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A Modem Communications Primer - Modems: The Inside Story

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3/8/2011 10:44:00 PM
Computers are essentially solitary beasts that prefer to keep their own company. However, that’s not to say that PCs don’t have a social side as well; you just have to work a bit to dig it out. There are three ways to go about this:
  • Add network interface cards to the computers, and then sling some cable around (or use wireless devices) to set up a local area network (LAN). 

  • Use a special cable (or infrared) to connect the serial ports of two computers, and then use a direct cable connection to exchange files between them. I don’t discuss this in the book, but it’s easy enough to establish the connection. Select Start, All Programs, Accessories, Communications, New Connection Wizard. In the first New Connection Wizard dialog box, click Next, activate Set Up an Advanced Connection, and click Next. Then activate Connect Directly to Another Computer, click Next, and follow the rest of the dialog boxes.

  • Attach a modem to your computer and use it to connect to remote systems.

The third method is the focus of this article. As an appetizer, this section presents a bit of background info that serves to get you comfortable with the underlying principles of modem communications. This knowledge will make it much easier for you to set up and work with your modem, and it will be invaluable when you need to troubleshoot the inevitable communications problems.

Modems: The Inside Story

Modems are, by now, a ubiquitous feature of the PC terrain, but they remain more mysterious than the other peripherals. Perhaps it’s the alphabet soup of modem standards, or the inherent complexities of modem-to-modem communications, or just all those strange sounds modems make when they converse with one another. To help you penetrate the mysteries of the modem, this section examines the inner workings of these electronic marvels.

The Modulation/Demodulation Thing

When you speak into a telephone, a diaphragm inside the mouthpiece vibrates. This vibration is converted into an electromagnetic wave that mirrors the amplitude and frequency of the original analog wave created by your voice. This wave travels along the telephone lines, and at the destination, electromagnets in the receiver vibrate another diaphragm that reproduces your voice.

Note that this process is entirely analog, from the original sound wave of your voice, to the electromagnetic wave that traverses the phone system, to the reconstituted sound wave created by the receiver. Computers, of course, are resolutely digital, so this analog state of affairs just won’t do. For a computer to send data along a telephone line, the individual bits that make up the data must be converted into some kind of analog wave.

This digital-to-analog process is called modulation. In essence, the 1s and 0s that compose digital data are converted into signals (or symbols) that can be represented as tones that fall within the frequency range of the human voice (between 300Hz and 3,000Hz). These tones can then be sent along regular telephone lines, where they’re converted back into their original digital format. This reconversion process is called demodulation. The device that modulates the data, sends the resulting tones, and demodulates the tones on the receiving end is a modulator/demodulator, or as it’s more familiarly known, a modem. Now you know why modems make such a racket while they’re communicating with each other: It’s all those tones exchanged back and forth.


Although most telephone systems are analog, digital phone lines are cropping up with increasing frequency. These lines work by sampling the voice, much like the way a sound card samples analog audio. The samples are then sent across the lines as bits without the need of modulation or demodulation, and so without the need of a modem.

The Difference Between Baud and Bits Per Second

The speed at which modems transmit data is called the data transfer rate, and it’s measured in bits per second (bps). The current standards for the data transfer rate are 14,400bps on the low end and 56,000bps on the high end. Another measure of transmission speed does, however, exist—the baud rate—and the two terms are often confused.

The baud rate defines the number of symbols (which might be variations in, say, voltage or frequency, depending on the modulation standard being used) per second that can be exchanged between two modems. Each of these symbols, however, can incorporate multiple bits of data. For example, a 2,400-baud modem might be able to cram six bits of data into a symbol, thus resulting in a data transfer rate of 14,400bps.

In the old days, modems incorporated only a single bit per baud, so the bps and baud rates were synonymous. Now, however, all modems support multibit baud rates, so the only true measure of a modem’s transmission speed is bps.

Understanding Modem Standards

For modems to communicate with each other successfully, they must speak the same language—language in this sense meaning, among other things, the type of modulation used, the data transfer rate, how errors are handled, and whether any data compression is used.

At one time, there were almost as many modem languages as there were modem manufacturers, resulting in what I call the Tower of Babel problem in communications. In other words, you could never be sure that the modem you were trying to connect with would have the faintest idea what your modem was saying. To solve this problem, the major players in the data communications game put together a series of modem standards to help ensure compatibility between devices from different manufacturers. These standards cover three aspects of modem communications: modulation, error correction, and data compression.


