This is an edited and condensed excerpt from The Modem Reference, written by Michael A. Banks and recommended by the Associated Press, The Smithsonian Magazine, Jerry Pournelle in Byte, et al. The right to reproduce this article is granted on the condition that all text, including this notice and the notice at the end of the article, remain unchanged, and that no text is added to the body of the article. Thanks! --MB Copyright (c), 1988, 1989, 1990, Michael A. Banks All Rights Reserved (From Chapter 4) MODEMS Chapter 2 covered the basics of telecomputing equipment and software, and Chapter 3 explained the technical aspects of personal computer telecommunications. This prepared you the in- depth look at modems in this chapter. The major topics in this chapter are modem types, modem features and options, and important factors to consider when buying a modem. I've included tips on using modems, too. If you don't yet have a modem, or are considering an upgrade to one with more features or higher speed, read this chapter; it takes the mystery out of modems. MODEM TYPES Modems come in two basic physical types: acoustic and direct connect. Most computer systems operate with either type, and each has its advantages and disadvantages, as described below. Acoustic Modems The simplest type of modem is the acoustic modem (sometimes called an acoustic coupler). This kind of modem was originally developed to circumvent now-defunct telephone company regulations involving devices connected to telephone lines. No part of an acoustic modem is electrically connected to a telephone or telephone lines. Instead, the modem communicates with the telephone via sound waves. A typical acoustic modem configuration (as shown in Figure 4.1) consists of the modem itself (which may be a stand-alone desktop unit or an internal card), and rubber microphone and speaker cups that connect the modem to a standard desk telephone handset. (NOTE: You'll find it difficult or impossible to use an acoustic modem with "designer"-style telephones, cordless telephones, and other telephones that have a non-standard handset. The speaker cups fit snugly over a standard telephone mouthpiece and earpiece, to filter out outside noise.) Another style of acoustic modem provides an appropriately-shaped cradle for the telephone handset. Figure 4.1 Acoustic Modem Configuration In operation, an acoustic modem converts a computer's digital signals to audio tones. These tones are emitted by the modem's speaker cup and picked up by the microphone in the telephone's mouthpiece. The telephone then sends the signals over telephone lines. On the receiving end, the modem's microphone cup picks up the tones from the telephone's earpiece. The signals are sent over a connecting to the modem, which converts them back to digital data. An acoustic modem is usually less expensive than a direct connect modem. And, because an acoustic modem requires no special plugs or outlets, it is convenient to use with hardwired hotel or pay telephones. (These are the most practical and popular applications for acoustic modems.) On the negative side, almost all acoustic modems require manual operation (i.e., you must dial the telephone number and listen for the remote system to answer, hang up the telephone manually, etc.). And, because they are less popular than direct connect modems, you'll have a difficult time finding acoustic modems with the enhancements offered by direct connect modems. Actually, it's hard to find acoustic modems nowadays. This is because the situations that made acoustic modems necessary have pretty much ceased to exist. However, acoustic couplers are available for use with direct-connect modems. (The modem must have an interface for the acoustic coupler.) These are most useful if you frequently take your modem and computer on the road. Direct Connect Modems A direct connect modem bypasses the telephone set altogether by connecting directly with a telephone line via a standard RJ-11 jack. As described in Chapter 2, an RJ-11 jack is the receptacle on a telephone into which the telephone line (equipped with an RJ-11 plug) is plugged. Though more expensive than acoustic modems, direct connect modems are very popular for several reasons. Most direct connect modems feature auto dial capability (the ability to dial a phone number without using a telephone set) as well as other features not usually found in acoustic modems. A direct connect modem may also allow you to connect both the modem and the telephone set to a telephone line simultaneously, which means you don't have to plug and unplug the phone line to switch between modem and voice communications. And, because data are not transmitted via sound waves, there is less potential for garbled signals. There are two classes of direct connect modems--internal and external. Internal direct connect modem. An internal direct connect modem consists of an IC board, or "card," mounted in a computer's expansion slot. A typical internal modem contains the modem plus a serial port and all required connectors--including a modular jack for a telephone line. (Many laptop computers feature built-in internal modems.) Figure 4.2 shows a typical internal direct-connect modem. Figure 4.2 Internal Direct-Connect Modem An internal modem has several advantages over an external modem, not the least of which is reducing desktop clutter. Once an internal modem is installed, there are no cables in the way, and the modem is less susceptible to external physical disturbance than an external modem. If you have a "luggable" or laptop computer, using an internal modem means one less piece of hardware to carry around. And because an internal modem draws power from its host computer's power supply, it doesn't require an extra electrical outlet. (The computer's power supply should be hefty enough to handle the modem's requirements, of course.) Finally, an internal modem leaves a computer's serial port free for other applications. The major disadvantage of an internal modem is that, because it is an integral part of a computer, it is machine-specific and function only with the type of computer for which it is designed. So, if you upgrade to