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Modem Overview

Modem is an acronym for Modulator Demodulator. A modem is a device that converts data from digital computer signals to analog signals that can be sent over a phone line. This is called modulation. The analog signals are then converted back into digital data by the receiving modem. This is called demodulation.

A modem is fed digital information, in the form of ones and zeros, from the CPU. The modem then analyzes this information and converts it to analog signals, that can be sent over a phone line. Another modem then receives these signals, converts them back into digital data, and sends the data to the receiving CPU.

Analog vs. Digital

"Analog" refers to information being presented continuously, while "digital" refers to data defined in individual steps. Analog information's advantage is its ability to fully represent a continuous stream of information. Digital data, on the other hand, is less affected by unwanted interference, or noise. In digital computers, data is stored in individual bits, which have a value of either 1 (on) or 0 (off). If graphed, analog signals are shaped as sine waves, while digital signals are square waves. Sound is analog, as it is always changing. Thus, in order to send information over a phone line, a modem must take the digital data given it by the computer and convert it into sound, an analog signal. The receiving modem must convert these analog signals back into the original digital data.

Data Compression

In order to transmit data at a speed greater than 600 bits per second (bps), it is necessary for modems to collect bits of information together and transmit them via a more complicated sound. This allows the transmission of many bits of data at the same time. Computers are capable of transmitting information to modems much faster than the modems are able to transmit the same information over a phone line. This gives the modem time to group bits together and apply compression algorithms to them. Two common compression protocols by which this is accomplished are MNP-5, which has a compression ratio of 2:1, and v.42bis, which has a 4:1 compression ratio. MNP-5, however, produces a great deal of overhead in its compression, so it actually makes the transmission of pre-compressed (ZIP, for example) files much longer. V.42bis can sense if compression is unnecessary, so can speed up transfer of pre-compressed files as well. If V.42bis is unavailable, it is best to disable MNP-5 data compression when transferring pre-compressed files.

Error Correction

Error correction is the method by which modems verify that the information sent to them has been undamaged during the transfer. Error-correcting modems break up information into small packets, called frames. The sending modem attaches a checksum to each of these frames. The receiving modem checks whether the checksum matches the information sent. If not, the entire frame is resent. Though error correction may slow down data transfer on noisy lines, it does provide greater reliability. MNP2-4, as well as V.42, are error correction protocols. These protocols determine how the modems verify data. As with data compression protocols, for an error correction protocol to be used, it must be supported by both modems in the connection.

Flow Control

Often, one modem in a connection is capable of sending data much faster than the other can receive. Flow control allows the receiving modem to tell the other to pause while it catches up. Flow control exists as either software, or XON/XOFF, flow control, or hardware (RTS/CTS) flow control. With software flow control, when a modem needs to tell the other to pause, it sends a certain character, usually Control-S. When it is ready to resume, it sends a different character, such as Control-Q. Software flow control's only advantage is that it can use a serial cable with only three wires. Since software flow control regulates transmissions by sending certain characters, line noise could generate the character commanding a pause, thus hanging the transfer until the proper character (such as Control-Q) is sent. Also, binary files must never be sent using software flow control, as binary files can contain the control characters. Hardware, or RTS/CTS, flow control uses wires in the modem cable or, in the case of internal modems, hardware in the modem. This is faster and much more reliable than software flow control.

UARTs and Data Buffering

UARTs, or Universal Asynchronous Receiver/Transmitters, are the method by which computers send information to a serial device, such as a modem. A UART is an integrated circuit (chip) that converts parallel input into serial output. The CPU communicates with the serial device by writing in the UART's registers. UARTs have FIFO, or First-In, First-Out buffers through which this communication occurs. First-In First-Out means that the first data to enter the buffer is the first to leave. Without the FIFO, information would be scrambled when sent by a modem.

