Analog Services

A modem is standard equipment in every new computer ”and with good reason. It's what most people use to connect to the Internet. Consider a computer without a modem, and you might as well buy a vacuum cleaner.

Today's modem is a world apart from those of only a few years ago, and the difference is speed. Nearly every new modem sold operates at a top speed of 56,000bps ”fast enough that most people refuse to pay the higher charges demanded for faster-still all-digital services. Today's modems are fast enough for e-mail and most surfing. They falter only when you want an edge in playing online games , want to download streaming video, or want to exchange photos with relatives and friends .

Background

A true modem is a necessary evil in today's world of telecommunications because we still suffer from a telephone system that labors under standards devised even before electronics were invented, at a time when solid-state digital circuitry lay undreamed, almost a hundred years off. The telephone system was designed to handle analog signals only because that's all that speaking into a microphone creates. Over the years, the telephone system has evolved into an elaborate international network capable of handling millions of these analog signals simultaneously and switching them from one telephone set to another, anywhere in the world.

In the last couple decades, telephone companies have shifted nearly all their circuits to digital. Most central office circuitry is digital. Nearly every long distance call is sent between cities and countries digitally. In fact, the only analog part of most telephone connections is the local loop , the wires that reach out from the telephone exchange to your home or office (and likewise extend from a distant exchange to the telephone of whomever you're calling).

The chief reason any analog circuitry remains in the telephone system is that there are hundreds of millions of plain-old telephone sets (which the technologically astute call simply POTS ) dangling on the ends of telephone wires across the country. As long as people depend on POTS, they need an analog network to connect to.

Modem Technology

Of all modern communications systems, the one with the most severe limits on bandwidth is the telephone channel. Instead of the full frequency range of a good-quality stereo system (from 20 to 20,000Hz), a telephone channel only allows frequencies between 300 and 3000Hz to freely pass. This very narrow bandwidth works well for telephones because frequencies below 300Hz contain most of the power of the human voice but little of its intelligibility. Frequencies above 3000Hz increase the crispness of the sound but don't add appreciably to intelligibility.

What works well for voice is horrible for data. Although intelligibility is the primary concern with voice communications (most of the time), data transfer is principally oriented to bandwidth. The comparatively narrow bandwidth of the standard telephone channel limits the bandwidth of the modulated signal it can carry, which in turn limits the amount of digital information that can be squeezed down the phone line by a modem. The result is that dial-up telephone lines are poor data channels ”but too often they are the only ones we have.

Shannon's Limit

Engineers can use a variety of technologies to squeeze more data into a narrow-bandwidth channel, but they still face an ultimate limit on the amount of data they can squeeze through a tight channel such as an analog telephone line. This ultimate limit combines the effects of the bandwidth of the channel and the noise level in the channel. The greater the noise, the more likely it will be confused with the information that has to compete with it. This theoretical maximum data rate for a communication channel is called Shannon's Limit. This fundamental law of data communications, discovered by Claude L. Shannon working at Bell Labs in 1948, states that the maximum number of digital bits that can be transmitted over a given communication path in one second can be determined from the bandwidth (W) and signal-to-noise ratio (S/N, expressed in decibels) by using the following formula:

In telephone circuits, the analog-to-digital converters used in telephone company central offices contribute the noise that most limits modem bandwidth. In creating the digital pulse-coded modulation (PCM) signal, they create quantization distortion , which produces an effective signal-to-noise ratio of about 36 dB. Quantization distortion results from the inability of the digital system with a discrete number of voltage steps (256 in the case of telephone company A/D converters) to exactly represent an analog signal that has an infinite number of levels. At this noise level, Shannon's Limit for analog data on telephone lines is about 33,600 bits per second. Modern modems for dial-up data communications can be faster only by sidestepping some of the bandwidth issues.

Channels

Communications are supposed to be a two-way street. Information is supposed to flow in both directions. You should learn something from everyone you talk to, and everyone should learn from you. Even if you disregard the potential for success of such two-way communication, one effect is undeniable: It cuts the usable bandwidth of a data communication channel in one direction in half because the data going the other way requires its own share of the bandwidth.

