Modem Standards

For two modems to communicate, they must share the same protocol. A protocol is a specification that determines how two entities will communicate. Just as humans must share a common language and vocabulary to speak with each other, two computers or two modems must share a common protocol. In the case of modems, the protocol determines the nature of the analog signal the device creates from the computer's digital data.

Bell Labs (which set standards for early 300bps modems) and the CCITT are two of the bodies that have set standards for modem protocols. CCITT is an acronym for Comité Consultatif International Téléphonique et Télégraphique, a French term that translates into English as the Consultative Committee on International Telephone and Telegraph. The organization was renamed the International Telecommunication Union (ITU) in the early 1990s, but the protocols developed under the old name are often referred to as such. Newly developed protocols are called ITU-T standards, which refers to the Telecommunication Standardization Sector of the ITU. Most modems built in recent years conform to the standards developed by the CCITT/ITU.

The ITU, headquartered in Geneva, Switzerland, is an international body of technical experts responsible for developing data communications standards for the world. The group falls under the organizational umbrella of the United Nations, and its members include representatives from major modem manufacturers, common carriers (such as AT&T), and governmental bodies. The ITU establishes communications standards and protocols in many areas, so one modem often adheres to many different standards, depending on its various features and capabilities.

All modems sold today support the following ITU protocols:

• ITU V.90 (modulation)

• ITU V.42 (error correction)

• ITU V.42bis (data compression)

However, earlier modems supported many industry-standard and proprietary protocols for modulation, error correction, and data compression.

Most modems today also support the proprietary Microcom Network Protocol (MNP) MNP10 and MNP10EC error-correction standards to provide better connection during conventional wired and wireless (cellular) communication sessions. The latest modems also support the newest ITU standards: V.92 (modulation) and V.44 (data compression). All current protocols are discussed later in this chapter.

Note

To learn more about earlier industry-standard and proprietary protocols, see Chapter 18 of Upgrading and Repairing PCs, 11th Edition, available in electronic form on the disc packaged with this book.

Note

The term protocol is also used to describe software standards that must be established between different computers to allow them to communicate, such as TCP/IP.

Modems are controlled through AT commands, which are text strings sent to the modem by software to activate the modem's features. For example, the ATDT command followed by a telephone number causes the modem to dial that number using tone dialing mode. Applications that use modems typically generate AT commands for you, but you can control a modem directly using a communications program with a terminal mode or even the DOS ECHO command.

Because almost every modem uses the AT command set (originally developed by modem-maker Hayes), this compatibility is a given and should not really affect your purchasing decisions about modems. The basic modem commands might vary slightly from manufacturer to manufacturer, depending on a modem's special features, but the basic AT command set is all but universal.

Note

A list of the basic AT commands can be found in the Technical Reference on the disc included with this book. However, the best source for the commands used by your modem is the manual that came with the device.

Although most modem users will never need to review these commands, if you use MS-DOSbased communication programs or some specialized Windows programs, you might be required to enter or edit an initialization string, which is a series of AT commands sent to the modem before dialing. If these commands are not correct, the modem will not work with these programs.

Bits and Baud Rates

When discussing modem transmission speeds, the terms baud rate and bit rate are often confused. Baud rate (named after a Frenchman named Emile Baudot, the inventor of the asynchronous telegraph printer) is the rate at which a signal between two devices changes in 1 second. If a signal between two modems can change frequency or phase at a rate of 300 times per second, for example, that device is said to communicate at 300 baud.

Thus, baud is a signaling rate, not a data-transmission rate. The number of bits transmitted by each baud is used to determine the actual data-transmission rate (properly expressed as bps or Kbps). Modern dialup modems transmit and receive more bits per baud than the original 300bps modems (which also ran at 300 baud).

Note

To learn more about bits versus baud, see Chapter 18 of Upgrading and Repairing PCs, 11th Edition, available in electronic form on the disc packaged with this book.

Modulation Standards

Modems start with modulation, which is the electronic signaling method used by the modem. Modulation is a variance in some aspect of the transmitted signal. By modulating the signal using a predetermined pattern, the modem encodes the computer data and sends it to another modem that demodulates (or decodes) the signal. Modems must use the same modulation method to understand each other. Each data rate uses a different modulation method, and sometimes more than one method exists for a particular rate.

Regardless of the modulation method, all modems must perform the same task: Change the digital data used inside the computer (ON-OFF, 1-0) into the analog (variable tone and volume) data used by the telephone company's circuits, which were built over a period of years and were never intended for computer use. That's the "mo(dulate)" in modem. When the analog signal is received by the other computer, the signal is changed back from the analog waveform into digital data (see Figure 17.7). That's the "dem(odulate)" in modem.

