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Normes et vitesses des modem téléphoniques
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12A. Phone modem

12a.1. Introduction - 12A.2. Communication speed - 12A.3. Telephone Modem - 12A.4. ISDN Modem - 12A.5 Fax-modem, Voice modem - 12A.6. Problems related to connections of modems

12A.1 Introduction

A modem is the principal use of the serial connections. At the beginning, the serials standards were even completely dedicated to these peripherals with standard RS232. A modem converts the numerical data coming from a computer into signals on a basic  carrier frequency, and inversely. The carrying one is a fixed frequency sinusoid on which "is stuck" the signal to transmit.

A modem can be internal or external. A current internal model (PCI) manages a new port series. As the majority of the PC include at the beginning Com1 and Com2, there remains Com3 and Com4. For a division of the interruptions, you cannot use a mouse series in COM 1 and one modem in Com3. On the other hand, an external modem connects on an existing connection of the PC.

An external modem is preferable (but more expensive) for the risks related to the storms and the facility of installation. In the case of a storm per telephone line (40% of the cases of the lightnings), an internal modem (even PC not lit) is destroyed, but generally causes breakdowns on the level of the mother chart or even in a random way on the printers, screens... On the other hand, in the case of modems external, only the modem is destroyed.

Currently, the majority of the modems intern are PCI and Plug & Play. One finds modems intern "Hardware" and "Software". In the case of the modems software, no interruptions is provided, it is the processor which takes care of the majority of the tasks. Be wary of the problems of COM.

One finds 2 types of standard modems: RTC or ISDN. Connections ADSL will be seen in the hardware course 2.

CTN (Commutated Telephone Network). Maximum speed is of 56 kbps (kilo bits per seconds) in reception and 33.600 kbps in emission

The modem

ISDN (Integrated services digital network): it requires the installation of numerical telephone lines. The basic access to 144 kb/s comprises 2 ways with 64 Kb/s (2 channels B) and a way with 16 Kb/s (channel D). By programming with the installation, one can work in 128 KPS by adding the 2 channels. Other side of the coin, one uses 2 telephone calls.

12A.2 the speed of communication

The speed of the modems is expressed in bps (bit per seconds) or in baud. The concept of baud refers to a number of packages of data sent of only one blocks. If blocks it contains only one bit, bps and baud correspond.

Currently, modems RTC function to 55.600 bps. Thus divide by 10 to obtain the approximate flow in byte or byte (KB/s)

12A.3. Modem RTC.

The modems are taken again here working on a normal telephone line. Each modem answers two types of standards: a standard speed and a standard of correction of error. Currently, the modem are taken again in 55.600 bps, in V90, like correction of error: V42 and MNP2-4, like compression in V42 (a) and MNP5. You will find below some information concerning the old and current standards, as well as the average transfer time of a file of 100k.

Modulation Speed (bits a second) Approximate transfert delay for a file of 100 KB
V.22 1200 bps 14 min.
V.22 (a) 2400 bps 7 min.
V.32 9600 bps 2 min.
V.32Bis 14.100 bps 1 min.
V34 28.800 bps 30 S.
  33.600 bps  
US Standard Robotic X2 33.600 bps - > 55.600 bps 33,6 Kb in emission, 55,6 Kb in reception
K56: Rockwell K56Flex norme 33.600 bps - > 55.600 bps 33,6 in emission, 55,6 Kb in reception
V90 33.600 bps - > 55.600 bps 33,6 Kb; in emission, 55,6 Kb in reception
V92 33.600 bps - > 55.600 bps 48 Kb in emission, 55,6 Kb in reception
V.42 Correction of error
V.42bis Data compression
V.44 Data compression for V92
MNP2 Correction of error
MNP4 Data compression
MNP5 Correction of error

There are 3 types of modem in 55.600 bps. Indeed, the transfer of information on a telephone line is limited in 33.600 bps. The transfer is done in 33.600 kps in sending and 55.600 kps in reception (seen by the user). 2 manufacturers of chipsets for modem developed in 1997 their own standard. They were incompatible enters and required that server Internet is same standard as the modem. Many a modem of the time go down again to 33.600 Kps since since, the V90 standard gathered the 2 manufacturers (semi-1998). In the 3 cases, the transfers are done to 55.600 bps in reception, but in 33.600 bps in emission (towards Internet).

Certain modem are flashables. You can download a small program on Internet which modifies your modem 33,600 Kps in 55.600 bps.  You with the site of the manufacturer of the modem refer. Attention, the programs to the American standard are not compatible with the European standard.

You cannot use a modem bought in France for example on the Belgian network. Nevertheless, in July 1998, the Council of the European Communities approved a named regulation RTC 21. This defines a standard common to all the countries of the European Community. Once a modem recognized under this standard, it can be used in all the countries of the European Community. Initials EC guarantee that the modem accepts this standard. Generally this standard is taken again only for external modems.

The V92 standard makes it possible to receive a message on your screen in the event of phone call and to take again Internet after having answered (if your supplier of access installed the function).

12A.4. Modem ISDN

You can also connect modems ISDN via a special radio-telephone box. These specific modems function to 128 Kps in the 2 directions by coupling 2 lines by software programming of the communication (and to pay 2 communications), that is to say 64KPS in normal mode ISDN. Speed is 2 times faster than for a traditional modem.

12A.5. Fax modem - Voice modem - message +.

With the exception of some specific modems ISDN, you can always install a software fax to receive (but especially to send) faxes. For recall, the speed of connection of a fax is limited to 9,60 Kps (14.400 bps for some professional models). The speed of sending of the large faxes of offices is related on the compression and not to the speed of transfer. Certain models offices send a A4 page in less than 3 seconds.

Modems VOICE are currently found. Those, are compatible sound. They can thus directly send your voice on the telephone line, or even be used as responder.

The modems "Message +" are external modems including a memory. They can receive faxes and be used as responder, even extinct PC. Two principal firms divide this market: US Robotics (3COM) and French firm OLITEC.

12A.6. Problems related to the installation of a modem, connection of telephone wire in the catches

In addition to the problems of COM (interruptions and addresses), new the modem internal include a specific driver which emulate a port series. Without this driver, the modem is unusable.

Second problem, WinModem. These internal modems use neither addresses, nor interruption. All the work of sending and reception is made by software. In addition to the fact that at equivalent speed, they are slower, they cannot work under DOS. They are generally established in the gone good PC.

As a modem is connected on the telephone line, this connection also should be checked. The standard of tension on a telephone line in Belgium (the tensions are different with France and thus modems no usable except CE). The telephone lines Belgacom border 55 V uninterrupted between 2 wire except connection, much less at the time of a call (5 V). If the tension of catch of line is too weak (too long length of wire), the modem cannot take the line. Moreover in certain arranged installations, the wire are not placed at the good place in the catch. Better is worth to reverse in the catch. Also check the catch. Certain poor contacts in the catches does not make it possible to connect 2 catches one to the top of the other.

Wiring of a telephone jack

Telephone catch for Belgium

Wiring of a telephone jack Belgium

Multi-stage Telephone Catch

For France. France Telecom advises the wiring of 8 wire of the jack telephones, but only wire 1 and 3 are to be connected

In relation:

The continuation of the course hardware 1 > 13. Colors parameters 

The hardware course for networks and servers

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