The first UART was National Semiconductor's INS-8250. This was soon upgraded to the faster 16450. Both the 8250 and 16450 only had a 1-byte FIFO, however. This meant that if information was traveling too fast for the CPU to handle it, it would be overwritten. It is nearly impossible for either of these UARTS to handle the transfer speeds of today's modems, most notable under Desqview and Windows, where the processor is kept busy, allowing data in the FIFO to be overwritten. The solution to this is the modern 16550A UART, which has fixed a bug in the original 16550. The 16550 has a 16-byte FIFO. Thus, up to 16 bytes can be written to the UART's registers before anything is deleted. This allows the CPU to catch up if it has been busy dealing with other tasks. 16550 UARTs are necessary for high speed communications under Windows or for DOS applications running in a Windows shell.

 MODEM TYPES

Frequency Shift Keying (FSK)

Bell 103 Modem

This modem supports asynchronous rates up to 300 baud using Frequency Shift Keying (FSK) modulation. Different carrier frequencies are used at each end of a 103 modem link, allowing full-duplex operation on a 2-Wire switched voice circuit.

The Originating modem transmits signals of either 1070 Hz (Space) or 1270 Hz (Mark). The Answering modem transmits signals of either 2025 Hz (Space) or 2225 Hz (Mark).

This modem was predominant until the early 1980s, when the Bell 212 modems became available.

V.21 Modem

This modem supports asynchronous transmission at rates up to 300 baud. Modulation is Frequency Shift Keying (FSK). In this modulation scheme, different carrier frequencies are used between the Originating and Answering modems. A Space is transmitted by a change in carrier frequency of +100 Hz. A Mark is represented by a change in carrier frequency of -100 Hz. For the carrier frequency of 1080 Hz, a Space is represented by a 1180 Hz signal and a Mark is represented by a 980 Hz signal. For the carrier frequency of 1750 Hz, a Space is represented by a 1850 Hz signal and a Mark is represented by a 1650 Hz signal.

This modem modulation scheme is specified in CCITT (now ITU-T) Recommendation V.21, naturally.

Bell 202 Modem

This Bell System modem was designed to support asynchronous data at rates of up to 1200 baud on 2-wire dial-up circuits, and up to 1800 baud on conditioned leased lines. Operation is half-duplex and the modem modulation scheme used is Frequency Shift Keying (FSK).

A Mark is represented by a frequency of 1200 Hz, while a Space is represented by a frequency of 2200 Hz.

This modem specification also describes an optional "Reverse" channel, for slow, general purpose use. While the specification calls for this "Reverse" channel to operate at 5 BPS, many modem manufacturers implement 75 to 150 baud versions.

Phase Shift Keying (PSK)

201 Modem

These Bell System modems support synchronous data rates up to 2400 BPS. Full-duplex is available, but the modem operates in a Half-Duplex mode when using a 2-Wire switched voice circuit. This modem uses a modulation scheme that encodes data by using four specific "phase shifts" of the transmitted carrier. This type of modulation is known as DPSK (Differential Phase Shift Keying), but is sometimes called QPSK (Quad Phase Shift Keying).

In this modulation scheme, two bits (called a "dibit") are represented with a single phase change:

00 = 45 degrees
10 = 135 degrees
11 = 225 degrees
01 = 315 degrees

The actual modem modulation rate is 1200 BAUDS, with each BAUD capable of supporting two data bits.

The CCITT (now "ITU-T") has specified a compatible modulation scheme in Recommendation V.26, Alternative "B".

V.26 Modem

CCITT Recommendation V.26, Alternative "A" describes a modulation type that is similar to the Bell 201 series of modems, except that different phase shift values are specified to encode the "dibit":

00 = 0 degrees
10 = 90 degrees
11 = 180 degrees
01 = 270 degrees

The actual modem modulation rate is 1200 bauds, with each baud capable of supporting two data bits.

In the second release of the V.26 Recommendation (called "V.26bis"), fallback operation to 1200 BPS is defined. V.26bis also recommends that the modulation type used be "Alternative B", as described above (Bell 212). The third iteration of this Recommendation (called "V.26ter") incorporates echo cancelation techniques within the modem to delineate the Originating and Answering modem signals. V.26ter allows full-duplex operation on a 2-Wire, switched, voice circuit.

