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). 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.

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. That's the "dem(odulate)" in modem.

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.

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.

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. 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.

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 4–compatible 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.

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 modem—at 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.

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.

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.

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.