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A Modem Overview
The word “Modem” is derived from two words; “MODulator” and “DEModulator”. From a data communications perspective, a modem is a device that converts the digital bit stream into analog signals that are suitable for transport over standard voice circuits.
Typical modems operate using Frequency Shift Keying (FSK), Phase Shift Keying (PSK), Amplitude Shift Keying (ASK), or a combination of basic schemes.
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.
Multi-dimensional Trellis coding is employed for robust Forward Error Correcting. A “Reverse Channel” option is also described in V.34!