Duplex

With modems, such a two-way exchange of information is called duplex communications . Often it is redundantly called full-duplex . A full-duplex modem is able to simultaneously handle two signals, usually (but not necessarily ) going in opposite directions, so it can send and receive information at the same time. Duplex modems use two carriers to simultaneously transmit and receive data, each of which has half the bandwidth available to it and its modulation.

Half-Duplex

The alternative to duplex communications is half-duplex . In half-duplex transmissions, only one signal is used. To carry on a two-way conversation, a modem must alternately send and receive signals. Half-duplex transmissions allow more of the channel bandwidth to be put to use but slow data communications because often a modem must switch between sending and receiving modes after every block of data crawls through the channel.

Echoplex

The term duplex is often mistakenly used by some communications programs for computers to describe echoplex operation. In echoplex mode, a modem sends a character down the phone line, and the distant modem returns the same character, echoing it. The echoed character is then displayed on the originating terminal as confirmation that the character was sent correctly. Without echoplex, the host computer usually writes the transmitted character directly to its monitor screen. Although a duplex modem generates echoplex signals most easily, the two terms are not interchangeable.

With early communications programs, echoplex was a critical setup parameter. Some terminal programs relied on modem echoplex to display your typing on the screen. If you had echoplex off, you wouldn't see what you typed. Other terminal programs, however, displayed every character that went through the modem, so switching echoplex on would display two of every letter you typed, lliikkee tthhiiss. Web browsers don't bother you with the need to select this feature. Most, however, work without echoplex.

Switching Modems

To push more signal through a telephone line, some modems attempt to mimic full-duplex operation while actually running in half-duplex mode. Switching modems are half-duplex modems that reverse the direction of the signal at each end of the line in response to the need to send data. This kind of operation can masquerade as full-duplex because most of the time communications go only in one direction. You enter commands into a remote access system, and only after the commands are received does the remote system respond with the information you seek. Although one end is sending, the other end is more than likely to be completely idle.

On the positive side, switching modems are able to achieve a doubling of the data rate without adding any complexity to their modulation. However, the switching process itself is time-consuming and inevitably involves a delay because the modems must let each other know that they are switching. Because transmission delays across long-distance circuits are often a substantial fraction of a second (most connections take at least one trip up to a satellite and back down, a 50,000 mile journey that takes about a quarter of a second even at the speed of light), the process of switching can eat huge holes into transmission time.

Most software modem protocols require a confirmation for each block of data sent, meaning the modem must switch twice for each block. The smaller the block, the more often the switch must occur. Just one trip to a satellite would limit a switching modem with an infinitely fast data rate, using the 128-byte blocks of some early modem protocols, to 1024 bits per second at the two-switches-per-second rate.

Asymmetrical Channels

Because of this weakness of switching modems, asymmetrical modems cut the waiting by maintaining a semblance of two-way duplex communications while optimizing speed in one direction only. These modems shoehorn in a lower speed channel in addition to a higher speed one, splitting the total bandwidth of the modem channel unequally.

Early asymmetrical modems were able to flip-flop the direction of the high-speed communications, relying on algorithms to determine which way is the best way. The modern asymmetrical technologies have a much simpler algorithm. Designed for Internet communications, they assume you need a greater data rate downstream (to you) than upstream (back to the server). This design is effective because most people download blocks of data from the Internet (typically Web pages rife with graphics) while sending only a few commands back to the Web server.

The latest V.90 modems operate asymmetrically at their highest speed. Cable modems and satellite connections to the Internet also use a variation on asymmetrical modem technology. These systems typically provide you with a wide bandwidth downlink from a satellite or cable system to permit you to quickly browse pages but rely on a narrow-channel telephone link ”a conventional modem link ”to relay your commands back to the network.

Connection-Enhancing Technologies

Getting the most from your modem requires making the best match between it and the connection it makes to the distant modem with which you want to communicate. Although you have no control over the routing your local phone company and long distance carrier give to a given call (or even whether the connection remains consistent during a given call), a modem can make the best of what it gets. Using line compensation , it can ameliorate some problems with the connection. Fallback helps the modem get the most from a substandard connection or one that loses quality during the modem link-up. Data compression helps the modem move more data through any connection, and error correction compensates for transitory problems that would result in minor mistakes in transmissions.