Figure 17.7. The modem at each computer changes digital (computer) signals to analog (telephone) signals when transmitting data or analog back to digital when receiving data.

The three most popular modulation methods are as follows:

• Frequency-shift keying (FSK). A form of frequency modulation, otherwise known as FM. By causing and monitoring frequency changes in a signal sent over the phone line, two modems can send information.

• Phase-shift keying (PSK). A form of phase modulation in which the timing of the carrier signal wave is altered and the frequency stays the same.

• Quadrature amplitude modulation (QAM). A modulation technique that combines phase changes with signal-amplitude variations, resulting in a signal that can carry more information than the other methods.

All modem protocols since ITU V.34 (33.6Kbps maximum speed) up through the current ITU V.90 and ITU V.92 standards (56Kbps maximum speed) are full-duplex protocols. A full-duplex protocol is one in which communications can travel in both directions at the same time and at the same speed. A telephone call, for example, is full duplex because both parties can speak at the same time. In half-duplex mode, communications can travel in both directions, but only one side can transmit at a time. A radio call in which only one party can speak at a time is an example of half-duplex communications.

These protocols are automatically negotiated between your modem and the modem at the other end of the connection. Basically, the modems start with the fastest protocol common to both and work their way down to a speed/protocol combination that will work under the line conditions existing at the time of the call.

The ITU V.90 and V.92 protocols are the industry-standard protocols most commonly used today; V.92 modems also support V.90.

Note

The 56Kbps standards that represent the highest current increment in modem communication speed require a digital connection at one end and are therefore not purely analog. Other high-speed communication technologies such as ISDN and cable network connections don't perform digital-to-analog conversions, so they should not be called modems, strictly speaking.

V.90

V.90 is the ITU-T designation for a 56Kbps communication standard that reconciles the conflict between the proprietary U.S. Robotics (3Com) x2 and Rockwell K56flex modem specifications developed in 1996 and 1997. The last ISA modems manufactured by major vendors typically support V.90, as do many PC Card and PCI modems built from 1998 to 2001.

See "56Kbps Modems," p. 1082.

V.92

V.92 is the ITU-T designation for an improved version of the V.90 standard that provides faster negotiation of the connection, call-waiting support, and faster uploading than is possible with V.90. Most PCI and PC Card modems sold by major vendors since mid-2001 to the present are V.92 compatible.

See "56Kbps Modems," p. 1082.

V.90 and V.92 are the current communication protocols supported by ISPs; any modem you want to use today should support at least the V.90 protocol.

Error-Correction Protocols

Error correction refers to the capability of some modems to identify errors during a transmission and to automatically resend data that appears to have been damaged in transit. Although you can implement error correction using software, this places an additional burden on the computer's expansion bus and processor. By performing error correction using dedicated hardware in the modem, errors are detected and corrected before any data is passed to the computer's CPU.

As with modulation, both modems must adhere to the same standard for error correction to work. Fortunately, most modem manufacturers use the same error-correction protocols.

V.42, MNP10, and MNP10EC Error-Correction Protocols

The current error-correction protocols supported by modems include Microcom's proprietary MNP10 (developed to provide a better way to cope with changing line conditions) and MNP10EC (an enhanced version developed to enable modems to use constantly changing cellular telephone connections).

V.90 and V.92 modems (as well as some older models) also support the ITU V.42 error-correction protocol, with fallback to the MNP 4 protocol (which also includes data-compression). Because the V.42 standard includes MNP compatibility through Class 4, all MNP 4compatible modems can establish error-controlled connections with V.42 modems.

This standard uses a protocol called Link Access Procedure for Modems (LAPM). LAPM, similar to MNP, copes with phone-line impairments by automatically retransmitting data corrupted during transmission, ensuring that only error-free data passes between the modems. V.42 is considered to be better than MNP 4 because it offers approximately a 20% higher transfer rate due to its more intelligent algorithms.

Note

For information about the MNP 14 protocols, see Chapter 18 of Upgrading and Repairing PCs, 11th Edition, included in electronic form on the disc packaged with this book.

Data-Compression Standards

Data compression refers to a built-in capability in some modems to compress the data they're sending, thus saving time and money for modem users. Depending on the type of files the modem is sending, data can be compressed to nearly one-fourth its original size, effectively quadrupling the speed of the modemat least in theory. This assumes that the modem has V.42bis data compression built in (true since about 1990) and that the data hasn't already been compressed by software. Thus, in reality, the higher throughput caused by data compression applies only to HTML and plain-text files on the Web. Graphics and Zip or EXE archives have already been compressed, as have most PDF (Adobe Acrobat Reader) files. Another factor that influences the throughput of a modem is the type of UART chip used by the serial port included in an internal modem or connected to an external modem, or the use of a USB port instead of a serial port.

