MoDem

Technology

It is an electronic device , out of independent case or chart to be inserted in a computer, which makes it possible to make circulate (reception and sending) Donnée S Numérique S on a channel Analogique. It carries out the modulation: coding of the numerical data, synthesis of an analogical signal which is in general a Fréquence modulated Porteuse. The operation of demodulation carries out the operation reverses and makes it possible the receiver to obtain the numerical information.

One speaks about modems to indicate the apparatuses intended to make communicate numerical machines (computers, embarked systems) through an analog network (Phone network commutated, Electrical communication, networks Radios…).

All these categories of modem are very often used to reach Internet (or to send or receive Télécopie S, to connect itself to services Minitel…), or to even make numerical telephony…!

Since the end of the year 1990, many standards of telecommunications appeared and, therefore as many new types of modems: ISDN (or ISDN), ADSL, GSM, GPRS, Wifi, Wimax…

History

The modems were used for the first time in the WISE air defense system at the end of the Années 1950. The goal was to connect final located on air bases, sites of Radar S and the centers of operation and WISE control to the exchanges scattered with the E. - U. and with the Canada. WISE used a system of dedicated lines but the equipment at their ends was similar to the modern modems.

IBM was the leading vendor of WISE for the computers and the modems. A few years later, a meeting between CEO of American Airlines and a regional manager of IBM made it possible to give rise to mini-WISE used like automatic system of Billeterie, for which the terminals placed in the agencies selling the tickets, were connected to a central computer charged to manage the availabilities and the Calendrier. The system, known under the name of Saber was a relative far away from the modern system SABER.

During years the development of news Technologie S of Communication allowed a broad multiplication of the modems in an indirect way. The France was for example during nearly one decade the Pays having the number of modems per capita most important, that via the quasi-omnipresence of the Minitel. The Fax has him also played in this evolution.

Characteristics

The main feature of a modem, it is its transmission speed. This one is expressed out of bits a second (bit/s or bps) or in kilobits a second (kbit/s or kbps). When it connects the modem made a recognizable noise by no matter whom. (http://www.lazylaces.com/56Kmodem/)

Note:

one finds in certain Kbit/s documents instead of kbit/s; the correct symbol is K; indeed, K is used in data processing to account for 1024 (2 ¹⁰).
not to confuse bps (bits a second) and bps ( bytes a second, càd. bytes a second)

There were modems working with 150,300,600,1200 bit/s, 4,8,9,6,14,4,28,8,33,6 kbit/s but, for now several years, it has been the standard 56 kbit/s which is installed out of standard. At these speeds, one arrives close to the theoretical limits of flow of information for a phone line using only one carrier frequency. For higher flows, systems using of carrying the multiples were developed, such ADSL. These techniques require obviously the use of specific modems (which one buys, or which is provided by a supplier of access to Internet).

Types of modulation

Various types of modulation are used in the modems:

amplitude modulation (AM, Amplitude Modulation ); this one for example will be carried out using an analogical multiplier receiving on an entry the carrying one, on the other the numeric signal to be transported;
the frequency modulation (FSK, Frequency Shift Keying ); the various frequencies can be obtained using a VCO ( Voltage-Controlled Oscillator , little precis) or by digital processing of a signal produced by a crystal controlled quartz clock (division of frequency, numerical synthesis…) ;
differential phase modulation (DPSK, Differential Phase Shift Keying ): at the end of each cycle of carrying, a phase shift of 180° represents a bit 0, not phase shift a bit 1; this can be obtained while placing on the outlet side of the oscillator generating carrying reverser and a switch which selects, with each passage by 0 carrying, either the direct exit, or the reversed exit;
amplitude modulation combined and phase (QAM, Squaring Amplitude Modulation ): one creates two sinusoids of the same frequency but out of phase of 90°; the two signals are combined in their giving adequate amplitudes; one of the standards specifies 16 possible combinations (3 levels of amplitude, 12 dephasings); the whole of the combinations constitute what one calls a constellation; a constellation of 16 points makes it possible to transmit the state of 4 bits simultaneously, it is what allows a rate higher than the other systems described above, which transmit only one bit at the same time;
an alternative of the QAM is coding out of lattice; here a constellation of 32 states is used, which should allow the simultaneous transmission of 5 bits; but the 5th bit is a bit of checking, which ensures a reinforced protection against the errors of transmission; one thus transmits, as in QAM, the state of 4 bits.
the modems 56 kbit/s are designed to work in the environment of the numerical networks; they use the pulse modulation coded (PCM, Pulse Code Modulation ) to convert the analogical signal into numerical sequence: the amplitude is measured 8000 times a second, with a resolution of 8 bits; the theoretical output should reach 64 kbit/s, but the real flow generally ranges between 40 and 56 kbit/s, depending on the state of the line of transmission.