a different kind of computer, you'll have to buy a new modem. Another disadvantage of an internal modem is the fact that you cannot monitor a call's status via external indicators. Finally, an external modem does require a slot--which leaves you with one less slot to devote to another device. External direct connect modem. A typical external direct connect modem (as shown in Figures 4.3 and 4.4) is a thin rectangular box with a power line, one or two modular jacks, and an RS-232C connector. The RS-232C connector accommodates a cable from the computer, and is usually mounted on the back panel. (As explained in Chapter 3, an RS-232C connector can be a 5-, 9-, or 25-pin male or female connector. The shape of the connector is either that of an elongated letter "D" or a circle.) An external modem usually has a row of LEDs mounted on its front panel. These are the modem's status lights. Status lights let you monitor the modem's operation and the status of a call. An external modem may also have controls of various kinds mounted on its front panel. Figure 4.3 External Direct-Connect Modem Figure 4.4 External Direct-Connect Modem ("Economy" Model) The main advantage of external modems over internal modems is the fact that they can be used with more than one kind of computer. And there are other advantages. If an external modem needs repairs, you can fix it (or take it into the shop) without disassembling the computer. Getting at the modem's DIP switches is easy, too. (That is, you don't have to open your computer; you may have to open the modem, although you can access DIP switches on some modems externally.) And, if you feel comfortable being able to monitor the status of a call visually, an external modem's status lights are a welcome feature. (Note that most portable modems have neither DIP switches nor status lights.) Finally, external modems cost less than internal modems. This is because they are not specially-engineered to operate with a certain computer, and because--unlike internal modems--they do not have built-in serial ports. On the negative side, an external modem requires a separate power source--more often than not, a transformer that plugs into a wall outlet. (There are battery-powered modems, and modems that take their power directly from a telephone line, but these are rare.) Too, using an external modem means having to find room for one more piece of equipment on your desk or work table. (Note that the comments about external modems apply in general to portable modems as well as desktop modems. Chapter 18 has more to say about probable modems.) Several modem manufacturers (notably, AT&T and Tandy) have experimented with combining a telephone set with a direct-connect modem, as illustrated in Figure 4.5. This type of modem is convenient and saves space. Figure 4.5 A Telephone/Modem Combination Modems are also classified by whether they communicate in asynchronous or synchronous mode, and whether they can communicate over common telephone lines to access online services and BBSs or work over dedicated lines at ultra high speeds (dialup vs. leased line). These distinctions are described below. Asynchronous/Synchronous Modems If you use dialup services, you'll probably never need a synchronous modem. All BBSs and popular dialup services (including those discussed in this book) provide asynchronous service. The same is true of the major packet switching networks. Leased line (see below) and mainframe connections usually require synchronous communications. If your applications are dialup, make sure the modem you buy is capable of asynchronous operation. If you have applications in both areas, you'll be happy to know there are a number of modems that operate in either asynchronous or synchronous mode. (The differences between asynchronous and synchronous operation are explained in Chapter 3.) Dialup and Leased Line Modems Dialup modems. Simply described, a dialup modem is one that operates within a bps range (0 to 9600) that is effective for data transmission via voice-grade telephone lines. (9600 bps is the upper limit for reliable data communications within the frequency bandwidth available on voice telephone lines.) A dialup modem also uses communications parameters (parity, data bits, number of stop bits, etc.) that are compatible with other dialup modems and packet switching networks. Most (but not all) dialup modems operate in asynchronous mode. Some high-end modems, such as the Hayes Smartmodem 2400, are switchable between asynchronous and synchronous modes. Dial up modems are, in general, less expensive than leased line modems. Virtually all modems sold for use with personal computers are asynchronous dialup modems. Leased line modems. Leased line modems use extremely high bps rates (9600 and up) over dedicated two- or four-wire telephone lines, or other direct-connect lines shielded against outside interference. (These are also called "conditioned" lines.) Leased line modems may operate in synchronous or asynchronous mode, or switch between modes. Leased line modems usually transmit data in full duplex (two-way) mode. A typical leased line modem is shown in Figure 4.6. Figure 4.6 Leased Line Modem Leased line modems are useful in LAN (Local Area Network) and private network applications (such as in commercial applications where large amounts of data pass rapidly over conditioned or four-wire lines). These modems almost always include the more esoteric of the modem options discussed on the following pages, such as data encryption, automatic dialback, and data compression. (This is not to say that dialup modems cannot include these features.) Unless you are setting up your own network, or intend to use a terminal (or a personal computer as a terminal) to communicate with a mainframe or mini-computer in a nearby location, you won't need a leased line modem. Analog and Digital Modems The modem applications discussed in this book concern analog modems (that is, modems that transmit and receive data over communications links in analog form). You'll be interested to know, however, that digital modems can transmit data via special digital networks in true digital form. They don't