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Types of modems


The speed of the modem

The modems of: 55.600 bps?

The communications norms

The internal speed PC - modem

The commands Hayes

The UART

The RDSI

Updated page
7/3/2000


Author:
Juan Herrerías Rey
 

To install a modem


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What is... a modem?


 

The UART

It is a question of the chip that controls the ports series of the computer. In case of the internal modems there does not have special importance, since they usually bring his prepared for the speed that they need. The day pupils, nevertheless, since already existing COM gets connected to one of the ports in the computer, they depend on the speed of the UART of this one.

The UART is designated by a number of reference. Although several models have existed in almost twenty years of life of the PCs, the most common have been 8250 and 16550. 8250 there is the classic model, which used even come up the epoch of the 486; it is capable of handling without problems modems even of 14.400 bps, but it turns out to be insufficient for more rapid modems.

16550 there is a model much more advanced that goes so far as to provide internal speeds PC - modem of up to 115.200 bps, more than sufficient for modems of 28.800, 33.600 and 55.600 bps. In addition to a more complex design, it has buffers of memory in which to keep information until it could be processed.

Other models are 16450, which it is a transition model between both and which like much he can handle modems of 28.800 bps, although already with certain yield decreases, and the diverse variants and progress of 16550 (16550AF and many others of number of top reference).

To identify the model of present UART in a computer they usually use programs software that detect the hardware, although the results not always are exact. One of these programs is the MSD of Microsoft, which comes with the last versions of TWO, as well as in the CD of Windows 95 (although he does not settle for defect and must be done to hand). It is a program for TWO, as almost all those who realize these tasks, and although it is slightly trustworthy it presents the advantage of that almost sure that we arrange of him.

Pantallazo of the utility MSD of Microsoft showing the screen of ports COM. Seemingly we have an UART 16550AF.

Both in the MSD and in other programs, if the program detects an UART 16550 or Superior is almost sure that it has been right; nevertheless, if it detects 8250 it is possible that it is not correct and in fact it is a more advanced model. Other programs that detect the hardware of the ordering sound CheckIt, Agsi, PCConfig or Hardware Info; all of them can be located and be obtained in Internet, in the direction www.shareware.com or by means of Yahoo or other searchers.

To change an UART that turns out to be insufficient to install an external modem of certain speed, we will have to change the card that controls the ports COM. In the above mentioned card, generally ISA, is where there will be the UART and the chips for the support of parallel port, as well as sometimes for the disk drive and the hard disks IDE. In the most modern computers these skills are integrated in the motherboard, or at least the support for hard disks. In many cases it will not be necessary to replace the card, but it will be enough to disable by means of a few jumpers in the same one the support of ports COM and to to enable it in the new card, which we would add without removing the ancient one. These cards (also known since of I/O) they are very cheap, less than 5.000 pts, but they turn out to be more and more difficult to be owed to the integration of these components in the motherboard.

Certainly, I have understood that some internal modems lack UART or do not form it appropriately, therefore it is as if it did not exist and they try to use that of the computer, what can give problems of yield, of conflict between devices or complicate the configuration of the modem. If the yield of his internal modem does not look like the correct one and the UART of his computer is an ancient model, perhaps be his case. Anyway, this does not seem to happen in modems "of mark".

 

The RDSI

Namely the Digital Network of Integrated Services (ISDN in English), or what is the same: the line of digital phone. What it distinguishes to these lines is not the cable, which in most cases is the same, but the method of using it: digitalmente uses the telephone line instead of similarly, which implies that the information quantity transmitible for the line is major.

To digitize any thing always implies losing an infinitesimal quantity of information of the original analogical sign, but in case of the voice it turns out to be impossible to perceive for any human being (and for many machines). In return we obtain a major band (more information fits) and major purity of sign (once digitized information does not get lost).

A common digital line has a band of 128 kilobps; yes: 128.000 bits per second!, that can be distributed in two channels of 64 Kbps. This way, we can have two phone lines, or a line of phone and an Internet connection of 64.000 bps, or an Internet connection of 128.000 bps. And there are real and absolute numbers, not as 55.600 bps of the modems about which we were speaking earlier...

The snag is usually economic, a skylight. These lines cost more money on having hired them and monthly, in addition to that some Internet providers ask for an extra one those who want to get connected at these speeds, what is logical if we think that only one client who gets connected to 64.000 bps occupies band as two or three with normal modems of between 14.400 and 33.600 bps, and the provider pays enough for the above mentioned band.

Otherwise, to install and to handle an Internet connection for RDSI is not much more difficult that to do it with one normally, and often there is even offered a bundle that includes the hiring of the line with Phone companhy (or Retevisión or the one who is), the card RDSI, the Internet connection and the installation to domicile of everything.

Finally to mention that these connections are realized by means of a device similar to a modem that, on having been almost always an intern, receives the generic name of card RDSI and that does not turn out to be excessively expensive. He has his own special UART capable of reaching this 128.000 bps, therefore it should not depend on the capacities of the computer; but due to the big quantity of information to handle and to that it is supposed that we look for a suitable yield (if not, better so much money does not wear out), the computer will have to be moderately powerful, 486 or preferably a Pentium.

 


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