Note

To learn more, see "Can Non-56Kbps Modems Achieve Throughput Speeds Above 115,200bps?" in the Technical Reference section of the disc packaged with this book.

As with error correction, data compression can also be performed with software. Data can be compressed only once, so if you are transmitting files that are already in a compressed form, such as Zip archives, GIF or JPEG images, or Adobe Acrobat PDF files, there will be no palpable increase in speed from the modem's hardware compression. The transmission of plain-text files (such as HTML pages) and uncompressed bitmaps, however, is accelerated greatly by modem compression.

MNP5 and V.42bis

Current data-compression standards in modems include Microcom's MNP 5 and the ITU V.42bis protocols. V.42bis is a CCITT data-compression standard similar to MNP Class 5, but it provides about 35% better compression. V.42bis is not actually compatible with MNP Class 5, but nearly all V.42bis modems include the MNP 5 data-compression capability as well.

V.42bis is superior to MNP 5 because it analyzes the data first and then determines whether compression would be useful. V.42bis compresses only data that needs compression. MNP 5, on the other hand, always attempts to compress the data, which slows down throughput on previously compressed files.

To negotiate a standard connection using V.42bis, V.42 also must be present. Therefore, a modem with V.42bis data compression is assumed to include V.42 error correction. When combined, these two protocols result in an error-free connection that has the maximum data compression possible.

V.44

At the same time that the V.92 protocol was introduced by the ITU in mid-2000, a companion data-compression protocol called V.44 was also introduced by the ITU. V.44 uses a new lossless LZJH compression protocol designed by Hughes Network Systems (developers of the DirecWAY satellite broadband Internet service) to achieve performance more than 25% better than that of V.42. Data throughput with V.44 can reach rates of as much as 300Kbps, compared to 150Kbps200Kbps with V.42bis. V.42bis was developed in the late 1980s, long before the advent of the World Wide Web, so it is not optimized for web surfing the way V.44 is. V.44 is especially designed to optimize compression of HTML text pages.

Note

V.44 is the latest compression algorithm to be based in part on the work of mathematicians Abraham Lempel and Jakob Ziv in the late 1970s. Lempel and Ziv's work also has been used in the development of LZW (Lempel-Ziv-Welch) compression for TIFF image files, GIF compressed image files, PKZIP-compatible compression, and other data compression methods.

Proprietary Standards

In addition to the industry-standard protocols for modulation, error correction, and data compression that are generally defined and approved by the ITU-T, several protocols in these areas were invented by various companies and included in their products without any official endorsement by any standards body. Some of these protocols have been quite popular at times and became pseudo-standards of their own. The only proprietary standards that continue to enjoy widespread support are the Microcom MNP standards for error correction and data compression. Others, such as 3Com's HST, CompuCom's DIS, and Hayes' V-series, are no longer popular. For details about the MNP classes, see "MNP Classes" in the Technical Reference section of the disc packaged with this book.

Note

For more information about older proprietary protocols, see Chapter 18 of Upgrading and Repairing PCs, 11th Edition, included in electronic form on this book's disc.

56Kbps Modems

At one time, the V.34 annex speed of 33,600bps (33.6Kbps) was regarded as the absolute speed limit for asynchronous modem usage. However, starting in 1996, modem manufacturers began to produce modems that supported speeds of up to 56,000bps. These so-called "56K" or "56Kbps" modems are now universal, although the methods for breaking the 33.6Kbps barrier have changed several times. To understand how this additional speed was achieved, you must consider the basic principle of modem technologythat is, the digital-to-analog conversion.

As you've learned, a traditional modem converts data from digital to analog form so it can travel over the Public Switched Telephone Network (PSTN). At the destination system, another modem converts the analog data back to its digital form. This conversion from digital to analog and back causes some speed loss. Even though the phone line is physically capable of carrying data at 56Kbps or more, the effective maximum speed because of the conversions is about 33.6Kbps. An AT&T engineer named Claude Shannon came up with a law (Shannon's Law) stating that the maximum possible error-free data communications rate over an all-analog PSTN is approximately 35Kbps, depending on the noise present.