have to convert data to analog form as analog modems do. Applications for digital modems involve specialized digital telephone networks of the type described in the preceding chapter. Special note on digital telephone systems. Many large offices have their own internal telephone network. One incoming number handles multiple calls to various extensions, as well as telephone traffic from one office to another. Such a system is called a "Private Branch Exchange," or "PBX" for short. There is usually no problem with using a modem through a conventional PBX, unless the system has a Call-Waiting-type feature. However, many modern PBX systems are digital systems. If you wish to telecompute via a digital PBX, you may have to install an analog (standard) telephone line. (This is would be a separate line with its own number, one that doesn't go through the PBX.) Try your modem with the PBX system before installing a new line, though; chances are, your modem should operate with the PBX. An alternative to having a dedicated phone line installed is to buy a digital-to-analog converter. Also known as analog-to-digital converters, or "A-to-D" or "D-to-A" converters for short, these devices handle changing a modem's analog output to a format compatible with a digital PBX. They also convert incoming data from a digital PBX to the analog format recognized by dialup modems. Check with your computer dealer or phone company on the availability of A-to-D converters. Selecting a modem deserves at least as much thought and consideration as you put into selecting your computer. Even bargain-priced modems are not inexpensive, and you have to live with your mistakes (unless, that is, your income and spending habits can easily absorb a useless expenditure of several hundred dollars--or more). So, take your time and choose carefully! You may find the "do it yourself" modem comparison chart in Table 4.2 helpful in selecting your modem. (Make a copy of the chart if you'd rather not mark in this book.) Write the model names of the modems you're considering in the blank row at the top of the chart, highlight the features in the left hand column that interest you, and check off those features as they apply to each modem model whose name you enter at the top of the page. MODEM FEATURES AND OPTIONS The number of modem features and options available is surprisingly large, and the source of more than a little confusion (especially since communications software packages offer many of the same features). I'll describe the more important features here, to acquaint you with what's available and to help you develop a "shopping list" of modem features. You'll also find this section handy when looking over a list of specifications for a particular modem. (Spec sheets tend to list buzz words without explaining what those words mean.) Not all software packages can use all the features offered by some modems. Some features may require communications software designed for use with the modem in question. Too, some modem features (like built-in error-checking protocol) will not operate unless you're calling a modem with the same features. You should also be aware that some features are available only with dialup modems, while others are available only with leased line modems. Unless otherwise noted, the features discussed in the following pages are available for dialup modems (sometimes as extra-cost options) as well as for leased line modems. Not every modem offers every feature discussed here. The features aren't listed in order of importance. Importance is subjective and depends on your application. Finally, some of the capabilities discussed in the following pages are also available as communications software features. Where appropriate, I've noted when this is the case. Notes on "Intelligent" Modems and Features Dialup modems are frequently advertised as "intelligent." This term usually alludes to the ability to perform certain functions automatically, such as dialing numbers, answering the telephone, etc. The "intelligence" consists of programming in ROM or hardware features. Advanced functions offered by intelligent modems are normally performed in response to commands issued from the keyboard or (more typically) by your communications software, or in response to signals from the system you have dialed up. As with people, modem intelligence varies; an "intelligent" modem may offer any number of the functions discussed here. If you see a modem labeled "intelligent" (or "full-featured"), don't count on it having everything you want. Be sure to get a complete list of a modem's features before buying it. Variable Communications Parameters Unless you plan to use a modem to dial up only one other system, don't buy a modem with fixed communication parameters. Such a modem will not respond to software commands to change parameters. For maximum utility, these parameters should be variable: * speed * duplex (echo) * parity * data bits * stop bit(s) Automatic parameter adjustment. Some top of the line modems (both leased line and dialup) sense and adjust to the communications parameters in use by a remote system. This capability is often called "automatic feature negotiation." AutoDial/Auto Answer Autodial. One of the most popular modem features is "autodial" (or, "auto dial")--the ability to dial a number without the use of a telephone set. With autodial, all your communications software has to do is send a command telling the modem to dial a number. (The command may come from your keyboard, or from a communication program's autologon script file.) Once it receives the dial commands and number, the modem opens the phone line, dials the number and--if there is an answer on the other end--lets the answering computer know a computer is calling. Most autodial modems can dial using either tones or pulses. Auto answer. Auto answer capability means that the modem can detect an incoming ring (via a variation in telephone line voltage), "answer" the phone, keep the line open, and let your software know there is an incoming call. (The communications software--assuming it has the capability--takes over from there and sends the appropriate commands to the