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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Almost all modems today claim to be Hayes-compatible, a phrase which has come to be as meaningless as IBM-compatible when referring to PCs. It does not refer to any communication protocol, but instead to the commands required to operate the modem. Because almost every modem uses the Hayes command set, this compatibility is a given and should not really affect your purchasing decisions about modems.

Not all modems that function at the same speed have the same functionality. Many modem manufacturers produce modems that have different feature sets at different price points. The more expensive modem usually supports such features as distinctive ring support and caller ID. When purchasing a modem, be sure that it supports all the features that you need.

The basic modem commands don’t vary from modem manufacturer to manufacturer, as much as they did. Some modems, most notably U.S. Robotics, allow you to query the command set by simply sending AT$ to the modem.

Baud Versus Bits Per Second (bps)

Baud rate and bit rate often are confused in discussions about modems. Baud rate is the rate at which a signal between two devices changes in one 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.

Sometimes a single modulation change is used to carry a single bit. In that case, 300 baud also equals 300 bits per second (bps). If the modem could signal two bit values for each signal change, the bps rate would be twice the baud rate, or 600 bps at 300 baud. Most modems transmit several bits per baud, so that the actual baud rate is much slower than the bps rate. In fact, people usually use the term baud incorrectly. We normally are not interested in the raw baud rate, but in the bps rate, which is the true gauge of communications speed.

Modulation Standards

Modems start with modulation, which is the electronic signaling method used by the modem (from modulator to demodulator). 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.

The three most popular modulation methods are:

  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.
Bell 103
Bell 103 is a U.S. and Canadian 300 bps modulation standard. It uses FSK modulation at 300 baud to transmit bit per baud. Most higher-speed modems will still communicate using this protocol, even though it is obsolete.
Bell 212A
Bell 212A is the U.S. and Canadian 1200 bps modulation standard. It uses differential phase-shift keying (DPSK) at 600 baud to transmit 2 bits per baud.
V.21
V.21 is an international data-transmission standard for 300 bps communications, similar to Bell 103. Because of some differences in the frequencies used, Bell 103 modems are not compatible with V.21 modems. This standard is used primarily outside the United States.
V.22
V.22 is an international 1200 bps data-transmission standard. This standard is similar to the Bell 212A standard, but is incompatible in some areas, especially in answering a call. This standard was used primarily outside the United States.
V.22bis
V.22bis is a data-transmission standard for 2400 bps communications. Bis is derived from the Latin meaning second, indicating that this data transmission is an improvement to or follows V.22. This data transmission is an international standard for 2,400 bps and is used inside and outside the United States. V.22bis uses QAM at 600 baud and transmits 4 bits per baud to achieve 2,400 bps.
V.23
V.23 is a split data-transmission standard, operating at 1,200 bps in one direction and 75 bps in the reverse direction. Therefore, the modem is only pseudo-full-duplex, meaning that it can transmit data in both directions simultaneously, but not at the maximum data rate. This standard was developed to lower the cost of 1200 bps modem technology, which was expensive in the early 1980s. This standard was used primarily in Europe.
V.29
V.29 is a data-transmission standard at 9,600 bps, which defines a half duplex (one-way) modulation technique. This standard generally is used in Group III facsimile (fax) transmissions, and only rarely in modems. Because V.29 is a half-duplex method, it is substantially easier to implement this high-speed standard than to implement a high-speed full-duplex standard. As a modem standard, V.29 has not been fully defined, so V.29 modems of different brands seldom can communicate with each other. This does not affect fax machines, which have a fully defined standard.
V.32
V.32 is a full-duplex (two-way) data transmission standard that runs at 9,600 bps. It is a full modem standard, and also includes forward error-correcting and negotiation standards. V.32 uses TCQAM (trellis coded quadrature amplitude modulation) at 2,400 baud to transmit 4 bits per baud, resulting in the 9,600 bps transmission speed.
The trellis coding is a special forward error-correction technique that creates an additional bit for each packet of 4. This extra check bit is used to allow on-the-fly error correction to take place at the other end. It also greatly increases the resistance of V.32 to noise on the line.
In the past, V.32 has been expensive to implement because the technology it requires is complex. Because a one-way, 9600 bps stream uses almost the entire bandwidth of the phone line, V.32 modems implement echo cancellation, meaning that they cancel out the overlapping signal that their own modems transmit and just listen to the other modem’s signal. This procedure is complicated and was at one time costly. Advances in lower-cost chipsets then made these modems inexpensive, and they were the de facto 9,600 bps standard for some time.
V.32bis
V.32bis is a 14,400 bps extension to V.32. This protocol uses TCQAM modulation at 2,400 baud to transmit 6 bits per baud, for an effective rate of 14,400 bps. The trellis coding makes the connection more reliable. This protocol is also a full-duplex modulation protocol, with a fallback to V.32 if the phone line is impaired. It is the communications standard for dialup lines because of its excellent performance and resistance to noise. I recommend the V.32bis-type modem.
V.32fast
V.32fast, or V.FC (Fast Class) as it is also called, was a new standard being proposed to the CCITT. V.32fast is an extension to V.32 and V.32bis, but offers a transmission speed of 28,800 bps. It has been superseded by V.34.
V.34
V.34 has superseded all the other 28.8Kbps standards. It has been proven as the most reliable standard of communication at 28.8Kbps. A recent annex to the V.34 standard also defines optional higher speeds of 31.2 and 33.6Kbps, which most of the newer V.34 modems will be capable of. Many existing V.34 modems designed using sophisticated Digital Signal Processors (DSPs) can be upgraded to support the new 33.6Kbps speeds by merely installing a software upgrade in the modem.
This is accomplished by downloading the Modem ROM upgrade from the manufacturer, and then running a program they supply to “flash” the modem’s ROM with the new code.
V.90
V.90 is the latest analog modem standard. V.90 allows for a theoretical maximum download speed of 56 Kbps. However, in the U.S., regulatory restrictions currently limit the download speed to 53 Kbps. See the section “56K Modems” later in this chapter for more detail on this.


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