However, because many parts of the United States's urban telephone system is digitalbeing converted to analog only when signals reach the telephone company's central office (or central switch)it's possible to "break" Shannon's Law and achieve faster download rates. You can, in some cases, omit the initial digital-to-analog conversion and send a purely digital signal over the PSTN to the recipient's CO (see Figure 17.8). Thus, only one digital-to-analog conversion is necessary, instead of two or more. The result is that you theoretically can increase the speed of the data transmission, in one direction only, beyond the 35Kbps specified by Shannon's Lawto nearly the 56Kbps speed supported by the telephone network. Prior to the new ITU V.92 standard, the transmission in the other direction was still limited to the V.34 annex maximum of 33.6Kbps. However, both the modem and the ISP must have support for the ITU V.92 standard to overcome this limitation for uploading speeds.

Figure 17.8. V.90-based 56Kbps connections enable you to send data at standard analog modem rates (33.6Kbps maximum) but enable you to receive data nearly twice as fast, depending on line conditions.

See "ITU V.92 and V.44Breaking the Upload Barrier," p. 1085, for more information on how the V.92 standard enables faster uploading.

56Kbps Limitations

Thus, 56Kbps modems can increase data transfer speeds beyond the limits of V.34 modems, but they are subject to certain limitations. Unlike standard modem technologies, you can't buy two 56Kbps modems, install them on two computers, and achieve 56Kbps speeds. One side of the connection must use a special digital modem that connects directly to the PSTN without a digital-to-analog conversion.

Therefore, 56Kbps modems can be used at maximum speeds only to connect to ISPs or other hosting services that have invested in the necessary infrastructure to support the connection. Because the ISP has the digital connection to the PSTN, its downstream transmissions to your computer are accelerated. If both sides of the connection support standards predating V.92, your communications back to the ISP are not accelerated.

On a practical level, this means you can surf the Web and download files more quickly, but if you host a web server on your PC, your users will realize no speed gain because the upstream traffic is not accelerated unless you and your ISP both use V.92-compliant modems. If you connect to another regular modem, your connection is made at standard V.34 annex rates (33.6Kbps or less).

Also, only one digital-to-analog conversion can be in the downstream connection from the ISP to your computer. This is dictated by the nature of the physical connection to your local telephone carrier. If additional conversions are involved in your connection, 56Kbps technology will not work for you; 33.6Kbps will be your maximum possible speed.

Note

Although most advertising for 56Kbps modems refers to them as simply "56K" modems, this is inaccurate. "K" is most often used in the computer business to refer to kilobytes. If that were true, a "real" 56K modem would be downloading at 56,000 bytes per second (or 448,000 bits per second)!

With the way the telephone system has had to grow to accommodate new exchanges and devices, even neighbors down the street from each other might have different results when using a 56Kbps modem.

Caution

56Kbps modem communications are highly susceptible to slowdowns caused by line noise. Your telephone line might be perfectly adequate for voice communications and even lower-speed modem communications, but inaudible noise easily can degrade a 56Kbps connection to the point at which there is only a marginal increase over a 33.6Kbps modem, or even no increase at all. If you do have a problem with line noise, getting a surge suppressor with noise filtration might help.

Hotel connections through telephones with data jacks typically provide very slow connections with any type of modem. Even if you have a V.90- or V.92-compliant 56Kbps modem, you will be lucky to achieve even a 24Kbps transmission rate. The analog-to-digital conversions that occur between your room's telephone and the hotel's digital PBX system eliminate the possibility of using any of the 56Kbps standards the modem supports because they depend on a direct digital connection to the central switch (CS).

As an alternative, more and more hotels and motels provide Ethernet-based access to broadband Internet service; a growing number even provide wireless Ethernet access with Wi-Fi/IEEE-802.11b hardware. Depending on the location, you might be able to use your normal Ethernet card or connect to a USB adapter provided by the hotel. If you want high-speed Internet access as part of the package with your next hotel or motel stay, contact the specific lodging location or chain website for details and pricing.

Early 56Kbps Standards

To achieve a high-speed connection, both modems and your ISP (or other hosting service to which you connect) must support the same 56Kbps technology. The first 56Kbps chipsets were introduced in late 1996:

• U.S. Robotics's x2 used Texas Instruments (TI) chipsets.

• Rockwell's K56flex was supported by Zoom and other modem makers.

These rival methods for achieving performance up to 56Kbps were incompatible with each other and were replaced in 1998 by the ITU's V.90 standard.

Note

For more information about K56flex and x2, see Upgrading and Repairing PCs, 11th Edition, available in electronic format on the disc supplied with this book.

Unfortunately, the 56Kbps name is rather misleading, in regards to actual transmission speeds. Although all 56Kbps modems theoretically are capable of this performance on top-quality telephone lines, the power requirements for telephone lines specified in the FCC's Part 68 regulation limit the top speed of these modems to 53Kbps. The FCC has been considering lifting this speed limitation since the fall of 1998, but it remains in place to date.