Structure of a modem

A modem comprises the following blocks:

a modulator, to modulate carrying which is transmitted by the phone line
a demodulator, to demodulate the received signal and to recover information under digital form
a circuit of conversion 2 wire/4 wire: the signal of the modulator is sent towards the phone line whereas the signal arriving by the phone line is acicular towards the demodulator; it is thanks to these circuits, laid out on both sides of the phone line, that the transmissions can be made in full duplex ( full duplex , càd. in the two directions at the same time)
a circuit of interface to the phone line (DAA, Dated Access Arrangement ) primarily made up of a transformer of insulation and limitings device of Surtension S

These circuits would be sufficient to transmit information in manual mode; all the operations such as unhooking of the line, composition of the number… are then carried out by the user. In order to allow an automated operation, where all the tasks are carried out under the control of a communication software, the modems generally comprise some auxiliary circuits:

a circuit of composition of the phone number; one can generally specify composition by impulses or tonalities (DTMF, Dual Thunders Multiple Frequency )
a detection circuit of ringing; this circuit prevents the computer when the modem is called by a distant computer
a detector of tonalities, which detects the various tonalities indicating that the line free, is occupied, in disturbance…

To manage all these blocks, the modem is connected to the computer by various lines, which are below detailed.

Signals of control of a modem

The various signals exchanged between a computer (DTE, Dated Final Equipment ) and a modem (DCE, Data Communications Equipment ) are specified in standard RS.232/V.24:

the data to be transmitted arrive at the DCE by the line Emission
the data received by the DCE appear on the line Réception
DSR ( Data Set Ready , ready modem) is at the active level when the DCE is fed and connected to a phone line
DTR ( Data Final Ready , ready computer) is active when the DTE is ready
RTS ( Request To Send , request for emission) is activated by the DTE when he wants to send data
CTS ( Clear To Send , ready to emit) is activated by the DCE when he the connection established and is ready to receive the data to transmit
CD ( Carrier Detect , carrying detected) is activated by the DCE when it receives carrying coming from another DCE
IH ( Ring Indicator , call sign) is activated by the DCE when it receives a signal of ringing
the standard envisages also two connections of mass, for the signal (obligatory), the other for a shielding (optional).

The connector envisaged initially was a connector 25 pins, the DB-25. However, as of many pins were unutilised, the current trend is to use connectors with less pins, the such DB-9 which count 9 pins.

For information concerning the stitching of the connectors, to see

Typical procedure of emission

As example, let us show how these various signals can be used:

before starting a data transmission, the computer has checks that the modem has is under tension by checking the level of DSR
the computer has gives the order to the modem has to form the phone number
the modem called, that we will name B, detects the ringing and prevents the computer B to which it is connected by activating its line IH
when the computer B is ready to receive the data, it activates its line RTS
the modem B activates its carrying
then the modem has detects the carrying one and prevents the computer has by activating CD
the computer has activate RTS to ask whether it can begin the transmission
the modem has answers by activating CTS, and the transmission of the data can begin

Null modem

We explained above the way of transmitting information between two distant computers, connected by a phone line: one connects to each computer a modem, which is connected to the phone line.

But what to make to transmit information between two computers being in the same part? And well, it is enough to disconnect the two modems, and to place between the 2 computers a case provided with two connectors DB-25 or DB-9; this case, whose function is to replace the 2 modems, is called null modem ( No Modem ). Inside the case, the pins of the two connectors are connected in the following way:

the pin emission has is connected to the pin reception B
the pin emission B is connected to the pin reception has
pins CTS and RTS are shorted-circuit on each side
DTR has goes towards DSR B
DTR B goes towards DSR has
the mass signal has goes towards the mass signal B
sometimes RTS has also goes towards CD B and RTS B towards CD has
sometimes, DTR has also goes towards IH B and DTR B towards IH has

Various standards for the modems

The UIT-T (International union of Telecommunications - standardization of Telecommunications; this organization was called until in 1992 CCITT, International Advisory committee Telephone and Telegraphic) a certain number of opinion concerning the operation of the modems emitted. These opinions specify the operating conditions of the apparatuses: authorized, standard transmission speeds of modulation, systems of possible compression and/or detection of errors; they constitute actually standards which are respected by many manufacturers. Hereafter some important standards.

V.21 Standard: flow of 300 bps in full duplex ( full-duplex , 2 directions simultaneously); modulation FSK; asynchronous mode; frequencies used: 980 and 1180 Hz (for the 0 and the 1) in a direction, 1650 and 1850 in the other direction
V.22bis Standard: flow of 2400 bps in full duplex; asynchronous mode or synchronous; carrier frequencies 1200 Hz in a direction, 2400 Hz in the other; modulation QAM with a constellation of 16 points
V.32 Standard: flow of 9600 bps in full duplex; carrying with 1800Hz at the same time for the emission and the reception (thus required to envisage suppressors of echoes to avoid an interference enters the signals being propagated the 2 directions); modulation with 2400 bauds, with a constellation of 32 points; one thus transmits 5 bits per interval, but the 5th bit being redundant, effective speed is 4 X 2400, that is to say 9600 bps.
V.32bis Standard: flow of 14,4 kbps; modulation QAM or lattice
V.32terbo Standard: flow 14,4,16,8 or 19,2 kbps; modulation DPSK and lattice
Standard V.FAST: flow of 28,8 kbps
V.34 Standard: flow of 28,8 kbps
V.34bis Standard: flow of 32 kbps
V.90 Standard: flow of 56kbps for the downlink ( downstream , towards the user), but 33,6kbps for the uplink ( upstream , towards the network); there is thus an asymmetrical connection, as for ADSL
Norme V.92: flow going down until 56Kbps, and flow going up until 48Kbps. V92 adds also some additional functions (ex: V44 more powerful than V42, taken phone call…).