You might still see some modems described as Hayes compatible. This is a holdover from the days when Hayes modems were the market leader, so other modems had to fall in line with the Hayes standard to gain consumer acceptance. In this case, however, the standard had nothing to do with modem communications. Instead, it defined a command set used by applications to control the modem. For example, the command ATDT (attention dial tone) tells the modem to get a dial tone. By now, however, every modem supports this command set (which is usually just called the AT command set because most of the commands begin with AT), so being Hayes compatible is no longer a big deal.

Modulation Standards

When a modem modulates digital data into a carrier wave, the receiving modem must understand how this modulation was performed in order to reverse the procedure during demodulation. This is, for obvious reasons, the most crucial aspect of modem compatibility, so having modulation standards is critical. These standards are set by a United Nations umbrella group called the International Telecommunication Union-Telecommunications Standardization Section (ITU-TSS; it was formerly another mouthful: the Consultative Committee on International Telephone and Telegraph, or CCITT). The ITU-TSS consists of representatives from modem manufacturers, telephone companies, and government agencies.

As modem technology improved, new standards had to be hammered out, so numerous modulation standards have been implemented over the years. Here’s a review of the most common ones.


When speaking the name of any modulation standard, the V. part is pronounced as vee dot. So, for example, the standard V.90 is pronounced vee dot ninety.

V.22This is a 1,200bps standard that was used mostly outside of the United States and Canada. (The corresponding standard used in the United States and Canada was called Bell 212A, which was a standard implemented by Bell Labs.)
V.22bisThis is a 2,400bps standard, and the first of the international standards. (The bis part is French for again or encore.)
V.29This is the standard for half-duplex (that is, one-way) communication at 9,600bps. It’s used for Group III fax transmissions and so is the standard facsimile implementation in fax/modems.
V.32This is the standard for full-duplex (that is, two-way) communications at 9,600bps. This standard incorporates a technique called trellis coding that enables on-the-fly error checking and reduces the effect of line noise.
V.32bisThis standard defines full-duplex transmission at 14,400bps. It’s basically the same as V.32, except that the number of bits per signal change was upped from four in V.32 to six in V.32bis (both standards operate at 2,400 baud).
V.32fast or V.FCThese standards upped the V.32 and V.32bis transmissions to 28,800bps, but they’ve been replaced by V.34.
V.34This is the standard for full-duplex transmission at 33,600bps.
V.90This is the standard for full-duplex transmission at 56,000bps for downloads (data coming into your computer) and 33,600bps for uploads (data going out of your computer).
V.92This is the standard for full-duplex transmission at 56,000bps for downloads and 48,000bps for uploads. V.92 also boasts a Quick Connect feature that cuts modem connect time in half, as well as a Modem On Hook (MOH) feature that enables you to initiate and receive voice phone calls while a modem connection is active. V.92 represents the current state of the art for analog modem communications. Despite their latest-and-greatest status, V.92 modems are relatively inexpensive, so you should shoot for V.92 if you’re in the market for a modem.

Most experts used to believe that V.34 represented the ceiling for analog transmission speed. However, modem companies continued to push the transmission rate envelope. U.S. Robotics and 3Com, for example, introduced x2 technology, which allowed for 56Kbps rates over standard phone lines. A competing technology called K56—also known as K56Flex—was also available, and eventually the combined standard V.90 was established. How was this speed increase achieved? V.34 treats the entire network as though it were analog, so it’s limited to 33,600bps. However, V.90 takes advantage of the fact that most of the network involved in modem communications is digital. In particular, the connection between a service provider and the switched telephone network is, in most cases, entirely digital. This means that no download modulation is necessary. In other words, the data that is downloaded to your modem doesn’t have to go through the costly digital-to-analog conversion, so it can make the most use out of the wider bandwidth on digital telephone networks. So, with V.90, downloaded data can achieve theoretical rates of up to 56,000bps, while uploaded data still transfers at 33,600bps. V.92 gets rid of most of the upload modulation as well, so uploaded data can transfer at up to 48,000bps.

Does it work? Well, in practice, you’re not likely to see true 56Kbps transmission rates due to line noise and other factors. However, rates in the 35Kbps to 50Kbps range are achievable, thus making V.92 a viable alternative.