V.90

V.90 was introduced on February 5, 1998, and was ratified by the ITU-T on September 15, 1998. Its ratification ended the K56flex/x2 standards "war": Shortly thereafter, most modem manufacturers announced upgrade options for users of x2 and K56flex modems to enable these products to become V.90 compliant.

Some modem vendors offer upgrades for K56flex and x2 modems to the V.90 standard. If you purchased your modem before the V.90 standard became official, see your modem vendor's website for information about upgrading to V.90.

ITU V.92 and V.44Breaking the Upload Barrier

56Kbps protocols, such as the early proprietary x2 and K56flex and the ITU V.90 standard, increased the download speed from its previous maximum of 33.6Kbps to 56Kbps. However, upload speeds, which affect how quickly you can send email, page requests, and file transfers, were not affected by the development of 56Kbps technologies. Upload speeds with any of these 56Kbps technologies are limited to a maximum of 33.6Kbps. This causes severe speed lags for both pure dialup users and those who depend on dialup modems for upstream traffic, such as users of one-way broadband solutionsfor example, one-way (Telco Return) cable modems, one-way DirecWAY, and one-way (Telco Return) fixed-base wireless Internet services. Other shortcomings of existing 56Kbps technology include the amount of time it takes the user's modem to negotiate its connection with the remote modem and the lack of uniform support for call-waiting features.

In mid-2000, the ITU unveiled a multifaceted solution to the problem of slow connections and uploads: the V.92 and V.44 protocols (V.92 was previously referred to as V.90 Plus).

V.92, as the name implies, is a successor to the V.90 protocol, and any modem that supports V.92 also supports V.90. V.92 doesn't increase the download speed beyond the 56Kbps barrier, but offers these major features:

• QuickConnect. QuickConnect cuts the amount of time needed to make a connection by storing telephone line characteristics and using the stored information whenever the same phone line is used again. For users who connect to the Internet more than once from the same location, the amount of time the modem beeps and buzzes to make the connection will drop from as much as 27 seconds to about half that time. Bear in mind, though, that this reduction in connection time does not come about until after the initial connection at that location is made and its characteristics are stored for future use.

• Modem-on-Hold. The Modem-on-Hold feature allows the user to pick up incoming calls and talk for a longer amount of time than the few seconds allowed by current proprietary call-waiting modems. Modem-on-Hold enables the ISP to control how long you can take a voice call while online without interrupting the modem connection; the minimum amount of time supported is 10 seconds, but longer amounts of time (up to unlimited!) are also supported by this feature. Modem-on-Hold also allows you to make an outgoing call without hanging up the modem connection. Modem-on-Hold, similar to previous proprietary solutions, requires that you have the call-waiting feature enabled on your telephone line and also requires that your ISP support this feature of V.92.

• PCM Upstream. PCM Upstream breaks the 33.6Kbps upload barrier, boosting upload speed to a maximum of 48Kbps. Unfortunately, because of power issues, enabling PCM Upstream can reduce your downstream (download) speed by 1.3Kbps2.7Kbps or more. PCM Upstream is an optional feature of V.92, and ISPs who support V.92 connections might not support this feature.

Modems that support V.92 typically also support the V.44 data-compression standard. V.44, which replaces V.42bis, provides for compression of data at rates up to 6:1that's more than 25% better than V.42bis. This enables V.92/V.44 modems to download pages significantly faster than V.90/V.42bis modems can at the same connection speed.

Note

Although Modem-on-Hold is good for the Internet user with only one phone line (because it allows a single line to handle incoming as well as outgoing calls), it's not as good for ISPs because when you place your Internet connection on hold, the ISP's modem is not capable of taking other calls. ISPs that support Modem-on-Hold might need to add more modems to maintain their quality of service if this feature is enabled. More modems are necessary because the ISP won't be able to count on users dropping their Internet connections to make or receive voice calls when Modem-on-Hold is available.

When will you be able to enjoy the benefits of V.92/V.44? Although most major modem vendors have been offering V.92/V.44-compliant modems since late 2000, ISP interest in this standard has been tepid. Only one national ISP, Navipath (which provided support for many local and regional ISPs), offered V.92 access in 2001, and Navipath went out of business in September 2001. Prodigy began to offer V.92 service early in 2002.