Line Compensation

Although a long-distance telephone connection may sound unchanging to your ear, its electrical characteristics vary by the moment. Everything, from a wire swaying in the Wichita wind to the phone company's automatic rerouting of the call through Bangkok when the direct circuits fill up, can change the amplitude, frequency, and phase response of the circuit. The modem then faces two challenges: not to interpret such changes as data and to maintain the quality of the line to a high-enough standard to support its use for high-speed transmission.

Under modern communications standards, modems compensate for variations in telephone lines by equalizing these lines. That is, two modems exchange tones at different frequencies and observe how signal strength and phase shift with frequency changes. The modems then change their signals to behave in the exact opposite way to cancel out the variations in the phone line. The modems compensate for deficiencies in the phone line to make signals behave the way they would have in absence of the problems. If, for example, the modems observe that high frequencies are too weak on the phone line, they will compensate by boosting high frequencies before sending them.

Modern modems also use echo cancellation to eliminate the return of their own signals from the distant end of the telephone line. To achieve this, a modem sends out a tone and listens for its return. Once it determines how long the delay is before the return signal occurs and how strong the return is, the modem can compensate by generating the opposite signal and mixing it into the incoming data stream.

Echo Cancellation

When satellite rather than fiber optic technology dominated the long-distance telephone market, the half-second delay imposed by the long distance the signals traveled from earth to satellite and back created annoying "echoes" on the line. You spoke, and a fraction of a second later, you heard your own voice, delayed by the satellite hop, as it looped through the entire connection.

Switching to fiber optic long-distance connections minimizes the problem, at least to your ears. But modems can be confused even with the resulting short-delay echoes. A delayed pulse or phase-shift can sound like good data to a modem. To prevent such problems, modern modems use echo cancellation , which automatically subtracts the original signal from what the modem hears after the echo delay, thus canceling out the echo. To properly cancel the echo, the modem must be trained ”during the initial handshake with a distant modem, it checks for echo and measures the delay. It later uses the discovered values for its echo cancellation.

Fallback

Most modems use at most two carriers for duplex communications. These carriers are usually modulated to fill the available bandwidth. Sometimes, however, the quality of the telephone line is not sufficient to allow reliable communications over the full bandwidth expected by the modem, even with line compensation. In such cases, most high-speed modems incorporate fallback capabilities. When the top speed does not work, they attempt to communicate at lower speeds that are less critical of telephone line quality. A pair of modems might first try 56,000bps and be unsuccessful . They next might try 53,000 or switch from high-speed to conventional modem technology with a fallback to 33,600bps.

Most modems fall back and stick with the slower speed that proves itself reliable. Some modems, however, constantly check the condition of the telephone connection to sense for any deterioration or improvement. If the line improves , these modems can shift back to a higher speed.

Multiple-Carrier Modems

Most modems rely on a relatively complex form of modulation on one or two carriers to achieve high speed. However, one clever idea (now relegated to a historical footnote by the latest modem standards) is the multiple-carrier modem, which uses relatively simple modulation on several simultaneous carrier signals. One of the chief advantages of this system comes into play when the quality of the telephone connection deteriorates. Instead of dropping down to the next incremental communications rate, thus generally cutting data speed in half, the multiple-carrier modems just stop using the carriers in the doubtful regions of the bandwidth. The communication rate may fall off just a small percentage in the adjustment. (Of course, it could dip by as much as a normal fallback modem as well.)

Data Compression

Although there's no way of increasing the number of bits that can cross a telephone line beyond the capacity of the channel, the information-handling capability of the modem circuit can be increased by making each bit more meaningful. Many of the bits that are sent through the telecommunications channel are meaningless or redundant ”they convey no additional information. By eliminating those worthless bits, the information content of the data stream is more intense , and each bit is more meaningful. The process of paring the bits is called data compression .

The effectiveness of compression varies with the type of data that's being transmitted. One of the most prevalent data-compression schemes encodes repetitive data. Eight recurrences of the same byte value might be coded as two bytes, one signifying the value and the second the number of repetitions. This form of compression is most effective on graphics, which often have many blocks of repeating text. Other compression methods may strip out start, stop, and parity bits.