The smaller transmission speed is high, is the difference between the various states of the line. The error rate thus tends to increase, particularly when the line of transmission is disturbed. This brought the settling of standards to detect and correct the errors, such as the V.42 standards and MNP 1 with MNP 4 (these last standards were developed by the Microcom firm).

In addition, like, with the V.90 standard, one arrives close the speed of theoretical transfer maximum of a standard phone line, one developed techniques allowing to increase the flow while proceeding, before the sending, with a compression of the data.

the V.42bis standard, which uses the technique of compression BTLZ, makes it possible to multiply by 4 the effective transmission speed of a line
standard MNP 5 makes it possible to double the flow
standard MNP 6 described the procedure of establishment transmission speed; each modems starts by being connected at its lowest speed (generally 2400bps), then augmentt gradually its rate until the other modem does not follow any more
standard MNP 7 is a protocol of compression of a factor 3
standard MNP 9 tries to increase the band-width rather while placing the ACK (acknowledgments of delivery) in the packages of data that separate
standard MNP 10 uses a compression MNP 5 or V.42bis, but manages to still increase the flow; if, in consequence of bad conditions of transmission (noise, parasites…), the modems reduced their rate, MNP 10 enables them to increase rate again if the state of the line improves

Orders AT

The firm Beam, manufacturer of modems, developed a protocol for the ordering of an external modem starting from a computer. The protocol defines various orders allowing for example:

to compose a phone number
to order the connection of the modem to the line (the equivalent to take down the telephone)
to know the state of the line: tonality of invitation to be transmitted, line occupied…
to specify the type of transmission and the protocol of connection to use
to regulate the sound volume of the loudspeaker interns modem
to send the characters simultaneously transmitted towards the screen
to post certain information concerning the modem
to handle the internal registers of the modem

Orders AT are orders which one can directly send to the modem, when this one is in mode Command , which occurs:

with the powering;
at the end of a communication;
after the execution of an order other than AT0 or AT&T;
at the time of the reception, whereas the modem is in mode on-line , of an escape sequence (constituted of the sending of 3 natures identical to the characters stored in an internal register of the modem);
at the time of the transition ONE towards OFF from line DTR (if D1, &D2 or &D3 are activated).

Hereafter, a summary of orders AT; all the orders must be preceded by AT. For example, ATA means " répondre".

Has to re-execute the preceding order
ATy=x to write X in the register interns there modem at present selected
AT? to see the contents of the register selected
to answer
Bn selects the Bell mode (American) or CCITT (European)
Cn controls carrying
DNN composes phone number N
In order of the echo
Hn controls the catch of line and the disconnection
In identification of the modem
Ln controls the volume of the internal loudspeaker of the modem
mn controls the volume of the internal loudspeaker of the modem
One return to the mode on-line
P selects classification by impulses
Qn controls code " résultat" returned by the modem after execution of the order; this code is made up either from 1 to 3 digits (short code), or of a more explicit character string (wide code)
Sn access to internal register N
&Dn option DTR ( Data Final Ready )
&Fn restores the configuration by defect of the modem
&Gn selects the tonality of guard
&Kn control of flow of information
&Mn selection of the mode
&Pn control of classification by impulses
&Qn selection of the mode
&Tn test and diagnosis
&Sn option DSR ( Data Set Ready )
&V to post the current configuration
&Wn to memorize the current configuration
&Yn selects the configuration by defect
&Zn=x memorizes phone number X in automatic register N
%En check fallback/fallforward
\ An selects the maximum size of block MNP
T selects classification by tonalities
Vn forms code " résultat"
Wn controls code of result of connection
Xn controls code " résultat" extended
Yn disconnection with long time
\ Bn to transmit station-wagon to the distant modem
\ Jn adjustment of rate DCE
\ kN controls station-wagon
&Cn control of the DCD ( Data Carrier Detect )
Zn reset

Examples of use of orders AT

Composition of phone number: ATDT (555) 555-5555
( Attention sometimes the sign " should be put; ; " not comma at the end of the number )

To answer: ATA
To hang up again: ATH

Examples of codes " résultat"

wide Form: OK, short form: 0, significance: confirmation of execution of an order
wide Form: CONNECT, short form: 1, significance: envoy when connection is established to 300bps
wide Forme: BOXING RING, short form: 2, significance: detection of the tonality of ringing
wide Form: BUSY, short form: 7, significance: occupied line
wide Form: CONNECT56000, short form: 82, significance: give rhythm transfer 56000bps

See too

Modulation
Phone network commutated
the MoDem is the name of a French political party.

Simple: Modem

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