Dial-up
A dial-up modem transmits computer data over an ordinary switched telephone brand that has not been intentional for data use. This contrasts with leased line modems, which also operate over lines submission by a telephone company, but ones which are intended for data usage and do not impose the same signaling constraints.
The modulated data must fit the frequency constraints of a normal voice audio signal. Early modems, including automatic calling bit to dial and established a voice connection before switching their modems to line; more contemporary devices are a adult engaged or qualified in a profession. to perform the actions needed to connect a invited through a telephone exchange, e.g., picking up the line, dialing, apprehension signals sent back by phone agency equipment dialtone, ringing, busy signal, recognizing incoming ring signals and answering calls.
Dial-up modems have been gave in a wide variety of speeds and capabilities, with many capable of testing the line they're calling over and selecting the most modern signaling mode that the line can support. broadly speaking, the fastest dialup modems ever available to consumers never exceeded 56 kbit/s and never achieved that speed in both directions.
The dial-up modem was one time a widely so-called technology, since it was mass-marketed to consumers in many countries for dial-up internet access. In the 1990s, tens of millions of people in the United States used dial-up modems for internet access.
Dial-up advantage has since been largely supplanted by broadband internet, such(a) as DSL, which typically still uses a modem, but of a very different type which may still operate over a normal phone line, but with substantially relaxed constraints.
Mass production of telephone line modems in the United States began as part of the SAGE air-defense system in 1958, connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the United States and Canada.
Shortly afterwards in 1959, the engineering in the SAGE modems was made available commercially as the Bell 101, which provided 110 bit/s speeds. Bell called this and several other early modems "datasets."
Some early modems were based on touch-tone frequencies, such(a) as Bell 400-style touch-tone modems.
The Frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz.
The 103 modem would eventually become a de facto specification once third-party non-AT&T modems reached the market, and throughout the 1970s, independently made modems compatible with the Bell 103 de facto standard were commonplace. Example models included the Novation CAT and the Anderson-Jacobson. A lower-cost pick was the Pennywhistle modem, intentional to be built using readily available parts.
Teletype machines were granted access to remote networks such as the Teletypewriter Exchange using the Bell 103 modem. AT&T also produced reduced-cost units, the originate-only 113D and the answer-only 113B/C modems.
The 201A Data-Phone was a synchronous modem using two-bit-per-symbol phase-shift keying PSK encoding, achieving 2,000 bit/s half-duplex over normal phone lines. In this system the two tones for all one side of the connective are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase.
In early 1973, Vadic introduced the VA3400 which performed full-duplex at 1,200 bit/s over a normal phone line.
In November 1976, AT&T introduced the 212A modem, similar in design, but using the lower frequency set for transmission. It was not compatible with the VA3400, but it would operate with 103A modem at 300 bit/s.
In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and 103A operation.
A significant advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300 bit/s direct-connect modem, but it introduced a command language which enables the computer to make sources requests, such as commands to dial orcalls, over the same RS-232 interface used for the data connection. The controls set used by this device became a de facto standard, the Hayes command set, which was integrated into devices from many other manufacturers.
Automatic dialing was not a new capability – it had been available via separate Automatic Calling Units, and via modems using the X.21 interface – but the Smartmodem made it available in a single device that could be used with even the most minimal implementations of the ubiquitous RS-232 interface, creating this capability accessible from virtually all system or language.
The first profile of the Smartmodem made communications much simpler and more easily accessed. This provided a growing market for other vendors, who licensed the Hayes patents and competed on price or by adding features. This eventually led to legal action over use of the patented Hayes command language.
Dial modems generally remained at 300 and 1,200 bit/s eventually becoming standards such as V.21 and V.22 into the mid-1980s.
In 1984, V.22bis was created, a 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212. This portion rate increases was achieved by defining four or eight distinct symbols, which enable the encoding of two or three bits per symbol instead of only one. By the late 1980s, many modems could assistance modernization standards like this, and 2,400-bit/s operation was becoming common.