At one time, many modem manufacturers had their own methods of compressing data so that you needed two matched modems to take advantage of the potential throughput increases . Today, however, most modems follow international compression standards so that any two modems using the same standards can communicate with one another at compressed-data speeds. The most efficient of these international standards is called V.44 .

These advanced modems perform the data compression on the fly in their own circuitry as you transmit your data. Alternately, you can precompress your data before sending it to your modem. Sort of like dehydrating soup, precompression (also known as file compression ) removes the unnecessary or redundant parts of a file, yet allows the vital contents to be easily stored and reconstituted when needed. This gives you two advantages: The files you send and receive require less storage space because they are compressed, and your serial port operates at a lower speed for a given data throughput.

Note that once a file is compressed, it usually cannot be further compressed. Therefore, modems that use on-the-fly compression standards cannot increase the throughput of precompressed files. In fact, using one on-the-fly modem data-compression system (MNP5) actually can increase the transmission time for compressed files as compared to not using modem data compression.

Error Checking and Error Correction

Because all high-speed modems operate closer to the limits of the telephone channel, they are naturally more prone to data errors. To better cope with such problems, nearly all high-speed modems have their own built-in error-checking methods (which detect only transmission errors) and error-correction methods (which detect data errors and correct the mistakes before they get passed along to your computer). These error-checking and error-correction systems work like communications protocols, grouping bytes into blocks and sending cyclical redundancy checking information. They differ from the protocols used by communications software in that they are implemented in the hardware instead of your computer's software. That means they don't load down your computer when it's straining at the limits of its serial ports.

It can also mean that software communications protocols are redundant and a waste of time. As mentioned before, in the case of switching modems, using a software-based communications protocol can be counterproductive with many high-speed modems, slowing the transfer rate to a crawl. Most makers of modems using built-in error-checking advise against using such software protocols.

All modem error-detection and error-correction systems require that both ends of the connection use the same error-handling protocol. In order that modems can talk to one another, a number of standards have been developed. Today, the most popular are MNP4 and V.42. You may also see the abbreviations LAPB and LAPM describing error-handling methods.

LAPB stands for Link Access Procedure, Balanced , an error-correction protocol designed for X.25 packet-switched services such as Telebit and Tymnet. Some high-speed modem makers adapted this standard to their dial-up modem products before the V.42 standard (described later) was agreed on. For example, the Hayes Smartmodem 9600 from Hayes Microcomputer Products included LAPB error-control capabilities.

LAPM is an acronym for Link Access Procedure for Modems and is the error-correction protocol used by the CCITT V.42 standard, described later in the chapter.

Combining Voice and Data

Having but a single telephone line can be a problem when you need to talk as well as send data. In the old days, the solution was to switch. You'd type a message to the person at the other end of the connection, such as "Go voice," pick up the telephone handset, and tell your modem to switch back to command mode so you could talk without its constant squeal.

In the early 1990s, several manufacturers developed the means of squeezing both data and voice down a single telephone line at the same time using special modem hardware. Three technologies ”VoiceView, VoiceSpan, and DSVD ”vied for market dominance .

Internet technology has made all three irrelevant. Instead of combining data and voice in modem hardware, the modern alternative is to combine them using software inside your computer. Your computer captures your voice with a microphone and digitizes it. Web software packages the voice information into packets that get sent to an ordinary modem exactly like data packets. At the other end of the connection, the receiving computer converts the voice packets back to audio to play through the computer's speakers . The modem connection doesn't care ”or even know ”whether its passing along packets of data or digitized audio.

Analog Standards

Neither men nor modems are islands. Above all, they must communicate and share their ideas with others. One modem would do the world no good. It would just send data out into the vast analog unknown, never to be seen (or heard) again.

But having two modems isn't automatically enough. Like people, modems must speak the same language for the utterances of one to be understood by the other. Modulation is part of the modem language. In addition, modems must be able to understand the error-correction features and data-compression routines used by one another. Unlike most human beings, who speak any of a million languages and dialects, each somewhat ill-defined, modems are much more precise in the languages they use. They have their own equivalent of the French Academy: standards organizations.

In the United States, the first standards were set long ago by the most powerful force in the telecommunications industry, which was the telephone company. More specifically , the American Telephone and Telegraph Company was the telephone company prior to is breakup announced on January 8, 1982, which resulted in seven local operating companies (in addition to AT&T). Before then, the Bell System created nearly all U.S. telephone standards, including two of the historically most important modem standards, Bell 103 and Bell 212A.