modem.) Selective Dialing Capability (Tone/Pulse) This is the ability to dial by either tone or pulse. While the majority of telephone exchanges in the U.S. handle tone dialing, it is a good idea to have both capabilities. Tone dialing. Tone dialing is dialing by sending DTMF signals over the phone line. DTMF is an acronym for "Dual Tone Modulated Frequency," which is the type of signal used by a touch tone system. If you've a good ear, you may hear two tones when you press a touch tone telephone button. The tones are carrier tones, modulated with information that identifies the number pressed. These tones go to a local telephone switching computer, which connects you with the number dialed. A modem with tone dialing capability can generate these tones. Pulse dialing. Pulse dialing is used with "rotary" style phones (the old ones with a true dial rather than push buttons). When a number is dialed with pulses, a series of clicks is sent through the telephone line--one click for the number 1, two clicks for the number 2, and so on, all the way up to ten clicks for 0. This is rather slow, but necessary with old telephone systems in some parts of the country. (You can try pulse dialing manually with your phone--even if it is a Touch Tone telephone. Lift the handset, then depress the "hangup button" rapidly for each number--once for 1, twice for 2, three times for 3, and so on. You can actually dial a number in this manner--which is the way a pulse dial modem does it.) You can often use tone dialing even if there is a rotary phone connected to the line. You'll often find locations that use rotary phones even though tone dialing is supported; it's a matter of economy. So, always try tone dialing before you try pulse dialing. The presence of a rotary phone doesn't mean tone dialing isn't supported. If dialing a number using tones has no effect, you can always switch your modem to dial using pulses (normally with the command ATDP). Tone dialers. If you have a manual-dial modem, you can "upgrade" it to autodialing, even if you have a rotary-dial telephone. All you need is a device known as a "tone dialer." As illustrated in Figure 4.7, a tone dialer is a handheld device with a speaker and numbered buttons. Figure 4.7 Tone Dialer When the buttons are pressed, the tone dialer's speaker emits the standard DTMF signals used by phone systems. These tones are picked up by a telephone's microphone and have the same effect as dialing with a telephone. The better tone dialers store several numbers for later playback. You can use this kind of tone dialer as an online "phone directory" to speed up manual dialing; all you have to do to dial a number is press a couple of keys. This gives you the convenience of an autodial modem with number storage. It also speeds up dialing and eliminates errors. Adaptive dialing. Modems with adaptive dialing sense whether they should use tone or pulse dialing, depending upon whether the local telephone system is equipped to handle tone dialing. This information is present in the dial tone sent by tone-dial telephone systems. When a modem with adaptive dialing receives a dial command without tone or pulse specification, it will try tone dialing first; if the dial tone is still present, it switches to pulse dialing. The U.S. Robotics Courier 2400e is one example of a modem that implements this feature. Default dial settings. Intelligent modems can be set to dial using tone or pulse only. When so programmed, a dial command sent to the modem doesn't have to include the tone or pulse specification, unless the non-default setting is to be used. Pause Capability The ability to pause during dialing is necessary if a modem is dialing through a PBX, certain long-distance services, or any phone system that pauses before a dial tone comes up. For example, if a PBX system requires you to dial 9 to get an outside line, you may have to wait one or two seconds after you dial 9 before the dial tone is heard. A communications program can tell a modem with pause capability to wait one second after dialing 9 before dialing the rest of the number. This is normally done by embedding the standard AT pause command (,) in the number, thus: 9,7221969. This sequence, sent to a modem as the number to be dialed, would tell the modem to dial 9, wait one second, then dial 7221969. Speaker A speaker is useful when you wish to monitor a dialup. With a speaker, you can hear if there's a busy signal, and thus not wonder why you're not getting an answer. And, if you get a voice response ("live," or, in the case of a changed number, a recording), you can respond appropriately. (The speaker is mounted inside the modem.) Volume control. A nice extra that's not available with all speaker-equipped modems is a volume control. If you buy a modem that doesn't have a volume control, you may wish it did after a few online sessions. Hearing a dial tone and the sound of the modem dialing is reassuring to some, but annoying to others. Most volume controls are external (manual). But many modems (among them the Hayes Smartmodem 2400 and the U.S. Robotics Courier 2400e), let you adjust speaker volume with software commands. Some modems offer both external and software volume controls. Communication Speeds As of this writing, 2400 bps is the "standard" communications speed for the vast majority of BBSs and online services, while a few offer 9600-bps service. (The larger online services and packet switching networks are moving in the direction of 9600-bps dialup service. Look for 9600-bps service to become the standard before the decade's out.) Despite all this, you'll still find systems limited to 300 or 1200 bps. And you will sometimes want to use 300 or 1200 bps on a commercial online service. So, while you'll want a modem with a maximum speed of at least 2400 bps, be sure it can handle 300- and 1200-bps communication, as well. Standards Modem standards are discussed in detail in Chapter 3. Suffice to say that if you use a modem in the U.S., it should conform to one of these standards: 300 bps Bell 103 (North America) 1200 bps Bell 212A (North America) 300/1200 bps CCITT V.22 and V.32 (International) 2400 bps CCITT V.22 bis (North America/International) Multi-speed modems are usually "downward compatible" with reference to standards. If a modem conforms to a recommended standard at its highest bps rate, it generally conforms to that standard's counterpart at lower rates. For instance, a 2400-bps modem that is compatible with the CCITT V.22 bis standard is compatible with the Bell 212A standard at 1200 bps (and probably the CCITT V.22 and V.32 standards, as well). Modems that accommodate both Bell and CCITT standards are offered by a number of manufacturers. Any modem you use in the U.S. should also be registered with the FCC (Federal Communications Commission) as meeting that body's standards. You'll find evidence of such registration on the modem's serial-number plate, and in the modem's manual. Don't buy a modem that isn't FCC registered. Dual Phone Jacks External direct-connect modems with autodial capability do not, of course, have to be connected to a telephone to function. But you may wish to have a telephone set connected to the telephone line along with the modem. This way, you can use either the phone or the modem without switching the telephone line from one to the other. One way to do this is with a "Y-jack" (sometimes called a Y-plug or Y-cable). This is a short cable with an RJ-11 jack, or receptacle at one end; the other end of the cable splits into two wires, each of which has an RJ-11 plug at its end. The telephone line is plugged into the single RJ-11 jack, and the two RJ-11 plugs are connected to your modem and your telephone set, respectively. You can use a "duplex jack" to do the same thing. (A duplex jack is a plastic block with an RJ-11 plug on one end and two RJ-11 jacks on the other, as shown in Figure 4.8.) Figure 4.8 Duplex Jack A more convenient approach is to buy a modem with two RJ-11 jacks. The incoming telephone line is plugged into one jack, and a short cord with an RJ-11 plug on each end (usually called the "telephone cable" and supplied with the modem) is plugged into the other jack. The free plug on the short cord is then plugged into the telephone set. The net effect is that the modem and telephone are connected to the same phone line in parallel. Automatic Hangup on Carrier Loss A few autodial/auto answer modems don't have this feature--and it's an important one to have. With automatic hangup, the modem automatically hangs up the phone (disconnects from the phone line) if the remote system hangs up or is accidentally disconnected. This feature is especially useful if you use a phone line for both voice and data; if it's not present, your phone may stay "off hook" after a remote system disconnects and prevent calls from coming in. A few modems with this capability will disconnect only when told to do so by communications software. Because of this, you must check carefully to make sure automatic hangup means the modem and not the software performs the operation. (There's nothing wrong with relying on your software to do this; but the feature should be available if you need it.) Visual Call Status and System Monitoring Most external desktop modems have status lights (usually LEDs--light-emitting diodes--rather than lights) on their front panels, as illustrated in Figure 4.9. These indicators provide a way to visually monitor the status of a call and the telecomputing system. Figure 4.9 A Modem with Front-Panel Indicators Status lights are usually marked with two- or three-letter abbreviations to indicate their purpose. The status indicators provided most frequently are: * AA "Auto Answer." This indicator is on when a modem with auto answer capability is set to answer incoming calls. * CD "Carrier Detect." This indicator lights when a modem detects a carrier tone from an answering computer during a call. (At the same time, the modem sends a "carrier detect" signal to its computer.) * HS When the "HS" indicator is on, it means that the modem is set to 1200 or 2400 baud. * MR "Modem Ready." MR lights when a modem is ready to receive or send data (normally on whenever there is power to the modem). * OH "Off Hook." This means that the telephone line connected to a modem is currently in use, as when the modem takes control of the line to make a call. * RD "Receiving Data." This indicator flashes with each bit of data your computer receives. * SD "Sending Data." This indicator flashes with each bit of data your computer sends. (Watch it as you type; it flashes each time you press a key.) * TR "Terminal Ready." This means your terminal is ready to dial out or receive calls. You'll find other, more specialized indicators on some modems. The more elaborate communications software packages (or ancillary programs) provide the same kinds of information onscreen. Call Status Monitoring If you don't have a speaker (or even if you do), it is nice to have a modem that detects and reports the status of a call by sending messages to your screen similar to these: Message Meaning ------- ------- BUSY (busy signal detected) CONNECT (connection established) CONNECT 300 (connection established at 300 bps) CONNECT 1200 (connection established at 1200 bps) CONNECT 2400 (connection established at 2400 bps) NO ANSWER (no response after a specified number of rings) NO DIAL TONE (no dial tone present) RINGING (the number is ringing) UNSUCCESSFUL (the call was aborted or interrupted) VOICE (a voice, rather than a modem, answered your call) Advanced communications programs react to such messages by disconnecting and/or redialing as appropriate when, for example, there's no answer or a voice answers the phone. More expensive call-monitoring modems handle these situations without waiting for software commands. This feature is "switchable" in many modems. This means the level of response can be adjusted to brief numeric codes, or turned off entirely. Auto Baud Rate Switching This feature is sometimes available whether or not a modem has call status monitoring. A modem with auto baud rate switching senses and matches the baud rate in use by a remote system when connection is established. Auto baud rate is especially convenient when you're dialing a BBS new to you; you don't have to worry about which baud rate