Data/FAX Modem Transmission Modulation Systems

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This is a table of many of the transmission systems used for data communication modems manufactured over the past thirty years. The name, operating speed, modulation technique, and the operating frequencies are listed. A second table briefly discusses the more common modulation techniques.

Name	Data/Fax	RX bps	TX bps	Baud Rate (Symbol Rate)	Modulation	Constel- lation Points	Originate Frequencies Marking/ Spacing In Hz	Answer Frequencies Marking/ Spacing In Hz
Bell 103	Data	300	300	300	FSK		1270/1070	2225/2025
Bell 108	Data	300	300	300	FSK		1270/1070	2225/2025
Bell 113	Data	300	300	300	FSK		1270/1070	2225/2025
Bell 202	Data	1200	1200	1200	FSK		1200/2400	1200/2400 (Half Duplex)
CCITT V.21	Data	300	300	300	FSK		980/1180	1650/1850
CCITT V.23	Data	1200(fwd)	75(rev)	1200/75	FSK		1300/2100(fwd)	390/450(rev)
Name	Data/Fax	RX bps	TX bps	Baud Rate (Symbol Rate)	Modulation	Constel- lation Points	Originate Carrier Frequency	Answer Carrier Frequency
Bell 212(A)	Data	1200	1200	600	DPSK	4	1200	2400
Bell 201	Data	2400	2400	1200	DPSK	4	1800	1800 (Half Duplex)
CCITT V.22	Data	1200	1200	600	DPSK	4	1200	2400
Bell 208	Data	4800	4800	1600	QAM	8	1800	1800 (Half Duplex)
CCITT V.22bis	Data	2400	2400	600	QAM	16	1200	2400
CCITT V.26	Data	2400	2400	1200	DPSK	4	1800	1800
Telebit PEP	Data	18000	18000	variable	DAMQAM (PEP)	2 to 5 bits per carrier	Up to 511 carriers with 7.8Hz spacing	Half Duplex
Name	Data/Fax	Max RX bps*	Max TX bps*	Baud Rate (Symbol Rate)	Modulation	Constel- lation Points	RX Carrier Frequency	TX Carrier Frequency
CCITT V.32	Data	9600	9600	2400	TCM	32	1800	1800
CCITT V.32	Data	7200	7200	2400	QAM	16	1800	1800
CCITT V.32	Data	4800	4800	2400	QAM	4	1800	1800
CCITT V.32bis	Data	14400	14400	2400	TCM	128	1800	1800
CCITT V.32bis	Data	12000	12000	2400	TCM	64	1800	1800
CCITT V.32bis	Data	9600	9600	2400	TCM	32	1800	1800
CCITT V.32bis	Data	7200	7200	2400	TCM	16	1800	1800
AT&T V.32terbo®	Data	19200	19200	2400	TCM	512	1800	1800
AT&T V.32terbo®	Data	16800	16800	2400	TCM	256	1800	1800
Telebit TurboPEP	Data	23000	23000	variable	TCM	2 to 7 bits per carrier	Up to 512 carriers	Half Duplex
ITU V.34	Data	33600	33600	3429	TCM		1800	1800
ITU V.34	Data	31200	31200	3200	TCM		1800	1800
CCITT V.34	Data	28800	28800	3200	TCM		1800	1800
CCITT V.34	Data	26400	26400	3200	TCM		1800	1800
CCITT V.34	Data	24000	24000	3200	TCM		1800	1800
CCITT V.34	Data	21600	21600	3200	TCM		1800	1800
CCITT V.34	Data	19200	19200	3200	TCM		1800	1800
CCITT V.34	Data	16800	16800	3200	TCM		1800	1800
Name	Data/Fax	Max RX bps*	Max TX bps*	Baud Rate (Symbol Rate)	Modulation	Constel- lation Points	RX Carrier Frequency	TX Carrier Frequency
Rockwell K56 1996	Data	53000	V.34	8000/V.34	PCM/TCM	RX 5 to 7 bits per sample TX uses V.34	RX uses PCM coding	V.34 for TX
Lucent V.flex® 1996	Data	53000	V.34	8000/V.34	PCM/TCM	RX 5 to 7 bits per sample TX uses V.34	RX uses PCM coding	V.34 for TX
Rockwell/ Lucent K56flex 1996	Data	53000	V.34	8000/V.34	PCM/TCM	RX 5 to 8 bits per sample TX uses V.34	RX uses PCM coding	V.34 for TX
USR X2 1996	Data	53000	V.34	8000/V.34	PCM/TCM	RX 5 to 7 bits per sample TX uses V.34	RX uses PCM coding	V.34 for TX
ITU V.90 1998	Data	53000	V.34	8000/V.34	PCM/TCM	RX 5 to 8 bits per sample TX uses V.34	RX uses PCM coding	V.34 for TX
ITU V.92 2000	Data	53000	48000	8000	PCM	5 to 8 bits per sample	PCM	PCM
Name	Data/Fax	RX bps	TX bps	Baud Rate (Symbol Rate)	Modulation	Constel- lation Points	Originate Frequencies Marking/ Spacing In Hz	Answer Frequencies Marking/ Spacing In Hz
CCITT V.21ch2	FAX	300	300	300	FSK		1650/1850	Half Duplex
CCITT V.17	FAX	14400	14400	2400	TCM	128	1800	Half Duplex
CCITT V.17	FAX	12000	12000	2400	TCM	64	1800	Half Duplex
CCITT V.17	FAX	9600	9600	2400	TCM	32	1800	Half Duplex
CCITT V.17	FAX	7200	7200	2400	TCM	16	1800	Half Duplex
CCITT V.27ter	FAX	4800	4800	1600	DPSK	8	1800	Half Duplex
CCITT V.27ter	FAX	2400	2400	1200	DPSK	4	1800	Half Duplex
CCITT V.29	FAX	9600	9600	2400	QAM	16	1700	Half Duplex
CCITT V.29	FAX	7200	7200	2400	QAM	8	1700	Half Duplex
CCITT V.29	FAX	4800	4800	2400	QAM	4	1700	Half Duplex