Error Correction Standards

One of the problems with analog telephone lines is that they suffer from line noise and other factors that can wreak havoc on the carefully crafted symbols sent by modems. To ensure that data arrives intact, the ITU-TSS has set up error correction standards. These standards enable the receiving modem to check the integrity of incoming symbols and, if it finds a problem, ask the originating modem to resend the data.

The current standard for error correction is V.42, which incorporates two protocols: Link Access Procedure for Modems (LAPM) and Microcom Networking Protocol (MNP) 4. Both protocols correct errors by asking that corrupted data be retransmitted. The default protocol is LAPM because it’s a bit faster than MNP 4.

Compression Standards

If you apply compression to a folder, NTFS compresses the files by replacing redundant character strings with tokens. Many modern modems can perform the same process on your outgoing data. In other words, the modem first uses a compression technique to reduce the size of the data and then converts the compressed data into symbols. This means that less data is sent, thus reducing upload times.


To apply compression to a folder, right-click the folder and then click Properties. In the General tab of the folder’s property sheet, click Advanced and then activate the Compress Contents to Save Disk Space check box. Note that with Windows XP you can only compress folders on NTFS partitions (although there are third-party utilities that enable you to compress folders on any file system).

Of course, the receiving modem must be able to decompress the data, so the ITU-TSS has implemented compression standards. The current standard is V.44, which can compress data up to 6:1. The old standard was V.42bis, which can compress data up to 4:1. (Most modems also support another compression scheme called MNP 5. However, this scheme provides a maximum compression ratio of only 2:1.)


Data compression sounds great, but it really works only on text transfers. Because binary files contain few redundant character strings, they can’t be compressed all that much, so you won’t see a significant increase in throughput. In fact, if you’re dealing with files that have already been compressed (such as ZIP files), data compression might lead to slower download times because compressing an already-compressed file generally increases the size of the file.

A Review of Modem Types

Modems come in various shapes and sizes, and most brand-name models provide similar features. If you’re looking to purchase a modem, your main criterion should be that the modem supports the ITU-TSS standards, especially one of the modulation standards (such as V.90). Also, many modems come with built-in fax capabilities  so look for V.29 compatibility as well.

After standards compliance, your next criterion will be the type of modem (or fax/modem) you need. Here’s a summary of the three main types:

ExternalThese modems are standalone boxes you connect to a serial port with a special cable. (You can also get USB modems that plug into a USB port.) Although external modems require a separate power source and tend to be more expensive than an equivalent internal modem, they have several advantages. For one, they can be transported between machines fairly easily. For another, most external modems have a series of LED indicators on their front panel that tell you the current state of the modem. These lights can be invaluable during troubleshooting. Here’s a summary of the LEDs that appear on most external modems and what each light represents:
 AAAuto Answer—When lit, indicates that the modem will answer incoming calls automatically.
 CDCarrier Detect—Lights up when the modem receives a valid data signal from a remote modem. This indicates that data transmission is possible, and the light remains on during the entire connection.
 CSClear to Send—Lights up when the modem has determined that it’s okay for an application to start sending data.
 MRModem Ready—Lights up when the modem’s power is turned on.
 OHOff Hook—Lights up when the modem takes control of the phone line (which is the modem equivalent of taking the telephone receiver off the hook).
 RDReceive Data—Lights up when the modem receives data.
 RSRequest to Send—Lights up when your computer has asked the modem whether it’s okay to start sending data.
 SDSend Data—Lights up when the modem sends data.
 TRTerminal Ready—Lights up when the modem receives a DTRdata terminal ready) signal from the computer. This means that the current communications program is ready to start sending data. (
InternalThese modems are cards you insert into a slot on your computer’s expansion bus. This type of modem is convenient because no external power source is required, it’s one less device taking up valuable desk space, and no external serial port is used up. Most modem jockeys, however, dislike internal modems because of the lack of LED indicators for troubleshooting. (As you’ll see later, though, Windows XP does provide an icon during modem connections that shows you the state of the RD and SD signals.)
PC CardThese are modems that use the credit-card–size, PC Card (PCMCIA) format and plug directly into a PC Card slot. If possible, look for PC Card modems that accept an RJ-11 jack directly because these kinds are more reliable than the “dongles” used by some PC Card modems.
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