According to the V.92 News & Updates page at Richard Gamberg's Modemsite (www.modemsite.com/56K/v92s.htm), many vendors of ISP equipment continue to drag their feet on V.92/V.44 support, in part because it often requires expensive upgrades to terminal equipment. Even after upgrading to support V.92/V.44, some existing terminal hardware is incapable of working with the desirable PCM Upstream feature. Additionally, many major modem vendors have produced so-called "V.92" modems that don't support major V.92 features. Check user reviews available at the Modemsite website and others before you buy a particular V.92 modem model.

Tip

Wondering whether it's time to get a V.92/V.44 modem? Before you update your current model or buy a new modem, do the following:

• Contact your ISP to see whether (or when) it plans to support V.92/V.44 and to determine which features will be supported.

• Check the V.92 ISPs listing at (www.v92.com) and contact ISPs in your area for more details.

Although the change from x2/K56flex to V.90 was a no-cost one for many modem owners, the upgrade from V.90 to V.92/V.44 isn't as painless. In many cases, only the most recent V.90 modems will be eligible for a free upgrade to V.92/V.44 firmware. Contact your modem vendor for details.

Can your existing V.90-compatible modem be upgraded to V.92/V.44? As with earlier 56Kbps modem standards, the answer will likely be, "It depends." Some Lucent LT Winmodem (Agere Systems) modem drivers for V.90 also might include V.92 commands; see Modemsite's Lucent modem section for the latest information (http://www.modemsite.com/56k/ltwin.asp). For modems based on other chipsets, check with your modem vendor.

As with earlier 56Kbps standards, you shouldn't worry about V.92/V.44 support until your ISP announces that it is supporting these standards. Because the V.92 standard has several components, find out which features of V.92 your ISP is planning to support before you look into a modem firmware update or modem replacement.

Fax Modem Standards

Even though the first experimental facsimile equipment was developed at the end of World War II, it took many years for faxing to become commonplace. Similarly, the first fax boards for computers were not introduced until the late 1980s as separate devices. Later, fax capabilities were incorporated into modems. Today, virtually all modems also meet the ITU-T Class 3 fax standards, enabling them to send data to and receive data from other ITU-T Class 3 fax machines and multifunction devices.

Many recent multifunction devices also support the newer ITU-T.30E recommendation for color faxing. Fax modems don't meet this standard as shipped from the manufacturer, but you can download free color fax software developed by HP (Impact ColorFax) that works with most fax modems. Find Impact ColorFax at the BlackICE Software website (www.blackice.com).

For more information about ITU-T fax protocols, see "ITU-T Fax Protocols" in the Technical Reference on the disc packaged with this book.

Dialup Modem Recommendations

A dialup modem for a PC can take the form either of an external device with its own power supply that plugs into a PC's serial port or USB port or of an internal expansion card you insert into a PCI or PC Card bus slot inside the computer. Very few ISA-slot dialup modems are still on the market because the majority of recent systems no longer have ISA slots.

External versions of dialup modems are slightly more expensive because they include a separate case and power supply and sometimes require you to buy a serial modem cable or USB cable. The decision as to which type to use should typically depend on whether you have a free bus slot or serial port, whether you have USB ports and Windows 98/Me/2000/XP, how much room you have on your desk, the capabilities of your system's internal power supply, and how comfortable you are with opening up your computer.

I often prefer external modems because of the visual feedback they provide through their LED indicator lights. These lights let you easily see whether the modem is still connected and transmitting or receiving data. However, Windows and many communication programs today include onscreen indicators that provide some of the same information.

In many situations, however, an internal modem is preferable. If you are using an older computer whose serial ports do not have buffered UART chips, such as the 16550, some more advanced controller-based internal modems include an onboard 16550 UART-equivalent controller chip. This onboard UART with the modem saves you the trouble of upgrading the UART serial port. However, controller-less (or "Windows") modems do not include such chips and rely on the processor for their power and speed, which can degrade the performance of other applications while online. External 56Kbps modems can be hampered from achieving their full speeds by the limitations of the computer's serial port. An external USB model or an internal model using the PCI slot might be preferable instead. Use Table 17.5 to see how internal and external units compare.

Table 17.5. External Versus Internal Modems

Features	External	Internal
Price	Higher	Lower
Extras required	Serial or USB cable	Nothing
Migration between systems	Easy	Difficult (must open case)
Power supply	External AC adapter brick	Nonepowered by host bus
Status monitoring	External signal LEDs	Tray icon or none
Interface	Serial or USB	PCI or ISA
Although late versions of Windows 95 OSR 2.x have USB support, many USB devices actually require Windows 98 or better. Use Windows 98/Me/2000/XP to achieve more reliable support for USB devices.

See "UARTs," p. 997.