(Information on printing color tables on color printers can be found here.)

* V.34, V.90, V.92, K56, F56flex, X2, V.flex, PEP and TurboPEP all allow assymetrical speeds, with the maximum operating speed in each direction dictated by line conditions.

Data/FAX Modem Transmission System Terminology

Term	Abbreviation	Meaning/Description
Frequency Shift Keying	FSK	The original and simplest transmission system designed for sending data by telephone voice circuits or radio rather than direct electrical current loops (or something like RS-232) between sender and receiver. In FSK, the presence of one frequency tone represents a "1" and the presence of another frequency represents a "0". To allow simultaneous transmission in both directions, two pairs of frequencies are commonly used. The frequencies are usually staggered to reduce the chance that a harmonic of one frequency (or a combination of sending and receiving frequencies) is incorrectly detected as the other frequency. Bell 103 and V.21 were the most popular systems to utilize FSK.
Phase Shift Keying	PSK	Similar to the transmission method used in FM radio, bits (and later Symbols) are represented as deviations in the phase of a constant frequency carrier. An unaltered signal represents the same bit value as the last bit, while a 50% reduction in frequency for one Hz period of the base frequency indicates that the next bit has a value opposite that of the last bit. If "C" represents the crest of a waveform and "V" represents the valley, the PSK pattern CV CV CV CC VC VC CC VV CV CC VC VC VC is used to transmit the bit sequence 0 0 0 1 0 0 1 1 0 1 0 0 0.
Differential Phase Shift Keying	DPSK	In this system, Symbols are represented by changes in the phase of the carrier, comparing its current state against its previous phase. If "C" represents the crest of a waveform and "V" represents the valley, the DPSK pattern CV CV CV CC VC VC CC VV CV CC VC VC VC transmits the bit sequence 0 0 0 1 1 1 0 1 1 0 0 0 0.
Amplitude Modulation	AM	Represents Symbols by different volume levels of a signal. AM forms the basis of several different modem transmission systems, but is always used in conjunction with another technique, such as Phase Encoding. AM by itself represents different bits (or symbols) as different signal amplitudes. The weaker the signal is, the harder it is to separate data from noise.
Quadrature Amplitude Modulation	QAM	The foundation of many high speed transmission systems, Symbols are encoded as deviations in the phase and amplitude of the carrier. This "two-dimensional" coding method increases the number of bits that can be contained in each symbol, limited only by media bandwidth and the sophistication of the receiver. If 'C' represented the crest of a strong signal while 'c' represented the crest of a weak signal, while 'V' and 'v' were the strong and weak valleys of a waveform, the QAM signal CCvcCVcvvCcvV (13 samples) could be used to send the bit pattern 11 11 01 10 11 00 10 01 01 11 10 01 00 (26 bits). Compared to PSK, AM and DPSK, twice as much data was conveyed in the same number of cycles of the waveform. By increasing the number of signal strength levels from two to a higher number, the number of bits that can be represented per Hz can be greatly increased without increasing the maximum frequency of the signal, which is usually constrained by the bandwidth of the transmission medium.
Trellis Coded Modulation	TCM	An enhancement of QAM, which adds a large number of redundant bits to each Symbol, a technique known as Forward Correction. The result is that a given Symbol has more invalid than valid bit combinations, and this means that bit errors have a higher probability of being detected at the transmission level. In TCM, the receiver uses the extra data from the previous Symbol to check the accuracy of the current Symbol. TCM is used in V.32, V.32bis and V.34 transmission systems, as well as some proprietary systems.
Pulse Coded Modulation	PCM	Arguably, Pulse Coded Modulation isn't a modulation at all. In the V.90 modem system, the transmitting modem is connected to the telephone network in a digital fashion, and simply emits an eight (or fewer) bit value for 1/8000th of a second that represents eight (or fewer) bits of data that the modem desires to transmit. No carrier is generated. The word (eight or seven bits, depending on channel type with seven being the case 99% of the time) is transmitted across the telephone network digitally, and when it reaches the line driver card that serves the destination telephone line, the surviving bits (some may be lost due to bit-robbing and other telephone network configuration) are converted into a voltage, using the appropriate translation tables of that country (µ-LAW or A-LAW). This voltage is held on that analog telephone line for 1/8000th of a second (125usec). It is up to the PCM modem receiving this now-analog signal to recognize a given voltage it receives for up to 1/8000th of a second as being directly equivalent to some combination of bits of actual data. Under perfect conditions, 8,000 samples transmitted in this fashion containing seven bits per sample would yield 56,000 bits per second of transmitted data in one direction for a North American voice-bearing channel. A North American clear-channel could carry 8,000 x 8, or 64,000 bits per second, but such channels are almost never used with analog telephone lines. (European values are different.) The voltage loss of the analog loop between the line card in the central office and the receiving modem, as well as other impairments of the loop are all evaluated during a training period. Bit combinations that cannot be uniquely identified by the receiver are skipped for the duration of this call, yielding a lower number of usable data bits transported per second. This training period cannot take into account impulse noise, only persistent impairments. Unexpected changes in line impairments forces a complete retrain to determine which bit combinations can successfully be recovered from this point forward. The V.90 system operates using a V.34 modulation in the analog-to-digital direction of the connection. In the V.92 system, the analog modem ascertains the exact moment when the central office line card samples the line for the signal to be sent to the remote party (largely by timing when changes on data it receives from the central office occur and by computing the length of the loop by timing how long it takes to hear the echo of the analog modems own transmissions), and so the analog modem is able to use PCM in both directions if the line is capable of supporting PCM transmissions in the first place. (If V.90 won't work, V.92 certainly won't.) V.90 is arbitrarily limited to 53,000 bits per second by the modem manufacturers, so that they can avoid having to certify that the V.flex, K56, K56flex, Aetherworks and X2 transmission systems (all predecessors of V.90) were all compliant with the maximum signal strength limits of Part 68 of the FCC Rules, which these systems could exceed under certain theoretical conditions. Rather than risk having K56flex or X2 (the only two pre-V.90 PCM modems to see wide distribution) delayed and possibly having other older modulation systems brought under similar scrutiny, PCM bit combinations that cause rail-to-rail voltage swings in adjacent samples are avoided to reduce the strength of interference the line may generate, but this also reduces the maximum possible bit rate of K56flex and X2 to around 53,333bit/sec, a limit that was kept in V.90 because of the same fear. The limit is set in the server-side modems. In 2000, the FCC indicated that they may provide a waiver to the Part 68 requirement for the V.90 (and V.92) transmission method without testing to see if a waiver was actually needed. As of January 2004, no waiver has appeared. However, based on actual use in countries where no limit was enforced by the server-side V.90 modems, less than one-tenth of one percent of V.90 modem owners could obtain and maintain the 56,000 bit rate, so having the 53,333 bps restriction lifted by the modem makers in the United States will not result in any significant number of modems obtaining higher PCM speeds.

This information is provided on an "AS IS" basis and contains no warranty. Most countries have regulatory authorities who will have the latest specifications and compliance information.

Data/FAX Modem Transmission Modulation Systems

Data/FAX Modem Transmission System Terminology

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