to use. Auto Redial Modems capable of auto redial store the most recently-dialed number in RAM, and will redial that number in response to a brief command. This is convenient if you call a system and receive a busy signal, and wish to try the system again in a short time. Modems with this feature can often be set to redial a number after a specified time if there's no carrier or a busy signal is detected. Many communications software packages offer this function, too. Number Storage Some modems with autodial capability also store numbers in non-volatile memory or EEPROM. This means the numbers are "remembered" even when power to the modem is off. Numbers are usually stored in a "directory" format, accessible by name. Given the proper command, the modem will retrieve a specified number from its memory and call it. Here again, many communications software packages offer this feature, eliminating the need for modem number storage. (I've found that using a communication program's "dialing directory" feature is easier than using that of a modem. This feature is most useful in a modem that you'll be using with different programs.) Parameter and Configuration Storage Modems capable of storing and retrieving numbers may also be able to store communications parameters and operating configurations. Parameter storage. Parameter storage is useful if you always use the same communications parameters. With the communications speed, duplex, parity, and number of data bits and stop bits stored in your modem's memory, all you have to do is tell the modem to dial a number. There's no need to set the parameters from your terminal. If you use autologon files with your communications program, however, you probably won't need parameter storage in your modem. Each time an autologon file is run, the communications program resets the modem parameters to those specified in the autologon file. Configuration Storage. If you change communications software frequently, and/or use your modem with more than one computer, you may have to change the modem's operating configuration to accommodate the new software or computer. The elements of a modem's operating configuration include carrier detect override, local echo, result code display, and more. (These elements vary from modem to modem; consult your modem's manual for details.) The operating configuration is normally set via a modem's DIP switches, and resetting it can become a tedious chore if you have to do it frequently. With configuration storage, you can use AT-type commands to reset the modem's operating configuration from your keyboard. (Incidentally, Touchbase Systems' "WorldPort 2400" portable modem is a prime example of the utility of configuration storage. Able to operate with virtually any computer that has an RS-232C interface, it is ideal if you want a modem to use with both a desktop and a portable computer. Connection is fast and easy, and it does away with DIP switches entirely by using parameter and configuration storage.) Automatic "Fall Back" When a poor connection exists as a result of a "noisy" telephone line, it is sometimes necessary for a modem to fall back to a lower transmission speed (perhaps 100 bps less). Many high speed modems cannot fall back once connection is made, but this feature is becoming more common--especially in modems with built-in error correction. Built-in Communications Software Having communications software built into a modem may seem a good idea. You don't have to install and load software from a disk, and you know the software will work with the modem. However, you'll probably find it more convenient to use disk-based communications software. Disk-based software is generally more flexible and easier to use than built-in software. (Interestingly enough, every modem user I've talked with who has a modem with built-in software uses a disk-based program, if possible.) Modems with built-in terminal software include many of the features for terminal software (as described in Chapter 5). Automatic Data/Voice Switching Lower-cost modems may make use of a telephone set's circuitry to generate DTMF signals for dialing, or require you to dial numbers manually. Such modems must be connected to a telephone set (rather than directly to a telephone line), and you have to switch to voice mode to dial out, then to data mode when the system you are calling answers. (Switching to data mode allows the modem to route incoming data to your computer.) Similarly, many acoustic modems require that you dial numbers for them, then flip from voice to data mode when the remote system answers. Manual voice/data switching seems to be the norm for these modems, but it is obviously less desirable than automatic switching, available on some modems that require a telephone set to dial a number. So, look for automatic voice/data switching on any modem that must be connected to a telephone set. (NOTE: You can avoid the data/voice switching problem entirely by buying a modem with auto dial capability!) Telephone line- and Battery-Powered Modems It is possible to buy a modem that draws its power from a telephone line, but this is not an important feature unless you have absolutely no extra electrical outlets available where you use your computer. The same is true for battery power. Unless you use a portable modem with a laptop computer, battery power is not an important feature. Note, too, that battery life is limited with portable modems. I've got as little as two hours use from a new alkaline battery with a portable modem. Call Duration Reporting 1200 and 2400 bps modems with this option can display the duration of calls in hours, minutes, and seconds on your computer screen. This provides an audit record of online activities. (This feature sometimes includes an optional real-time clock.) Inactivity Timer This is a handy feature that prevents online charges from piling up if you leave your computer and forget to sign off a commercial online service. Modems with this feature disconnect if there is no activity (data sent or received) after a preset length of time. This feature is sometimes offered by communications software, as well. Built-in Error Correction