With the globalization of business and technology, communication standards have become international, and the onus to set new standards has moved to an international standards organization that's part of the United Nations, the International Telecommunications Union (ITU) Telecommunications Standards Sector, which was formerly the Comite Consultatif International Telegraphique et Telephonique (in English, that's International Telegraph and Telephone Consultative Committee). All the current standards for modems fall under the aegis of the ITU. You can purchase copies of the ITU standards from the organization's Web site at www.itu.org.

Standards are important when buying a modem because they are your best assurance that a given modem can successfully connect with any other modem in the world. In addition, the standards you choose will determine how fast your modem can transfer data and how reliably it will work. The kind of communications you want to carry out will determine what kind of modem you need. If you're just going to send files electronically between offices, you can buy two nonstandard modems and get more speed for your investment. But if you want to communicate with the rest of the world, you will want to get a modem that meets the international standards. Table 13.1 summarizes major modem speed standards.

Table 13.1. Modem Speed Standards

Standard	Organization	Introduction	Baud Rate	Data Speed	Details
103	Bell	1963	300	300bps	FSK
V.21	CCITT	1965	300	300bps	International version of Bell 103 using different carrier frequencies
212A	Bell	1979	600	1200bps	QDPSK
V.22	CCITT	1980	600	1200bps	QDPSK, International version of Bell 212A
V.22bis	CCITT	1984	600	2400bps	QAM
V.32	CCITT	1984	2400	9600bps	QAM
V.32bis	CCITT	1991	2400	14,400bps	QAM
V.34	CCITT	1998	3429	33,600bps	QAM, highest true analog speed
V.90	ITU	1999	8000	56,000bps up, 33,600bps down	Level-sensitive digital encoding
V.92	ITU	2000	8000	56,000bps up, 48,000 down	Level-sensitive digital encoding

Nearly all new modems now sold operate at a top speed set by the V.92 modem standard, the international standard for modem communications at 56,000 bits per second (or 56Kbps) across dial-up telephone lines. It is the highest speed modem standard in use today and possibly the highest speed true modems will ever achieve. Note that this standard is essentially the same as ITU V.90. The new designation does not indicate an increase in speed. The chief changes between V.90 and V.92 include improved connection setup and handshaking, so it takes a modem less time to set up a V.92 connection. The V.92 standard includes (and is compatible with) the V.90 standard, and modems matching the two standards will interconnect using V.90 technology.

Strictly speaking, V.92 is not a modem standard because, at its top speeds, it involves no modulation or demodulation. When line conditions are not favorable, however, it shifts back to analog technology to cope.

It is an asymmetrical standard, with a maximum upstream data rate of 48,000bps and a maximum downstream data rate of about 56,000bps. It uses switching technology and cannot send and receive simultaneously. In theory, in many connections, its downstream falls short of its maximum rate because of line conditions. In addition, telephone regulations at one time prohibited the V.92 top operating speed because the last bit of speed pushed the power level on the telephone line above the allowed standards. Most sources listed a top practical speed of 53,000 bits per second, but your actual connection speed is now determined by line conditions and not law.

V.92 takes advantage of the digital technology already in use to shift voice calls across long-distance digital lines. The telephone standard for voice is to sample analog signals 8000 times a second using an eight-bit digital code. V.92 translates the digital code into voltage levels on the telephone line from your local phone company's digital switch to your home or office. Encoding digital information as voltage levels is a technology called pulse amplitude modulation (PAM). Because the V.92 standard requires modems to operate at frequencies in excess of normal voice circuits, its range is limited ”V.92 connections require your modem to have no more than three miles of telephone wire between it and your telephone company's switch.

It should take 256 voltage levels, which are technically called quantization levels , to encode the eight-bit data stream. However, noise and line conditions can often mask changes of 1/256th of the voltage on an ordinary telephone line. To avoid problems with noise, the V.92 system uses only 128 quantization levels to encode data, allowing a seven-bit digital code with an 8000Hz sampling rate or a 56,000bps data rate.