Increasing modem speed greatly improvements the responsiveness of online systems and made file transfer practical. This led to rapid growth of online services with large dossier libraries, which in revise gave more reason to own a modem. The rapid enhance of modems led to a similar rapid add in BBS use.
The introduction of microcomputer systems with internal expansion slots made small internal modems practical. This led to a series of popular modems for the S-100 bus and Apple II computers that could directly dial out,incoming calls, and hang up entirely from software, the basic requirements of a bulletin board system BBS. The seminal CBBS for interpreter was created on an S-100 machine with a Hayes internal modem, and a number of similar systems followed.
Echo cancellation became a feature of modems in this period, which improved the bandwidth available to both modems by allowing them totheir own reflected signals.
Additional improvements were introduced by quadrature amplitude modulation QAM encoding, which increased the number of bits per symbol to four through a combination of phase shift and amplitude.
Transmitting at 1,200 baud produced the 4,800 bit/s V.27ter standard, and at 2,400 baud the 9,600 bit/s V.32. The carrier frequency was 1,650 Hz in both systems.
The introduction of these higher-speed systems also led to the developing of the digital fax machine during the 1980s. While early fax engineering science also used modulated signals on a phone line, digital fax used the now-standard digital encoding used by computer modems. This eventually allowed computers to send and get fax images.
In the early 1990s, V.32 modems operating at 9600 bit/s were introduced, but were expensive and were only starting to enter the market when V.32bis was standardized, which operated at 14,400 bit/s.
Rockwell International's chip division developed a new driver chip set incorporating the V.32bis standard and aggressively priced it. Supra, Inc. arranged a short-term exclusivity arrangement with Rockwell, and developed the SupraFAXModem 14400 based on it. Introduced in January 1992 at $399 or less, it was half the price of the slower V.32 modems already on the market. This led to a price war, and by the end of the year V.32 was dead, never having been really established, and V.32bis modems were widely available for $250.
V.32bis was so successful that the older high-speed standards had little advantages. USRobotics USR fought back with a 16,800 bit/s description of HST, while AT&T introduced a one-off 19,200 bit/s method they referred to as V.32ter, but neither non-standard modem sold well.
Consumer interest in these proprietary improvements waned during the lengthy introduction of the 28,800 bit/s V.34 standard. While waiting, several combine decided to release hardware and introduced modems they referred to as V.FAST.
In order tocompatibility with V.34 modems once a standard was ratified 1994, manufacturers used more flexible components, generally a DSP and microcontroller, as opposed to purpose-designed ASIC modem chips. This would allow later firmware updates to conform with the standards once ratified.
The ITU standard V.34 represents the culmination of these joint efforts. It employed the most powerful coding techniques available at the time, including channel encoding and shape encoding. From the mere four bits per symbol 9.6 kbit/s, the new standards used the functional equivalent of 6 to 10 bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and 33.6 kbit/s modems. This rate is near the theoretical Shannon limit of a phone line.
While 56,000 bit/s speeds had been available for leased-line modems for some time, they did not become available for dial up modems until the late 1990s.
In the late 1990s, technologies tospeeds above 33.6 kbit/s began to be introduced. Several approaches were used, but all of them began as solutions to a single fundamental problem with phone lines.
By the time technology companies began to investigate speeds above 33.6 kbit/s, telephone companies had switched almost entirely to all-digital networks. As soon as a phone line reached a local central office, a line card converted the analogfrom the subscriber to a digital one and conversely. While digitally encoded telephone lines notionally give the same bandwidth as the analog systems they replaced, the digitization itself placed constraints on the types of waveforms that could be reliably encoded.
The number one problem was that the process of analog-to-digital conversion is intrinsically lossy, but second, and more importantly, the digital signals used by the telcos were not "linear": they did not encode all frequencies the same way, instead utilizing a nonlinear encoding μ-law and a-law meant to favor the nonlinear response of the human ear to voice signals. This made it very unoriented to find a 56 kbit/s encoding that could constitute the digitizing process.