Not all modems that function at the same speed have the same functionality. Many modem manufacturers produce modems running at the same speed but with different feature sets at different price points. The more expensive modems usually support advanced features, such as distinctive ring support, caller ID, voice and data, video teleconferencing, and call-waiting support. When purchasing a modem, be sure it supports all the features you need. You also should make sure the software you plan to use, including the operating system, has been certified for use with the modem you select.

If you live in a rural area, or in an older city neighborhood, your telephone line quality might influence your decision. Look at comparison test results carefully and pay particular attention to how well various modems perform with noisy lines. If your phone line sounds crackly during a rainstorm, that poor-quality line makes reliable modem communications difficult, too, and it can limit your ability to connect at speeds above 33.6Kbps.

Another feature to consider is the modem's resistance to electrical damage. Some brands feature built-in power protection to shield against damage from digital telephone lines (higher powered and not compatible with modems) or power surges. However, every modem should be used with a surge protector that allows you to route the RJ-11 telephone cable through the unit for protection against high-voltage surges.

All modems on the market today support V.90 or V.92, and even if your particular location can't support those speeds, your modem might still offer advanced features, such as voicemail or simultaneous voice and data. Keep in mind that V.90/V.92 connections seem to work better for many users if their modems also support x2. If you prefer a modem made by a vendor that also supports K56flex, try to buy a modem that contains both types of standards in its firmware (referred to by some vendors as "Dualmode" modems).

Choosing to Upgrade

If you bought your modem in 1997 or later, or if it was included in your computer, chances are good that it came with 56Kbps support or that you've upgraded it to some form of 56Kbps support. However, even though today's V.90/V.92 modems still have the same maximum speed, other changes in modem design have occurred that might make a modem upgrade desirable for you. And if you are still using a 33.6Kbps modem or even slower model, you should get a 56Kbps modem if your line quality can support it.

Dialup modems introduced from 2001 to the present still run at the 53Kbps FCC maximum with the potential to run at up to 56Kbps if the FCC regulations change, but they might offer one or more of the following advanced features:

• MNP10EC support. Improves performance on poor-quality phone lines by quickly adjusting line speed up and down with changing conditions. MNP10EC includes MNP10 and is superior.

• PCI bus. Newer systems don't have ISA slots. It can share IRQs. Unfortunately, most PCI modems are WinModems; see "Modems Without a UART (WinModems)," later in this chapter.

• USB connection. Most new systems include USB, but some no longer include serial ports. Requires Win98 or better.

• Game-optimized. Provides faster ping response for gaming but is not as useful for ordinary web surfing. These modems are more expensive than others.

• Modem-on-hold. Allows you to answer the phone and not lose the modem connection. Maximum hold time might be limited.

• V.92/V.44 support. Includes faster negotiation, modem-on-hold, and 56Kbps support. Not all ISPs have full V.92/V.44 support.

• Voice support. Allows the digitizing of received calls for answering machine functionality.

Modems Without a UART (WinModems)

Modems without a UART chip, sometimes referred to as WinModems after the pioneering U.S. Robotics version, can save you money at purchase time but can cause problems with speed and operating-system compatibility later.

For users wanting an inexpensive internal modem, a modem that doesn't use a traditional UART instead of a UART-equipped internal or external modem looks like a great deal, often costing less than $40, compared to $80 or more for a UART-equipped "hardware" modem. But, there is no free lunch for modem users. What can you lose with a modem that lacks a UART?

First, you need to realize that there are actually two types of UART-less modems: those that rely on Windows and the CPU for all operations (these modems are also called controllerless modems) and those that use a programmable digital signal processor (DSP) chip to replace the UART. Both types of modems use less power than traditional UART-based modems, making them better for use with notebook computers. Although both are "software modems," there's an enormous difference in what you're getting.

A Windows-based modem must run under Windows because Windows provides the brains of the modem, a cost-cutting move similar to that used by some low-cost host-based printers. You should avoid this if you're planning to try Linux, move the modem to a Macintosh, or use an old MS-DOSbased communications program. If you have no drivers for your modem/operating system combination, you'll have no luck using the software modem.

Software modems that lack a DSP have a second major strike against them: They make your CPU do all the work. Although today's computers have much faster CPUs than those required for typical software modems (Pentium 133 minimum), your modem can still slow down your computer if you multitask while downloading or surfing the Web.

Most of the modems bundled with computer systems are software modems, and the major chipsets used include Lucent LT (now Agere Systems), Conexant (formerly Rockwell) HCF, U.S. Robotics WinModem, ESS Technology's HSP-compliant chipsets, Intel's Modem Silicon Operation (formerly Ambient), and PCTel.

