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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?


 

Modem is a MOdulador-DEModulador acronym; that is to say, that is a device that transforms the digital signs of the computer into analogical telephone sign and vice versa, with what it allows to the computer to transmit and get information for the telephone line.

The chips that realize these functions are so standardized almost like those of the sound cards; many manufacturers use the same integrated, for example of the company Rockwell, and only they differ for other electronic elements or the casing.

 

Types of modems

The principal distinction that one usually does is between internal modems and external modems, although recently there have appeared a few modems called "modems software" or Winmódems, which have complicated a little the panorama.

  • Interns: they consist of a card of expansion on which there are ready the different components that form the modem. They exist for diverse types of connector:
    • ISA: due to the low speeds that manage in these devices, for many years this connector was used exclusive, nowadays in desuetude.
    • PCI: the most common format at present.
    • AMR: in some very modern badges only; bargain sales but slightly advisable for his low yield.
    The principal advantage of these modems resides in his biggest integration with the computer, since they do not occupy space on the table and take his electrical feeding of the proper computer. Also, there are usually some more bargain sales due to lacking casing and transformer, especially if it is PCI (although in this case they are almost all of the type "modem software"). On contrary, they are slightly more complex of installing and the information about his state can only be obtained by means of software.
  • A typical external modem
  • Day pupils: they are similar to previous but shoves in a casing that is placed on the table or the computer. The connection with the computer realizes generally by means of one of the ports series o "COM", by what there is used the UART of the computer, which will have to be capable of providing enough communication speed; at present models exist already for port USB, of even more simple connection and configuration.
    The advantage of these modems resides in his easy transportabilidad between computers, in addition to which we can know the state the modem (marking, with / without line, transmitting...) by means of a few lights that they usually have in the hood. On the contrary, there are one more utensil, they need a plug for his transformer and the UART must be 16550 or Superior so that the yield of a modem of 28.800 bps or more it is the suitable one.
  • Modems PC-Card: there are modems that are used in portable; his size is similar to that of a slightly thicker credit card, but his capacities can be equal or more advanced posts than in the normal models.
  • Modems software, HSP or Winmódems: there are internal modems (at least I do not know external any, and doubt that it was possible to construct it) in which several electronic pieces have been eliminated, generally chips specialized, so that the microprocessor of the computer must fulfill his function by means of software. The normal thing is that they use as a connection a groove PCI (or an AMR), although not all the modems PCI are of this type.
    The advantage turns out to be clear: less pieces, more bargain sales. The disadvantages, which need very powerful microprocessors (like little a Pentium 133 MHz), that his yield depends on the number of open applications (nothing of multitask while the modem works or an authentic turtle will turn) and that the software that handles them only is free usually for Windows 95/98, hence the Winmódems appellative one. Obviously, they turn out to be slightly advisable but they are cheap...
  • Finished modems: the classic modems not HSP, good are internal or external. In them the yield depends almost exclusively on the speed of the modem and of the UART, not of the microprocessor.

 

The speed of the modem

There proves undoubtedly the parameter that better it defines to a modem, up to the point about which frequently one speaks simply of "a modem 33.600", or "14.400", without specifying more. These numbers are bits per second, bps.

It is necessary to bear in mind that they are bits, not bytes. In this context, a byte is composed of 8 bits; therefore, a modem of 33.600 bps will transmit (in the best conditions) a maximum of 4.200 bytes per second, or what is the same: he will need like slightly 6 minutes to transmit the content of a 1,44 MB diskette.

Certainly: in the best conditions only. The saturation of the lines, the low capacity that there provides the provider of access to Internet, the bad quality of the modem or of the line (noises, interferences, crossings...) usually do that the effective average speed is very much a 3.000-year-old minor bytes/s or less. To know which of these the factor is limitante turns out to be vital to improve our access to Internet.

Likewise, it is not necessary to confuse this nominal speed (that one supposes that it might reach the modem, for example 33.600 bps) with the speed of department, which is that that is indicated us to the beginning of an Internet connection; the last one is that that in principle, and in this moment, there has identified the modem of another side of the line like valid, and it has little to do with the yield that we will obtain.

This way, a connection in which the department speed has been 31.200 bps might end up by being much more rapid than other one in that the 33.600 have been reached. Only this value must be born in mind when it is abnormally low (like 14.400 with a modem of 33.600) or when we never reach the maximum speed (what can indicate that the modem, the line or the provider are of bad quality).

 

The modems of: 55.600 bps?

... In fact, of a few less. These modems, also known simply how of "56 K" (a rounding to the rise that does not correspond to the reality), they use a series of tricks to make use better of the telephone line and to be able to receive information at this speed... sometimes.

The problems of this technology are:

  • to another side of the line (for example in the servant of his Internet provider) there must exist a modem that is also of 55.600 bps, and in addition to the same type (since three different standards exist);
  • this speed is used only on having received information, on having ordered the maximum speed, it is 33.600 bps (although in Internet the most common thing is to receive it);
  • if into the way the sign is transformed multiple times (which can happen, for example, if it is at big distance of his provider, far from a downtown or uses a switchboard), it turns out to be impossible to use this technology;
  • the telephone line must be of high quality; if it does not get connected to 33.600 bps without problems, insurance that will not be able to do it to 55.600.

For all these motives, the real maximum speed there will be approximately 45.000 bps of average, supposing that all the factors collaborate and the lines are not saturated (which unfortunately is not very common). With regard to the ideal maximum distance up to the provider or the corresponding telephone head office, it is recommended to be a minor of 3,5 miles (approximately 5,6 Km).

Logo of Rockwell, creator of the norm K56flex

Nevertheless, he is worth acquiring a modem of this type if we know that our provider admits it and our line is of quality, since they cost little more than those of 33.600 bps and if they are not capable of reaching the 55.600 they will work like normal modems to 33.600 bps, that is not small. In this respect, remember that in Spain there is used for the most part the official norm V.90, but also the pseudo-norm K56flex, with which most of the modems are compatible, but not all.

Any way remember that the factor more limitante is usually a bad provider, and that it is always convenient to buy a modem of confirmed quality of 33.600 bps (an US Robotics, Diamond, Sitre, Zoom, Motorola...) before one of 55.600 of unknown father...

 


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