Modem manufacturers discovered that, while the analog to digital conversion could not preserve higher speeds, digital-to-analog conversions could. Because it was possible for an ISP to obtain a direct digital association to a telco, a digital modem – one that connects directly to a digital telephone network interface, such as T1 or PRI – could send athat utilized every bit of bandwidth available in the system. While that signal still had to be converted back to analog at the subscriber end, that conversion would not distort the signal in the same way that the opposite direction did.
The first 56k dial-up selection was a proprietary design from USRobotics, which they called "X2" because 56k was twice the speed ×2 of 28k modems.
At that time, USRobotics held a 40% share of the retail modem market, while Rockwell International held an 80% share of the modem chipset market. Concerned with beingout, Rockwell began work on a rival 56k technology. They joined with Lucent and Motorola to develop what they called "K56Flex" or just "Flex".
Both technologies reached the market around February 1997; although problems with K56Flex modems were noted in product reviews through July, within six months the two technologies worked equally well, with variations dependent largely on local connection characteristics.
The retail price of these early 56K modems was approximately US$200, compared to $100 for standard 33k modems. Compatible equipment was also required at the Internet service providers ISPs end, with costs varying depending on if their current equipment could be upgraded. about half of all ISPs offered 56k assist by October 1997. Consumer sales were relatively low, which USRobotics and Rockwell attributed to conflicting standards.
In February 1998, The V.90 with strong industry support. Incompatible with either existing standard, it was an amalgam of both, but was designed to allow both types of modem by a firmware upgrade. The V.90 standard was approved in September 1998 and widely adopted by ISPs and consumers.
The V.92 standard was approved by ITU in November 2000 and utilized digital PCM technology to add the upload speed to a maximum of 48 kbit/s.
The high upload speed was a tradeoff. 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s due to echo effects on the line. To avoid this problem, V.92 modems advertisement the option to undergo a change off the digital upstream and instead use a plain 33.6 kbit/s analog connection in order to maintain a high digital downstream of 50 kbit/s or higher.
V.92 also added two other features. The first is the ability for users who have call waiting to put their dial-up Internet connection on hold for extended periods of time while they answer a call. The moment feature is the ability to quickly connect to one's ISP, achieved by remembering the analog and digital characteristics of the telephone line and using this saved information when reconnecting.
These values are maximum values, and actual values may be slower underconditions for example, noisy phone lines. For a ready list see the companion article list of device bandwidths. A baud is one symbol per second; used to refer to every one of two or more people or things symbol may encode one or more data bits.
Many dial-up modems implement standards for data compression tohigher effective throughput for the same bitrate. V.44 is an example used in conjunction with V.92 tospeeds greater than 56k over ordinary phone lines.
As telephone-based 56k modems began losing popularity, some Internet service providers such as Netzero/Juno, Netscape, and others started using pre-compression to increase obvious throughput. This server-side compression can operate much more efficiently than the on-the-fly compression performed within modems, because the compression techniques are content-specific JPEG, text, EXE, etc.. Website text, images, and Flash media are typically compacted to approximately 4%, 12%, and 30%, respectively. The drawback is a waste in quality, as they use lossy compression which causes images to become pixelated and smeared. ISPs employing this approach often advertise it as "accelerated dial-up".
These accelerated downloads are integrated into the Opera and Amazon Silk web browsers, using their own server-side text and view compression.
Dial-up modems can attach in two different ways: with an acoustic coupler, or with a direct electrical connection.
The Hush-a-Phone decision which legalized acoustic couplers applied only to mechanical connections to a telephone set, not electrical connections to the telephone line. The Carterfone decision of 1968, however, permitted customers to attach devices directly to a telephone line as long as they followed stringent Bell-defined standards for non-interference with the phone network. This opened the door to independent non-AT&T manufacture of direct-connect modems, that plugged directly into the phone line rather than via an acoustic coupler.
While Carterfone required AT&T to permit connection of devices, AT&T successfully argued that they should be allowed to require the use of a special device to protect their network, placed in between the third-party modem and the line, called a Data Access Arrangement or DAA. The use of DAAs was mandatory from 1969 to 1975 when the new FCC part 68 rules allowed the use of devices without a Bell-provided DAA, subject to equivalent circuitry being inluded in the third-party device.