Except for U.S. Robotics, the other companies produce chipsets that can be found in the modems made by many manufacturers.

For best results, do the following:

• Make sure your modem uses a DSP. Typically, these modems don't require a particular CPU or a particular speed of CPU.

• Consider modems built around the Lucent/Agere LT chipset. These modems have a DSP, and Lucent/Agere's firmware is frequently revised to achieve the best results in a rapidly changing telephony environment.

• Use the modem manufacturer's own drivers first. But software modems can often use any manufacturer's drivers for the same chipset with excellent results; in particular, Lucent/Agere LT chipset modems typically can use any Lucent/Agere LT driver from any modem manufacturer.

• Don't delete the old software driver when you download and install new modem software. As with UART-equipped modems, the latest firmware isn't always the best.

• Look carefully at the CPU, RAM, and operating system requirements before you buy your modem.

Tip

Many manufacturers sell both traditional and UART-free modems. If you have an older system or want the option to use MS-DOS or Linux-based communications programs, the hardware modem with a traditional UART might cost more but be a better choice.

Finding Support for "Brand-X" Modems

Many computer users today didn't install their modems, or even purchase them as a separate unit. Their modems came bundled inside the computer and often have a bare-bones manual that makes no mention of the modem's origin or where to get help. Getting V.90 firmware updates, drivers, or even jumper settings for OEM modems such as this can be difficult.

One of the best websites for getting help when you don't know where to start is the "Who Made My Modem?" page (www.56K.com), which features

• Links to the FCC's equipment authorization database (enter the FCC ID to locate the vendor)

• Using ATI commands to query the modem chipset

• Lookup by chipset manufacturer

• Search engine tips

• Links to major modem and chipset manufacturers

Squeezing Performance from Your 56Kbps Modem

Although many users of 56Kbps modems have seen significant improvements in their connect speeds and throughputs over their previous V.34-type modems, many have not or have seen only sporadic improvements. According to the research of Richard Gamberg, available online at his Modemsite website (http://www.modemsite.com), a combination of five factors comes into play to affect your ability to get reliable connections in the range of 45Kbps53Kbps (the current FCC maximum):

• The modem

• The modem firmware/driver

• Your line conditions

• The ISP's modems

• The ISP's modem firmware

It's up to you to ensure that you match your modem's 56Kbps type to the 56Kbps standards your ISP supports, and that you use the best (not always the latest!) modem firmware and drivers, as discussed in the previous section.

Other modem adjustments recommended by Modemsite include

• Modifying existing modem .INF files used by Windows 9x to accurately reflect connection speed

• Disabling 56Kbps connections (!) when playing games to minimize lag times

Note

This last suggestion might seem odd, but "fast" modems are designed to push a large amount of data through for downloads, and gameplay over modems actually deals with small amounts of data instead. The lag time caused by 56Kbps data handling can make a regular "fast" modem seem slow. If you want a fast modem for both downloading and game playing, see "Choosing to Upgrade," earlier in this chapter.

The site also hosts a forum area for discussing modem configuration, reliability, and performance issues.

Telco "Upgrades" and Your Modem

In addition to the well-known analog-to-digital conversion issue that prevents some phone lines from handling 56Kbps modems at anything beyond 33.6Kbps, other local telephone company (telco) practices and services can either prevent 56Kbps from ever working or take it away from you after you've enjoyed it for a while.

If you were getting 45Kbps or faster connections with your 56Kbps modem but can no longer get past 33.6Kbps, what happened? Some local telephone companies have been performing network "upgrades" that improve capacity for voice calls but prevent 56Kbps modems from running faster than 28Kbps. The cause seems to be the telephone companies' change from a signaling type called RBS (Robbed Bit Signaling) to SS7 (Signaling System 7), which changes how data used by the modem for high-speed access is detected. Caller ID devices connected to your phone line use RBS or SS7 signals to obtain information from incoming phone calls. If you use a caller ID box on the same phone circuit as your modem (even if it's connected in another room), you might not be able to get fast connections, or you might experience frequent disconnects. If you notice a drop in connection speed or reliability after you install caller ID, disconnect the caller ID box from the wall jack while you're online and see whether the speed and reliability of your modem connections improve.

What else can you do? You can install the latest firmware available for your modem model or chipset. You can also check with your local telephone company to see whether it can update its firmware to solve the problem. Even if your modem has different firmware, checking on an upgrade might still be useful because this problem is likely to become widespread as telephone numbers, exchanges, and area codes continue to multiply like weeds and telephone network upgrades must keep pace.