A popular trend among modem manufacturers is to include built-in error-checking protocol as a feature with dial-up and leased line modems. Several hardware error-checking protocols are in use, including MNP (Microcom Network Protocol), X.25, LAP-B, AFT, and various other proprietary protocols, but the feature is not useful unless both modems involved in a data transfer use the protocol. It's difficult to recommend one protocol over the other, but, as of this writing, MNP seems to be the leader among dialup modems, as it has the largest installed user base. X.25 and X.400 are close behind, with X.25 being the inter-system E-mail communication standard and X.400 emerging as the international connection standard. In some instances, the use of a hardware error-checking protocol slows real-time communications slightly. These protocols typically use a system of sending groups of characters in "packets." Sometimes there is a brief delay before a character is sent because the modem waits to be sure no additional characters are coming before it sends a data packet. (Data packets and related topics are addressed in later chapters.) (NOTE: This type of protocol should not be confused with software error-checking protocols such as Xmodem, Zmodem, Kermit, et al.) SELECTING A MODEM By now, you should have a fairly good idea of whether you will be better off with an acoustic modem or an internal, external, or portable direct-connect modem. You should also know which features and options you want (or need). Deciding the configuration and features you want is only the first step in selecting a modem, however. You must also consider hardware compatibility, software compatibility, command set, price, and other factors. Hardware Compatibility First, a modem must be compatible with your computer system. One way to find out if a modem is compatible with your computer system is to read the modem's specifications, found on the modem package or in advertising material. You might also read magazine reviews, which usually tell you what kinds of the equipment you can use a modem with. If you're lucky, your computer will be compatible with the modem you're considering. In this case, you have only to make or buy the proper cable to connect the modem to your computer. The simplest way to ensure that a modem will work with your computer, however, is to buy a "system-ready" modem. A system-ready modem is typically a dial-up modem that operates with a variety of computers, but packaged with the appropriate cable and documentation for a specific brand/model of computer. Or, you might look at dial-up modems designed specifically for a computer brand/model (these are commonly internal modems, discussed earlier in this chapter). Some modems are labeled as compatible with any computer. Modems so labeled probably work with just about any computer that has a serial port. Of course, you may have to find or make the proper cable to connect the modem to your computer (see Chapter 3 for more information on cables). To do this, you will have study "pinout" diagrams for the modem and your computer, and perhaps experiment a bit. (A pinout diagram is a diagram that illustrates a connector's pin assignments.) Remember, too, that a few modems communicate via a computer's parallel port. So be sure you're getting a modem with the appropriate connection--serial or parallel--for your computer. If you have any doubt as to whether a particular modem can be used with your computer, contact the manufacturer or a knowledgeable computer salesperson. (Don't count on information from computer salespeople being 100% accurate, though. It is an unfortunate fact that all too few computer salespeople are proficient in things technical; after all, most are salespeople first. I'm reminded here of a local branch of a national computer retail chain that frequently advertises for salespeople, requesting used-car salespeople as prime candidates ....) As a backup to information given to you by salespeople, check out manufacturers' brochures and specification sheets. You might also check with acquaintances who use your brand of computer. Chances are, you'll find someone who uses the modem(s) you are considering with the computer you use. Or, your friend(s) can advise you as to which modem is best for your computer. If nothing else, you'll learn a lot about other modems that are compatible with your computer. If you're buying modem accessory hardware, or plan to use modem features that require compatibility between two modems, your safest course is to buy all equipment from the same manufacturer. Software Compatibility Another important consideration in selecting a modem is software compatibility. That is, the modem you select should work with the communications software you intend to use. This may seem obvious, but not all modems respond to the same set of commands. And unless your software issues commands to which the modem responds, you'll have trouble using the two together. (I am, of course, speaking of the actual commands the software sends to the modem--not the commands you type at your keyboard.) Fortunately, most modems use the same command set (or portions thereof)--a de facto standard called the "AT" or "Hayes" command set, which we'll examine in a few paragraphs. It also happens that some software packages come with prepared command or script files that enable them to "talk" with a variety of modems. With this type of software, you merely select the appropriate brand and model of modem from a menu during installation. The program automatically uses the proper command set with the modem. (You will have to change a DIP switch setting or two on some modems.) Usually only the more popular modems are so accommodated--yet another factor to keep in mind when shopping for a modem and software. The best way to determine modem/software compatibility is to try the software with the modem before you buy either. If you can't do that, consult with someone who already uses the modem/software combination you're considering. Bundled software. You'll find communications software included with many modems (this is known as "bundled" software). You don't have to worry