As originally developed as the V.90 standard, modems used the PAM system only for downstream communications (that is, from the telephone company's switch to your modem). Upstream data (from your modem back to the phone company) relied on conventional modem technology under the V.34 standard, although limited to 31,200bps. The underlying assumption was that the phone company was better able to control digital signals and keep them within the required limits. Moreover, only the telephone company had direct access to the digital connection with long-distance trunk lines.

With the advent of the V.92 standard, this situation changed. Under V.92, modems use a technology called V.PCM upstream to let you use PAM to send data to other modems and services at a rate of 48,000 bits per second. The only difference between upstream and downstream is that the upstream signal is limited to only 64 quantization levels, thus allowing a six-bit code. The fewer voltage levels means that modem manufacturing inconsistencies and installation differences are less likely to cause signals exceeding the levels allowed by law.

To reduce the time needed for modems to connect, V.92 uses a technology called QuickConnect that takes advantage of the digital nature of the telephone system from the central office onward. The only part of the connection that needs line compensation is the local phone loop between where you use your modem and the telephone company's central office. Unless you're traveling with a notebook computer, this connection doesn't often change (and even when you're traveling, you're likely to use the same hotel phone for several connections at a time). Consequently, QuickConnect checks and remembers the settings for the local telephone loop and tries to reuse them if possible.

In truth, QuickConnect does not reduce the time required to determine the quality of a connection and compensate for it. Rather, it remembers the quality of your last connection under the assumption that you'll use the same telephone line for consecutive calls. The QuickConnect system stores in nonvolatile memory the equalizer and echo-cancellation settings as well as the digital characteristics of the line. When you place a subsequent call, your modem first examines the tone from the distant modem and compares it to the setting in memory. In case of a match, the modem starts with the stored settings to make a fast connection. If the modem detects a substantial change (such as you're using a notebook computer in a different hotel room), it walks through the standard V.90 handshaking procedure. The first time you use a V.92 modem in a given location, it will use the standard V.90 handshaking, requiring 20 to 30 seconds to adjust for the local phone line. Subsequent QuickConnect handshaking usually takes about half as long.

Recognizing how modems are used in normal telephone systems, V.92 explicitly recognizes the need to be able to pause modem communications so you can take another call. A feature called Modem-on-Hold lets you suspend data transfer without looking at your modem connection. In effect, you can put your modem on hold, take another call, and then return to your modem connection.

The V.92 standard is regarded as a refinement of V.90, which evolved out of two competing 56Kbps systems. K56flex is a proprietary technology that was independently developed and initially marketed as two different and incompatible systems by Rockwell and Lucent Technologies. In November, 1996, the two companies agreed to combine their work into a single standard. x2 is a proprietary technology developed by U.S. Robotics. Both K56flex and x2 used the same PAM digital encoding as was adopted for V.90 and V.92. They differed from each other (and the standard) only in the handshaking used to set up the connection.

In addition to speed standards, several other standards have been used for data compression and error correction. Before the widespread adoption of international standards, many U.S. companies used technologies developed by Microcom Corporation and formalized as Microcom Networking Protocol Standards Levels 1 through 9. Currently, however, the dominant modem compression and error-control standards are those set by the ITU. These include V.42, a world-wide error-correction standard (which also incorporates MNP4 as an "alternative" protocol), V.42bis (data compression that can yield compression factors up to four, potentially quadrupling the speed of modem transmissions), and V.44, an improved compression system.

Modem Hardware

A modem is a signal converter that mediates the communications between a computer and the telephone network. In function, a modern computer modem has five elements: interface circuitry for linking with the host computer; circuits to prepare data for transmission by adding the proper start, stop, and parity bits; modulator circuitry that makes the modem compatible with the telephone line; a user interface that gives you command of the modem's operation; and the package that gives the modem its physical embodiment.

Computer Interface

For a modem to work with your computer, the modem needs a means to connect to your computer's logic circuits. At one time, all modems used a standard or enhanced serial port to link to the computer. However, because the standard serial port tops out at a data rate that's too slow to handle today's fastest modems ”the serial limit is 115,200 bits per second, whereas some modems accept data at double that rate ”modem-makers have developed parallel- interfaced modems.

All modems, whether installed outside your computer, in one of its expansion slots, or in a PCMCIA slot, make use of a serial or parallel communications port. In the case of an internal computer modem, the port is embedded in the circuitry of the modem, and the expansion bus of the computer itself becomes the interface.

With an external modem, this need for an interface (and the use of a port) is obvious because you fill the port's jack with the plug of a cable running off to your modem. With an internal modem, the loss is less obvious. You may not even detect it until something doesn't work because both your modem and your mouse (or some other peripheral) try to use the same port at the same time.

In the case of serial modems, this interface converts the parallel data of your computer into a serial form suitable for transmission down a telephone line. Modern modems operate so fast that the choice of serial port circuitry (particularly the UART) becomes critical to achieving the best possible performance.

The serial and parallel ports built into internal modems are just like dedicated ports of the same type. They need an input/output address and an interrupt to operate properly. The Plug-and-Play system assigns these values to the modem during its configuration process. (Older modems required you to select these values with jumpers or switches.)

Ordinarily you don't need to know or bother with these values. Some communications software, however, may not mesh perfectly with the Windows system. It may demand you tell it the port used by your modem. The modem's properties sheet lists this value. You can check it under Windows by clicking the modem icon in Control Panel and then clicking the Properties tab.

Data Preparation

Modern modem communications require that the data you want to send be properly prepared for transmission. This pre-transmission preparation helps your modem deliver the highest possible data throughput while preventing errors from creeping in.

Most modem standards change the code used by the serial stream of data from the computer interface into code that's more efficient (for example, stripping out data-framing information for quicker synchronous transfers). The incoming code stream may also be analyzed and compressed to strip out redundant information. The modem may also add error-detection or error-correction codes to the data stream.

At the receiving end, the modem must debrief the data stream and undo the compression and coding of the transmitting modem. A micro controller inside the modem performs these functions based on the communications standard you choose to use. If you select a modem by the communications standards it uses, you don't have to worry about the details of what this micro controller does.

Modulator

The heart of the modem is the circuitry that actually converts the digital information from your computer into analog-compatible form. Because this circuitry produces a modulated signal, it is called a modulator .

User Interface

The fourth element in the modem is what you see and hear. Most modems give you some way of monitoring what they do either audibly with a speaker or visually through a light display. These features don't affect the speed of the modem or how it works but can make one modem easier to use than another. Indicator lights are particularly helpful when you want to troubleshoot communication problems.

Line Interface

Finally, the modem needs circuitry to connect with the telephone system. This line interface circuitry (in telephone terminology, a data access arrangement ) boosts the strength of the modem's internal logic-level signals to a level matching that of normal telephone service. At the same time, the line interface circuitry protects your modem and computer from dangerous anomalies on the telephone line (say, a nearby lightning strike), and it protects the telephone company from odd things that may originate from your computer and modem (say, a pulse from your computer in its death throes).

From your perspective, the line interface of the modem is the telephone jack on its back panel. Some modems have two jacks so that you can loop through a standard telephone. By convention, the jack marked "Line" connects with your telephone line; the jack marked "Phone" connects to your telephone.

Over the years, this basic five-part modem design has changed little. But the circuits themselves , the signal-processing techniques that they use, and the standards they follow have all evolved to the point that modern modems can move data as fast as the theoretical limits that telephone transmission lines allow.

Packaging

Internal modems plug into an expansion slot in your computer. The connector in the slot provides all the electrical connections necessary to link to your computer. To make the modem work, you only need to plug in a telephone line. The internal modem draws power from your computer, so it needs no power supply of its own. Nor does it need a case. Consequently, the internal modem is usually the least expensive at a given speed. Because internal modems plug into a computer's expansion bus, a given modem is compatible only with computers using the bus for which it was designed. You cannot put a computer's internal modem in a Macintosh or workstation.

External modems are self-contained peripherals that accept signals from your computer through a serial or parallel port and also plug into your telephone line. Most need an external source of power, typically a small transformer that plugs into a wall outlet and ”through a short, thin cable ”into the modem. At a minimum, then, you need a tangle of three cables to make the modem work. You have two incentives to put up with the cable snarl. External modems can work with computers that use any architecture as long as the computer has the right kind of port. In addition, external modems usually give you a full array of indicators that can facilitate troubleshooting.

Pocket modems are compact external modems designed for use with notebook computers. They are usually designed to plug directly into a port connector on your computer, eliminating one interface cable. Many eliminate the need for a power supply and cable by running from battery power or drawing power from your computer or the telephone line.

PC Card modems plug into that PCMCIA slots that are typically found in notebooks . They combine the advantage of cable-free simplicity of internal modems with the interchangeability of external modems (the PCMCIA interface was designed to work with a variety of computer architectures). The confines of the PCMCIA slot also force PC Card modems to be even more compact than pocket modems. This miniaturization takes its toll in higher prices, however, although the ability to quickly move one modem between your desktop and portable computer can compensate for the extra cost.

The confines of a PCMCIA slot preclude manufacturers from putting a full-size modular telephone jack on PC Card modems. Modem-makers use one of two workarounds for this problem. Most PC Card modems use short adapter cables with thin connectors on one end to plug into the modem and a standard modular jack on the other. Other PC Card modems use the X-Jack design, developed and patented by Megahertz Corporation (now part of 3Com). The X-Jack pops out of the modem to provide a skeletal phone connector into which you can plug a modular telephone cable. The X-Jack design is more convenient because you don't have to carry a separate adapter with you when you travel. On the other hand, the X-Jack makes the modem more vulnerable to carelessness. Yank on the phone cable, and it can break the X-Jack and render the modem useless. Yanking on an adapter cable will more likely pull the cable out or damage only the cable. On the other hand, the connectors in the adapter cables are also prone to invisible damage that can lead to unreliable connections.

Indicators

The principal functional difference between external (including pocket) and internal (including PC Card) modems is that the former have indicator lights that allow you to monitor the operation of the modem and the progress of a given call. Internal modems, being locked inside your computer, cannot offer such displays. Some software lets you simulate the lights on your monitor, and Windows will even put a tiny display of two of these indicators on your task bar. These indicators can be useful in troubleshooting modem communications, so many computer people prefer to have them available (hence, they prefer external modems).

The number and function of these indicators on external modems vary with the particular product and the philosophy of the modem-maker. Typically you'll find from four to eight indicators on the front panel of a modem, as shown in Figure 13.1.

Figure 13.1. Typical indicators on an external modem's front panel.

The most active and useful of these indicators are Send Data and Receive Data . These lights flash whenever the modem sends out or receives in data from the telephone line. They let you know what's going on during a communications session. For example, if the lights keep flashing away but nothing appears on your monitor screen, you know you are suffering a local problem, either in your computer, its software, or the hardware connection with your modem. If the Send Data light flashes but the Receive Data light does not flicker in response, you know that the distant host is not responding.

Carrier Detect indicates that your modem is linked to another modem across the telephone connection. It allows you to rule out line trouble if your modem does not seem to be getting a response. This light glows throughout the period your modem is connected.

Off-Hook glows whenever your modem opens a connection on your telephone line. It lights when your modem starts to make a connection and continues to glow through dialing, negotiations, and the entire connection.

Terminal Ready glows when the modem senses that your computer is ready to communicate with it. When this light is lit, it assures you that you've connected your modem to your computer and that your computer's communications software has properly taken control of your serial port.

Modem Ready glows whenever your modem is ready to work. It should be lit whenever your modem is powered up and not in its test state.

High Speed indicates that the modem is operating at its fastest possible speed. Some modems have separate indicators for each speed increment they support. Others forego speed indicators entirely.

Auto Answer lights to let you know that your modem is in its answer mode. If your telephone rings, your modem will answer the call (at least if its connected to the line that is ringing).

Table 13.2 summarizes the mnemonics commonly used for modem indicators and their functions.

Table 13.2. Modem Indicator Abbreviations and Definitions

Mnemonic	Spelled Out	Meaning
HS	High Speed	Modem is operating at highest speed.
AA	Auto Answer	Modem will answer phone.
CD	Carrier Detect	Modem is in contact with remote system.
OH	Off Hook	Modem is off hook, using the phone line.
RD	Receive Data	Modem is receiving data.
SD	Send Data	Modem is transmitting data.
TR	Terminal Ready	PC is ready to communicate.
MR	Modem Ready	Modem is ready to communicate.