technically-inclined users often find a way to disable the auto-disconnect program such that they can remain connected for days. This is particularly useful for downloading large files such as videos.

A 2008 Pew Internet and American Life Project study states that only 10 percent of American adults still use dial-up Internet access. Reasons for retaining dial-up access span from lack of infrastructure to high broadband prices.

Performance

Modern dial-up modems typically have a maximum theoretical transfer speed of 56 kbit/s (using the V.90 or V.92 protocol), although in most cases 40-50 kbit/s is the norm. Factors such as phone line noise as well as the quality of the modem itself play a large part in determining connection speeds. Some connections may be as low as 20 kbit/s in extremely "noisy" environments, such as in a hotel room where the phone line is shared with many extensions. Other things such as long loops, loading coils, pair gain, and digital loop carriers can also cripple connections to 20 kbit/s or lower.

Dial-up connections usually have latency as high as 400 ms or even more, which can make online gaming or video conferencing difficult, if not impossible. First person shooter style games are the most sensitive to latency, making playing them impractical on dial-up. However, some games such as Star Wars: Galaxies, The Sims Online, Warcraft 3, Guild Wars, Unreal Tournament, Halo: Combat Evolved, and Audition are capable of running on 56k dial-up.

An increasing amount of Internet content such as streaming media will not work at dialup speeds.

Analog telephone lines are digitally switched and transported inside a Digital Signal 0 once reaching the telephone company's equipment. Digital Signal 0 is 64 kbit/s, therefore a 56 kbit/s connection is the highest that will ever be possible with analog phone lines.

Using compression to exceed 56k

The V.42, V.42bis and V.44 standards allow modems to accept uncompressed data at a rate faster than the line rate. These algorithms use data compression to achieve higher throughput.

For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3 * 6 == 320 kbit/s if the offered data stream can be compressed that much. However, the compressibility of data tends to vary continuously, for example, due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video). A modem might be sending compressed files at approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any rate in this range.

Performance assessment

Many modems were manufactured as independent communications devices connected to the computer via a RS-232 cable. Modems are capable of independently managing the connection and monitoring signal quality, and can adjust the data rate as line conditions change.

In analog serial communications modems, once the connection is established, the data communications session consumes all available bandwidth. Often there is no backchannel capacity for the modem to communicate connection status to the end user or local computer. During the connection negotiation phase, modems transmit the connection speed to the attached computer in status reports. If the base data rate changes at a later time, there is no way to indicate this change to the local computer during the data communications session.

Although much more capable serial communications such as USB are now used, and in approximately 1992 the soft modem was developed that uses the internal computer CPU to handle modem communications, there is still no defined industry standard backchannel method available to indicate status information such as the current base rate and actual compression ratio, to the user of the local computer.

Compression by the ISP

As telephone-based 56 kbit/s modems began losing popularity, some Internet Service Providers such as Netzero, , and Earthlink started using pre-compression to increase the throughput and maintain their customer base. As an example, Netscape ISP uses a compression program that squeezes images, text, and other objects at the server, just prior to sending them across the phone line. The server-side compression operates much more efficiently than the "on-the-fly" compression of V.44-enabled modems. Typically website text is compacted to 5% thus increasing effective throughput to approximately 1000 kbit/s, and images are lossy-compressed to 15-20% increasing throughput to about 350 kbit/s.

The drawback of this approach is a loss in quality, where the graphics acquire more compression artifacts taking-on a blurry appearance, however the speed is dramatically improved and the user can manually choose to view the uncompressed images at any time. The ISPs employing this approach advertise it as "DSL speeds over regular phone lines" or simply "high speed dialup".

Replacement by broadband

Broadband Internet access (cable and DSL) has been replacing dial-up access in many parts of the world. Broadband connections typically offer speeds 700 kbit/s or higher for approximately the same price as dialup.

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 the required infrastructure. Some Dialup ISPs have responded to the increased competition by lowering their rates to as low as $5 a month making dialup an attractive option for those who merely want email access or basic web browsing.

Recession and its effect on service

News reports in 2009 have noted a resurgence of dial-up access in the U.S. resulting from a recessionary economy, as a more affordable way of accessing the internet. For example, the lowest cost dialup is approximately $7–8 per month, whereas the lowest cost DSL and Cable are about $18 (Verizondsl.com) and $45–55 (Comcast.com) per month.

Certainly high-speed DSL and Cable are available without local phone service, but the cost of this "naked" service is noticeably higher. AT&T offers basic DSL ("Direct Express") without a phone line for $35/month, negating any savings from canceling the phone service. Cable companies do not financially penalize a subscriber for not having a local phone, however cable internet services are usually more expensive if the customer does not subscribe to their television services.

List of dialup speeds

Note that the values given are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines).