Course: Introduction to Computer Communication Dr. D. Koren / University of Tel Aviv


Prepared by : Benny Assif, Yosy Breznitz, Bat-Sheva Ovadia, Yoram Shnapp


The modem is a device that converts digital information to analog by MODulating it on the sending end , and DEModulating the analog information into digital information at the receiving end.

This document begins with an introduction followed by the classification of modems according to their characteristics. Later, standards and protocols are discussed. Finally, the document overview today's status and future trends.


The need to communicate between distant computers led to the use of the existing phone network for data transmission. Most phone lines were designed to transmit analog information - voices, while the computers and their devices work in digital form - pulses. So, in order to use an analog medium, a converter between the two systems is needed. This converter is the MODEM which performs MODulation and DEModulation of transmitted data. It accepts serial binary pulses from a device, modulates some property (amplitude, frequency, or phase) of an analog signal in order to send the signal in an analog medium, and performs the opposite process, enabling the analog information to arrive as digital pulses at the computer or device on the other side of connection.

Modems, in the beginning, were used mainly to communicate between DATA TERMINALS and a HOST COMPUTER. Later, the use of modems was extended to communicate between END COMPUTERS. This required more speed and the data rates increased from 300 bps in early days to 28.8bps today. Today, transmission involves data compression techniques which increase the rates, error detection and error correction for more reliability.

In order to enable modems of various types and different manufacture to communicate, interface standards were developed by some standard organizations

Today's modems are used for different functions. They act as textual and voice mail systems, facsimiles, and are connected or integrated into cellular phones and in notebook computers enabling sending data from anywhere. The future might lead to new applications. Modem speeds are not expected to be increased much over today's 28.8 kbps. Further dramatic speed increases will require digital phone technology such as ISDN and fiber optic lines.

New applications might be implemented such as simultaneous voice and data. Videophones are an example of this.


The modems can be classified according to their characteristics:

Classifying Modems according to : Range

* Short Haul

Short haul modems are cheap solutions to systems of short ranges (up to 15 km), which use private lines and are not part of a public system. Short haul modems can also be used, even if the end-to-end length of the direct connection is longer than 15 km, when both ends of the line are served by the same central office in the telephone system. These lines are called "local loops". Short haul modems are distance-sensitive, because signal attenuation occurs as the signal travels through the line. The transmission rate must be lowered to ensure consistent and error-free transmission on longer distances.

Short haul modems tend to be cheaper than other modems for two reasons:

There are two main types of short haul modems:

  1. Analog modems, using a simple modulation method, without sophisticated devices for error control or equalizers. These modems usually operates at a maximum rate of 9600 bps, but there are some which supports higher rates (up to 64,000 bps).
  2. Line drivers increase the digital signal, which transmit to the communication channel and do not transmit the carrier signal, as conventional modems. Line drivers are very cheap and tiny and connect to the RS232 connector of the terminal (since they lack a power supply, they use the signal voltage of the DTE-DCE interface for DC power supply).

* Voice Grade (VG)

Voice-grade modems are used for unlimited destination, using a moderate to high data rate. These modems are expensive and their maintenance and tuning are sophisticated. Communication channels are leased lines and dial-up.

Voice-band telephone network is used for data transmission. A user-to-user connection may be either dedicated or dialed. The links in the connection are the same in the two cases, and the only difference for the user is that for some impairments (particularly attenuation and delay distortion), a dedicated (private or leased) line is guaranteed to meet certain specifications, whereas a dialed connection can only be described statistically.

* Wideband

Wideband modems are used in large-volume telephone-line multiplexing, dedicated computer-to-computer links. These modems exceed high data rates.

Classifying Modems according to : Line Type

* Leased, Private

Leased, private or dedicated lines (usually 4-wire) are for the exclusive use of "leased-line" modems - either pair (in a simple point-to-point connection) or several (on a multidrop network for a polling or a contention system). If the medium is the telephone network, their transmission characteristics are usually guaranteed to meet certain specifications, but if the link includes any radio transmission, the quality of it may be as variable as that of a switched (i.e. nondedicated) line.

* Dial up

Dial-up modems can establish point-to-point connections on the PSTN by any combination of manual or automatic dialing or answering. The quality of the circuit is not guaranteed, but all phone companies establish objectives. The links established are almost always 2-wire because 4-wire dialing is tedious and expensive.

* Two and Four-Wires Lines

A four-wire (4W) line is a pair of two-wire (2W) lines, one for transmitting and one for receiving, in which the signals in the two directions are to be kept totally separate. Perfect separation can be maintained only if the four-wire configuration is sustained from transmitter to receiver. The lines may be combined in a 4W/2W network (often called a hybrid or a hybrid transformer) at any point in the signal path. In this case impedance mismatches will cause reflections and interference between the two signals.

Classifying Modems according to : Operation Mode

* Half Duplex

Half duplex means that signals can be passed in either direction, but not in both simultaneously. A telephone channel often includes an echo-suppressor, allowing transmission in only one direction, this renders the channel half-duplex. Echo suppressors are slowly being replaced by echo cancelers, which are theoretically full-duplex devices.

When a modem is connected to a two-wire line, its output impedance cannot be matched exactly to the input impedance of the line, and some part of its transmitted signal (usually badly distorted) will always be reflected back. For this reason half- duplex receivers are disabled (received data is clamped) when their local transmitter is operative.

Half-duplex modems can work in full-duplex mode.

* Full Duplex

Full duplex means that signals can be passed in either direction, simultaneously. Full duplex operation on a two-wire line requires the ability to separate a receive signal from the reflection of the transmitted signal. This is accomplished by either FDM (frequency division multiplexing) in which the signals in the two directions occupy different frequency bands and are separated by filtering, or by Echo Canceling (EC).

The implication of the term full-duplex is usually that the modem can transmit and receive simultaneously at full speed. Modems that provide a low-speed reverse channel are sometimes called split-speed or asymmetric modems.

Full duplex modems will not work on half-duplex channels.

* Simplex

Simplex means that signals can be passed in one direction only. A remote modem for a telemetering system might be simplex and a 2-wire line with a common unidirectional amplifier is simplex.

* Echo Suppressor and Echo Canceler

At the junction between the local loop, which is usually a 2-wire circuit, and the trunk, which is a 4-wire circuit, echoes can occur. The effect of the echo is that a person speaking on the telephone hears his own words after a short delay. Psychological studies have shown that this is annoying to many people, often making them stutter or become confused. To eliminate the problem of echoes, echo suppressors are installed on lines longer than 2000 km. (On short lines the echoes come back so fast that people cannot detect them). An echo suppressor is a device that detects human speech coming from one end of the connection and suppresses all signals going the other way. The device compares the levels at its two input ports, and if it decides, for example that the other end is talking, it inserts an attenuator in the return (echo) path, and vice versa.

Echo suppressors have several properties that are undesirable for data communication. First, they prevent full- duplex data transmission, which would otherwise be possible, even over the 2-wire local loop (by allocating part of the bandwidth to the forward channel and part to the reserve channel). Even if half-duplex transmission is adequate, they are a nuisance because the time required to switch directions can be substantial. Double-talking totally confuses them, and the attenuation may be switched in and out repeatedly. Furthermore, they are designed to reverse upon detecting human speech, not digital data.

To reduce these problems, when echo suppressors detect a specific tone they shut down, and remain shut down as long as the carrier is present (this is an example of inband signaling, where control signals that activate and deactivate internal control functions lie within the band accessible to the user). This disabling is usually done during initial handshaking by one modem transmitting an answer tone in either 2100 Hz (CCITT standard) or 2225 Hz (modems following the old Bell 103 standard).

Echo suppressor are slowly being replaced by ECs, which allow a certain amount of double-talking and do not require "capture" time for any one talker to assume control of the connection.

Classifying Modems according to : Synchronization

* Asynchronous Modems

Most of the modems that operate in slow and moderate rates, up to 1800 bps, are asynchronous (using asynchronous data). Asynchronous modems operate in FSK modulation and use two frequencies for transmission and another two for receiving. Asynchronous modems can be connected in different options to the communication media:

In a 2-wire line, full duplex operation can be achieved by splitting the channel into two sub-channels.l

Figure Modem - 1

* Asynchronous data

Asynchronous data is not accompanied by any clock, and the transmitting and receiving modems know only the nominal data rate. To prevent slipping of the data relative to the modems' clocks, this data is always grouped in very short blocks (characters) with framing bits (start and stop bits). The most common code used for this is the seven-bit ASCII code with even parity.

* Synchronous Modems

Synchronous modems operates in the audio domain, at rates up to 28800 bps in audio lines, used in telephones systems (using synchronous data). The usual modulation methods are the phase modulation and integrated phase and amplitude (at higher rates than 4800 bps).

In synchronous modems, equalizers are used, in order to offset the misfit of the telephone lines. These equalizers are inserted in addition to the equalizers, that sometimes already exist in the telephone lines.

These equalizers can be classified into three main groups:

  1. Fixed/statistical equalizer - these equalizers offset the signal according to the average of the known attenuation in each frequency. Tuning the equalizer is sometimes done in the factory and stays fixed, usually they are used to operate at low rates in a dial up line.
  2. Manually adjusted equalizer - these equalizers can be tuned to optimal performance to a given line. These equalizers should be re-tuned when the line is replaced and periodically. Specially, it should be tuned frequently when the line is of a low quality and it's parameters are changed frequently. Tuning is done using a button inside the modem (or on the external board).
  3. Automatic equalizer - these equalizers are tuned automatically when the connection is established. Depending on the line quality in a specific moment, in a process of about 15ms to 25ms, after the first tuning, the equalizer samples the line continually and adjusts itself to the changed conditions, so the modem operates at each moment under optimal conditions. The fitness process operates, in some modems, at rates of 2400 times in a second.

Synchronous modems operate in the same manner asynchronous modems. However, synchronous modems operates at higher rates and since the requirements to transmit at these rates is increasing, most of the innovations are implemented for synchronous modems.

In synchronous modems the channel can be split for several consumers at various speeds. Modems who have this ability are called SSM - Split System Modem. These modems can use a simple split or a split using multipoint connection.

Synchronous data is accompanied by a clock signal. Synchronous data is almost always grouped in blocks, and it is the responsibility of the data source to assemble those blocks with framing codes and any extra bits needed for error detecting and/or correcting according to one of many different protocols (BISYNC, SDLC, HDLC, etc.). The data source and destination expect the modem to be transparent to this type of data, conversely, the modem can ignore the blocking of the data.

Classifying Modems according to : MODULATION

Communication channels like telephone lines are usually analog media. Analog media is a bandwidth limited channel. In the case of telephone lines the usable bandwidth frequencies is in the range of 300 Hz to 3300 Hz.

Data communication means moving digital information from one place to another through communication channels. These digital information signals have the shape of square waves and the meaning of "0" and "1"

If such digital signals were transmitted on analog media the square waves of the digital signals would be distorted by the analog media as shown in figure Modem-2. The receiver which receives these distorted signals will be unable to interpret accurately the incoming signals. These digital signals must be converted into analog signals so that the communication channels can carry the information from one place to another. The technique which enables this conversion is called modulation .

FIGURE Modem-2* Modulation

Modulation is a technique of modifying some basic analog signal in a known way in order to encode information in that basic signal. Any measurable property of an analog signal can be used to transmit information by changing this property in some known manner and then detecting those changes at the receiver end. The signal that is modulated is called the carrier signal, because it carries the digital information from one end of the communication channel to the other end.

The device that changes the signal at the transmitting end of the communication channel is called the MODULATOR. The device at the receiving end of the channel, which detects the digital information from the modulated signal, is called the DEMODULATOR .

A basic analog signal is a sinusoidal wave which can be written in mathematical form as follows :
S(t) = A * SIN ( 2* PI * F * T + PHI )
were A is the peak amplitude, F is the signal frequency and PHI is the phase of the signal . Modulation can use any of these three measurable and changeable properties of the sine wave for encoding purposes.
There are three modulation techniques, each of them changes one of the properties of the basic analog signal.

* AM - amplitude modulation

This technique changes the amplitude of the sine wave. In the earliest modems, digital signals were converted to analog by transmitting a large amplitude sine wave for a "1" and zero amplitude for a "0", as shown in figure Modem-3. The main advantage of this technique is that it is easy to produce such signals and also to detect them. This technique has two major disadvantages. The first is that the speed of the changing amplitude is limited by the bandwidth of the line. The second is that the small amplitude changes suffer from unreliable detection. Telephone lines limit amplitude changes to some 3000 changes per second. The disadvantages of amplitude modulation causes this technique to no longer be used by modems, however, it is used in conjunction with other techniques .

FIGURE Modem-3

* QAM - quadrature amplitude modulation

This technique is based on the basic amplitude modulation . This technique improves the performance of the basic amplitude modulation. In this technique two carrier signals are transmitted simultaneously. The two carrier signals are at the same frequency with a 90 degrees phase shift. The mathematical form of the transmitted signal will be as follows:
S(t) = A* SIN (Wc* t) + B* COS (Wc* t)
A, B, are the amplitude of the two carrier signals. Each of them can get a value from a known set of values. In this way a few bits can be transmitted in the period of one symbol time. For example consider the set of values {1 , 2 , 3 , 4 }. In this example 4 different values can represent 2 bits. During one symbol time 4 bits will be transmitted, "A" will represent 2 bits and another 2 bits will be represented by "B".

* FM - frequency modulation

In this technique the frequency of the carrier signal is changed according to the data. The transmitter sends different frequencies for a "1" than for a "0" as shown in figure Modem-4.This technique is also called FSK - frequency shift keying. The disadvantages of this technique are that again (as it was with amplitude modulation) the rate of frequency changes is limited by the bandwidth of the line, and that distortion caused by the lines makes the detection even harder than amplitude modulation. Today this technique is used in law rate asynchronous modems up to 1200 baud only.

FIGURE Modem-4

* CPM - continuous phase modulation

A modern technique which derives from basic frequency modulation. The only difference is that in the transition from one symbol to another the phase is continuously changed, there are no phase steps. Continuous phase means that the transmitted signal bandwidth is limited and faster data rates can be achieved for the same bandwidth.

* PM - phase modulation

In this modulation method a sine wave is transmitted and the phase of the sine carries the digital data. For a "0", a 0 degrees phase sine wave is transmitted ( PHI = 0 ). For a "1", a 180 degrees sine wave is transmitted ( PHI = 180 ) as shown in figure Modem-5. This technique, in order to detect the phase of each symbol, requires phase synchronization between the receiver's and transmitter's phase. This complicates the receiver's design.

FIGURE Modem-5

A sub method of the phase modulation is DIFFERENTIAL PHASE MODULATION. In this method, the modem shifts the phase of each succeeding signal in a certain number of degrees for a "0" (90 degrees for example) and a different certain number of degrees for a "1" (270 degrees for example ) as illustrated in figure Modem-6. This method is easier to detect than the previous one. The receiver has to detect the phase shifts between symbols and not the absolute phase. This technique is also called PSK - phase shift keying. In the case of two possible phase shifts the modulation will be called BPSK - binary PSK. In the case of 4 different phase shifts possibilities for each symbol which means that each symbol represents 2 bits the modulation will be called QPSK, and in case of 8 different phase shifts the modulation technique will be called 8PSK.

FIGURE Modem-6

* TCM - trellis coded modulation

A modern technique which uses the modulation techniques that was discussed previously like QAM or PSK in conjunction with coding in order to improve data rates.


Any technique of the various modulation methods discussed previously or even any combination of these methods (integrated modulation method) can be used for data transfer.

For example we shall look at the following table:

      relative          phase             bit                 symbol

     amplitude          shift             meaning             value 


          1                 45             0  0  0             "0" 

          1                135             0  0  1             "1" 

          1                225             0  1  0             "2"  

          1                315             0  1  1             "3" 

          2                 45             1  0  0             "4"  

          2                135             1  0  1             "5"  

          2                225             1  1  0             "6"  

          2                315             1  1  1             "7" 

In this example a combination of differential phase modulation and amplitude modulation is used. Each symbol is represented by a certain amplitude and phase shift. The transmitting modem is combining 3 succeeding bits in to one transmitted symbol. The receiving modem interprets each detected symbol to 3 succeeding bits. For the data sequence 10100101011001010, the transmitted symbol' sequence will be: 6 4 5 1 2.

* Data Rate

The number of signal changes transmitted per unit of time is called the data rate of the modem. That rate is usually expressed in terms of a unit known as a baud. The baud is the number of times per second the line condition can switch from "1" to "0". Data rate and transmission speed , which is expressed in terms of bits per second, usually are not the same, as several bits may be transmitted through the channel by the modem in each signal change (a few bits can be transmitted as one symbol).

Claude Shennon showed, in 1948, that the maximum capacity (bit rate) of a bandwidth limited transmission line with limited signal to noise ratio is:
C = W * log (1 + S/N) / log (2)
Where C is the maximum capacity, W is the limited bandwidth and S/N is the power of the signal to noise ratio.
A telephone line, for example, has a bandwidth of 3000 Hz and maximum S/N of about 1000 (30db). Thus the theoretically maximum data rate that can be achieved is about 30 K bps (bits per second). Earliest modems that work through telephone lines had 1.2 K bps. Today's modems reach data rates of 28.8 K bps.


Communication between two devices might work only when the interface is defined and agreed. For modems, the standards define techniques used for modulation, for error correction for data compression, and other attributes. There are some standard organizations for the development of interface standards. The ITU - International Telecommunications Union an agency of the United Nations (Geneva, Switzerland), ISO - International Standards Organization, and CCITT - International Telegraph and Telephone Consultative Committee a group of ITU.

The modem standards were developed during the years and published as V series of recommendations. In the United States the primary standards body is the ANSI - American National Standards Institute. Its committees, concerned with information processing and data communication, are designated X3 and X3S3, respectively. The organizations deal with De Jure standards.
There are also De Facto standards which were developed by a specific manufacturer, using new features in his products, while they were not defined yet by the standard committees. When the definitions were adapted by another manufacturers for compatibility they became a De Facto standard.
Some examples of De Facto standards follow. The Bell-100 standards established by the Bell System with their 100 and 200 series of modems; Bell Dataphone 103 of 300 bps, that was introduced at 1958 was the first modem used to transfer data over telephone lines. The modem command language, "AT" commands, invented by Hayes but used by all modem vendors ("AT" stands for "Attention" and each command begun with an "AT" command). This command language enabled the control of the modem operation set from a simple one, such as dialing a phone number and from a complicated one, such as answering the phone only after 15 rings. Since only a few modem manufactures existed at that time, "Hayse" became a leading company and the "Hayse" commands became standard. New modem manufactures imitated the "Hayse " commands. Most of the communication SW companies today offer "Hayse" compatibility. The MNP protocols that define error correction and data compression schemes, were developed by Microcom Inc. but are widely accepted throughout the modem industry.

The ITU (formerly CCITT) STANDARDS :


                 date          speed                   PSTN/

standard      (ratified)       (bps)       HDX/FDX     private   modulation 

V.21             1964           200        FDX(FDM)     PSTN        FSK 

V.22             1980          1200        FDX(FDM)     PSTN        PSK 

V.22 bis         1984          2400        FDX(FDM)     PSTN        QAM 

V.23             1964          1200        HDX          PSTN        FSK

V.26             1968          2400        HDX          Private     PSK

V.26 bis         1972          2400        HDX          PSTN        PSK

V.26 ter         1984          2400        FDX(EC)      PSTN        PSK

V.27             1972          4800        HDX          Private     PSK

V.27 bis         1976          4800        HDX          Private     PSK

V.27 ter         1976          4800        HDX          PSTN        PSK

V.29             1976          9600        HDX          Private     QAM

V.32             1984          9600        FDX(EC)      PSTN        QAM

V.32 bis         1991         14400                                 TCM

V.32 Ter                      19200                                 TCM

V.34 (    1994         28800                                 TCM

FDM means Frequency Division Multiplexing

EC    means Echo Canceler

There are other standards that deal with the modem's related functions. Some of these standards and protocols are listed here.

Data compression involves different methods such as Huffman coding and run length coding. The first method deals with the frequent characters being transmitted in less bits than the other characters. The second method transmits the bit value and the chain length, instead of a chain of identical succeeding bits. The main characteristic of data compression protocols is that they buffer the desired data to be transferred, compress it and only then transfer it to the second modem. The second modem must do the opposite work. The data compressing algorithms are similar to the ARC, ZIP or ARJ programs. They are located in the modem ROM and compress the data in real time. The compression depends on the characteristics of the data. For example, PC '.EXE' or '.COM' files may be compressed up to 40-50%. Text files may be compressed up to 100%.

Protocols used by modems to transfer files: The widespread protocols for transferring files are: Xmodem, Ymodem, Zmodem. (Mainframe computers may also use the Kermit protocol). The following is a brief description of the protocols. The Xmodem protocol divides the data into blocks. Each block contains a sequence number of the block, 128 Bytes of the data and 4 Bytes of checksum. The checksum is computed for the 128 Data Bytes. The protocol on the other side is synchronized by checking the sequence number of currently transferred block and then calculating the checksum of 128 bytes of data and comparing it to the transferred checksum. In the case of error, it requests to send the same block again. In the Ymodem protocol, each block contains 1024 Bytes. The checksum size is 4 Bytes. It is faster then the Xmodem protocol. It also may transfer a batch of files and information about each file to be transferred and its size. This helps the user on the other side to see the time left of the transfer. Zmodem is a public domain program written by Chuck Forsberg at Omen Technology. There are several main advantages of this protocol. The block size varies from 16 to 1024 Bytes. The protocol dynamically finds the optimal block size for the file transfer on the current phone line. It starts with a data block size of 1 KByte. It reduces it automatically in case the phone line is noisy or enlarges the block size when the line disturbance disappears. The transfer rate is fast with big block sizes but remember that in case of an error the whole block is retransmitted and time is wasted. Since this protocol adjusts the block size to the line quality it combines the best characteristics on each side. It resumes the transfer after line disconnection. The checksum size is 8 Bytes (CRC/32), therefore, it raises the protocol error correction authenticity.

Establishing Connection
Establishing a connection between two modems involves a handshaking process of sending and receiving coded signals to coordinate the connection. The FallBack method is used to find a common way of communication. The calling modem first tries to connect at its highest speed (or best error-correction or data compression scheme). If the called modem doesn't signal back that is can handle that protocol, the calling modem falls back to a slower speed or less effective scheme and tries to connect again. This cycle continues until a common ground is found or they run out of options.


Today in addition external modems, there are internal modems which are included as an additional board within the computer. There are advantages to each type.

Actual transfer rates are limited due to type of phone lines. Using slower phone trunks, international circuits where half the normal bandwidth is used, and the slow cellular connections where it might run at only 14.4 kbps (without compression) although the modem itself enable 28.8kbps.

More and more users are accessing the Internet and on-line services such as Compuserve, so, use of modems has increased dramatically. The more powerful processors such as Pentium and PowerPC in workstations and PC's, enable the modem h/w to be less complicated. Part of the functions done in the DSP or microcontroller might be performed by the host. So, modems might drop in price.

The advent of semiconductor modems will enable a wide range of applications to be implemented: Vending machines will call up when they need more goods to vend, or elevators will call when they require service, and so on.

The last approved standard of V.34 with 28.8 kbits/s speed will enable the Digital Simultaneous Voice and Data (DSDV) applications. DSVD is a modem specification that lets voice and data to be shared over a single dial-up connection. The data is multiplexed into packets, much like an ATM stream.

V.34 is approaching the theoretical speed limit of an analog line - estimated to be in the low 30kbps range. This may be the last of the new modem protocols which doubled the previous speeds.