about software compatibility when the modem and software come as a package--unless, that is, you find you don't like the bundled software. Figure 4.11 A "Bundled" Modem-and-Software Package If you don't want to use the software provided with a modem, you'll have to look for a program better suited to your needs and tastes. In this case, you should have a modem that responds to the aforementioned AT command set, again because the majority of communications programs use this command set. Command Set Conflicting "standards" proliferate throughout the computer industry, and telecomputing is no exception. Fortunately, the standards for modem commands have been pretty well settled. Most communications software publishers have adopted what is known as the "AT command set," or the "Hayes Standard AT command set" in recent years. This is partly because, early on, the best-selling modems used this command set and partly because a standard was needed. A modem labeled "AT compatible" or "Hayes compatible" uses (or ostensibly uses) the AT command set. Not every "Hayes compatible" modem is 100% compatible with this command set, but a modem labeled AT- or Hayes-compatible should be close enough in compatibility that software issuing standard AT commands can communicate with it. By the way, most modem manufacturers have adopted this command set, too. This way, their products are compatible with popular communications software packages. (A selling strategy, of course--the easier a product is to use, the better it sells.) The AT command set is so named because the command used to "wake up" (initiate) a modem is "AT," which stands for "attention." Other commands include "D" for "dial," "T" for "use tones when dialing," and "P" for "use pulse dialing." (Thus, the command string "ATDT" tells a modem to wake up and dial a number using tones.) Table 4.1 lists the basic standard AT commands. Table 4.1 Note that the commands listed in Table 4.1 are those commonly used in dialing and answering operations. Your modem may offer additional commands for configuration, setup, and self-diagnostics, particularly if it is a Hayes modem. Some modem manufacturers (such as U.S. Robotics and Hayes) use what is called an "extended AT command set." This is a set of specialized commands--based on the AT command set--used to access their modems' special features. The AT commands shown in Table 4.1 also operate with such modems, of course. The chances are good that any modem you are considering uses the AT command set. But, again, make sure about this before you buy it. Otherwise, you could find yourself stuck with a limited range of choices in communications software, unless you want to write your own! ((EXCLUDED: Several sections following provide more information on selecting a modem.) Selecting a modem deserves at least as much thought and consideration as you put into selecting your computer. Even bargain-priced modems can be costly, in terms of living with your mistakes. (Unless, that is, your income and spending habits can easily absorb a useless expenditure of several hundred dollars--or more). So, take your time and choose carefully! You may find the accompanying "do it yourself" modem comparison chart (Table 4.2) helpful in selecting your modem. Write the model names of the modems you're considering in the blank row at the top of the chart, highlight the features in the left hand column that interest you, and check off those features as they apply to each modem model whose name you enter at the top of the page. (Make a copy of the chart if you'd rather not mark in this book.) Table 4.2 Modem Comparison Chart ((OMITTED: The remainder of the chapter provides hints and tips on modem installation and use.)) If you found this excerpt useful, you may want to pick up a copy of the book from which it was excerpted, THE MODEM REFERENCE, recommended by Jerry Pournelle in Byte, The New York times, The Smithsonian Magazine, various computer magazines, etc. (Excerpts from this book accompany this file.) THE MODEM REFERENCE published by Brady Books/Simon & Schuster, and is available at your local B. Dalton's, WaldenSoftware, Waldenbooks, or other bookstore, either in stock or by order. Or, phone 800-624-0023 to order direct. In addition to explaining the technical aspects of modem operation, communications software, data links, and other elements of computer communications, the book provides detailed, illustrated "tours" of major online services such as UNISON,CompuServe, DELPHI, BIX, Dow Jones News/Retrieval, MCI Mail, Prodigy, and others. It contains information on using packet switching networks and BBSs, as well as dial-up numbers for various networks and BBSs, and the illustrations alluded to in this excerpt. You'll also find hands-on guides to buying, setting up,using, and troubleshooting computer communications hardware and software. (And the book "supports" all major microcomputer brands.) Want the lowdown on getting more out of your word processor?Read the only book on word processing written by writers, forwriters: WORD PROCESSING SECRETS FOR WRITERS, by Michael A. Banks& Ansen Dibel (Writer's Digest Books). WORD PROCESSING SECRETS FOR WRITERS is available at your local B. Dalton's, Waldenbooks, or other bookstore, either in stock or by order. Or, phone 800-543-4644 (800-551-0884 in Ohio) to order direct. Do you use DeskMate 3? Are you getting the most out of theprogram? To find out, get a copy of GETTING THE MOST OUT OF DESKMATE 3, by Michael A. Banks, published by Brady Books/Simon & Schuster, and available in your local Tandy/Radio Shack or Waldenbooks store now. Or, phone 800-624-0023 to order direct. Other books by Michael A. Banks UNDERSTANDING FAX & E-MAIL (Howard W. Sams & Co.) THE ODYSSEUS SOLUTION (w/Dean Lambe; SF novel; Baen Books) JOE MAUSER: MERCENARY FROM TOMORROW (w/Mack Reynolds; SF novel; Baen Books) SWEET DREAMS, SWEET PRICES (w/Mack Reynolds; SF novel; Baen Books) COUNTDOWN: THE COMPLETE GUIDE TO MODEL ROCKETRY (TAB Books) THE ROCKET BOOK (w/Robert Cannon; Prentice Hall Press) SECOND STAGE: ADVANCED MODEL ROCKETRY (Kalmbach Books) For more information, contact: Michael A. Banks P.O. Box 312 Milford, OH 45150