Bell 212 Modem

This Bell System modem supports either asynchronous or synchronous data rates up to 1200 BPS on switched 2-wire dial-up circuits. Operation is full-duplex with different carrier frequencies used between the Originating and Answering modems. The modulation method employed is DPSK, using "dibits" to represent up to four phase changes.

The actual modem modulation rate is 600 bauds, with each baud consisting of two data bits.

The 212 modem series also incorporates a 300 baud, Type 103, modem for compatibility with pre-existing Bell 103 modems.

This modem type was widely used in the early to mid 1980s, and is still commonly found in use today.

V.22 Modem

This modulation is described in CCITT Recommendation V.22 and FED-STD-1008! It utilizes Differential Phase Shift Keying (DPSK) and is designed for synchronous or asynchronous operation at 1200 BPS. Operation is full-duplex with different carrier frequencies used between the Originating and Answering modems.

The modem modulation rate is 600 bauds, with each baud representing two data bits.

This modem modulation scheme supports a "fallback" rate of 600 BPS.

00 = 90 degrees
10 = 0 degrees
11 = 270 degrees
01 = 180 degrees

V.22bis Modem

The "bis" qualifier is a French (also, Latin) term for "duo" or "twice". Thus, as the name would suggest, this modulation scheme is described in the second release of CCITT's V.22 Recommendation.

This modulation scheme supports transmission of full-duplex 2400 BPS synchronous or asynchronous data over a switched, 2-Wire, voice circuit. Alternatively, these modems may be employed on leased-lines as well. The modulation scheme used is QAM (Quadrature Amplitude Modulation). In this modulation scheme, the data stream is divided up into groups of four bits, known as "quadbits". The first two bits of each "quadbit" are encoded as a phase change, changing the "quadrant" (except in cases where the first two bits are "01"; in this case, the "quadrant" is not changed from its previous state). The second two bits of each "quadbit" define one of four signal states in the new "quadrant".

The modulation rate is 600 bauds, with each baud representing four data bits.

These modems support fallbacks to V.22 modulation schemes also. Most of the popular V.22bis PC modems support fallback operation to Bell 212 modulation also, depending upon the capabilities of both Originating and Answering modems.

Although the V.22bis specification was defined in 1984, practical deployment of these modems did not occur until the late 1980s.

V.27 Modem

CCITT Recommendation V.27 describes a modulation scheme that is capable of supporting 4800 BPS, full-duplex, synchronous data. Operation may be full-duplex on a 4-Wire leased line or half-duplex on a 2-Wire, switched, voice circuit. The modulation scheme is known as D8PSK (Differential 8 Phase Shift Keying) and operates by breaking the incoming data stream into groups of three bits ("tribit"). These "tribits" are represented by one of eight possible phase shifts:

001 = 0 degrees
000 = 45 degrees
010 = 90 degrees
011 = 135 degrees
111 = 180 degrees
110 = 225 degrees
100 = 270 degrees
101 = 315 degrees

The modulation rate is 1600 bauds, with each baud representing three data bits.

The second and third releases of the V.27 Recommendation (V.27bis and V.27ter, respectively) added the ability to fallback to a 2400 BPS rate using V.26, Alternative "A" modulation. Also, the start-up/training times are reduced in the V.27bis Recommendation.

V.29 Modem

This modulation scheme was first standardized by the CCITT in 1976. It uses a form of Quadrature Amplitude Modulation (QAM), which transports data in groups of four bits ("quadbits"). The first bit determines the amplitude of the signal while the next three bits represent one of eight phase changes. The phase shifts are similar to the "tribit" modulation scheme described in Recommendation V.27:

001 = 0 degrees
000 = 45 degrees
010 = 90 degrees
011 = 135 degrees
111 = 180 degrees
110 = 225 degrees
100 = 270 degrees
101 = 315 degrees

V.29 modulation is capable of transporting synchronous data at rates up to 9600 BPS. It operates full-duplex on a 4-Wire leased line or half-duplex on a 2-Wire, switched, voice line.

The V.29 modulation rate is 2400 bauds, with each baud representing four data bits.

V.29 modulation also incorporates fallback to 7200 BPS. In this mode, three bits ("tribit") are combined to represent one of eight possible phase changes. In this mode of operation, the modem's modulation rate is still 2400 baud, but each baud now represents three data bits.

V.29 modulation also incorporates fallback to 4800 BPS. In this mode, two bits ("dibit") are combined to represent up to four phase changes (0, 90, 180, and 270 degrees). The modem's modulation rate remains at 2400 baud, but each baud now represents only two data bits:

00 = 0 degrees
10 = 90 degrees
11 = 180 degrees
01 = 270 degrees

V.29 modems were highly popular in the 1970s and 1980s for use on 4-Wire leased lines and are still found in use today. The Group 3 FAX machines that are popular today operate in a half-duplex fashion using V.29 modulation.

V.32 Modem

First defined in 1984 by the CCITT, V.32 defines a modem that can support 9600 BPS asynchronous or synchronous data. Operation is full-duplex over a 2-Wire, switched, voice circuit. The modulation used may be Quadrature Amplitude Modulation (QAM), or QAM with Trellis coding. Trellis coding is actually a Forward Error Correcting (FEC) scheme.

The modulation rate is 2400 baud, in both "Nonredundant" and "Trellis" modes of operation. In the "Nonredundant" mode, each baud represents four bits. In the "Trellis" mode, each baud represents five bits; the four data bits, plus a coded, redundant bit that is the result of convolutional coding of the first two bits in the previous "quadbit" modulation process.

The use of echo cancellation techniques allows the same carrier frequency (1800 Hz) to be used at each end of a modem system.

Delays in the development of cost-effective echo cancelation techniques resulted in practical deployment of V.32 modems in the early 1990s.

V.32bis Modem

This standard modulation scheme was developed by the CCITT at the end of the 1980s (1988), although practical deployment of such systems did not occur until the early 1990s. This modulation method allows the transport of asynchronous or synchronous data at line rates up to 14400 BPS (14.4 KBPS). Operation is full-duplex over 2-Wire, switched, voice circuits, using echo cancelation techniques to differentiate between the Originating and Answering modem's signals.

The modulation rate is 2400 bauds, and use the Forward Error Correcting (FEC) capabilities of Trellis coding. Quadrature Amplitude Modulation (QAM) is employed, using groupings of seven bits. Only six of these bits contain actual user data, the remaining bit is the convolutional coded, redundant bit generated from the previous bits.

V.FC Modem

V.FC (or "V.Fast Class") is a non-standard 28.8 KBPS modulation scheme developed by, and proprietary to, Rockwell. It was first released in 1993 and has enjoyed some success. As a mature product, most of the "bugs" have been worked out, thus improving reliability. However, the majority of the industry has been awaiting the recent release of the new CCITT V.34 Recommendation.

It is capable of supporting full-duplex synchronous or asynchronous data and supports either 4-Wire leased lines or 2-Wire switched voice circuits.

V.34 Modem

Recently approved in the summer of 1994 was the new 28.8 KBPS modulation scheme described in CCITT (ITU-T) Recommendation V.34. During the development of this modulation method, this scheme was known as "V.FAST".

It is capable of supporting full-duplex synchronous or asynchronous data over 4-Wire leased lines or 2-Wire circuits.

The modulation rate (baud or "symbol" rate) can vary. The carrier frequency can vary. The V.34 Recommendation also describes a "line probing" process that allows the modem to automatically setup optimally for any type of line connection. The training time has been reduced, but the modem will recover automatically from most line disturbances.

V.90 modems

Multi-dimensional Trellis coding is employed for robust Forward Error Correcting. A "Reverse Channel" option is also described in V.34!