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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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56K Modems

In the ever-continuing quest for speed, a new breed of modems is coming to market. These modems allow for downstream communications—those going from the host to the client—up to 56Kbits/sec. This doubles 28.8K and not quite doubles the preceding standard of 33.6K/sec. However, in the U.S., regulatory restrictions currently limit the download speed to 53 Kbps.


Note:  
New technologies, which require central office (CO) upgrades, promise to bring multi-megabyte speeds to the same telephone wiring that is already run to your home. However, these technologies are still being tested and are not widely deployed yet.

To understand how this additional speed was captured, you need to understand a few basic principles of modem technology. In a traditional modem, circuit information is converted from digital form to analog, so it can travel over the Public Switched Telephone Network (PSTN), and finally back to a digital signal.

This conversion from digital to analog and back causes some speed loss. Even though the phone line is capable of carrying about 56K of information over it, the effective maximum speed because of conversions is about 33.6Kbits. A man by the name of Shannon came up with a law (Shannon’s Law) which states that the maximum speed over an analog phone circuit is 33.6K.

However, Shannon’s Law assumes that the telephone network is entirely analog. That is not the case in most of the United States today. In urban areas, most circuits are digital until they reach the CO which your phone line is connected to. The CO converts the digital signal into an analog signal before sending it to your home.

Considering the fact that the phone system is largely digital, you can—in some cases—remove the first step of translating the information from digital form to analog form for transmission over the digital PSTN.

The result is that you can, if you connect the host modem digitally, eliminate the restriction of 33.6K shown in Shannon’s Law. The result is that data can be transmitted at the full 56K capacity of the phone line in one direction. The other direction, from your computer to the host, will still operate at the 33.6K speed.

When looking to purchase an 56K modem, you may be surprised to know that you may not be able to use these modems at their top speed based on the PSTN between you and the host system.

There are some very specific requirements to make 56K modems work. They are:

  There can be only one digital-to-analog conversion in the network. This means that the connections between your CO and the CO which services the host must be all digital.
  The host must be connected digitally. One end of the connection must be connected to the PSTN.
  Both modems must support the 56K technology. Both modems must support the same 56K technology.

Most users find that their connections average between 42-48Kbps. Additional, this speed is “one-way.” Upload speeds are limited to 33.6 Kbps. A V.90 connection requires a special modem bank and special digital connections and the number being called. These modems fall back to 28.8-33.6 Kbps if the modem being dialed does not connect at 56K speeds due to incompatibilities in the modems or distance and switching incompatibilities in the phone lines.

Complicating matters in the “56K” arena is the fact that US Robotics (now a part of 3Com) and Rockwell released competing and incompatible standards for 56K modems prior to the establishment of the V.90 standard. The 3Com standard is ”X2“ and the Rockwell standard is ”56KFlex.“ Each standard requires its own type of modem bank to be installed at the number you are calling in order to connect at 56K speeds.

Although the V.90 standard has now been ratified by the ITU, significant compatibility and speed problems remain. The most significant long term problem for users of these modems is that some phone lines, even recently installed phone lines, may not support speeds over 33.6 Kbps. The maximum speed you can reach depends on the distance from the modem to the telephone switching office (shorter is better), the type of equipment installed that the telephone switching office, and the number of digital-analog conversions between the modem and the switching office. In the short term, incompatibility between the three versions (V.90, X2, and 56KFlex), lack of upgrades for particular modems, and service providers who have not upgraded their modem banks remain the biggest obstacles to connecting at speeds in the 34-53 Kbps range.

For more information on the latest developments in 56K modems, you may want to check the http://www.56k.com/ Web site. There are frequent updates and news items posted there regarding all three varieties of 56K modems.

Testing Serial Ports

You can perform several tests on serial ports. The two most common types of tests involve software only, or both hardware and software. The software-only tests are done with diagnostic programs such as Microsoft’s MSD, while the hardware and software tests involve using a wrap plug to perform loopback testing.

Microsoft Diagnostics (MSD)

MSD is a diagnostic program supplied with MS-DOS 6.x, Microsoft Windows, or Windows 95. Early versions of the program also were shipped with some Microsoft applications such as Microsoft Word for DOS.

To use MSD, switch to the directory in which it is located. This is not necessary, of course, if the directory containing the program is in your search path—which is often the case with the DOS 6.x or Windows-provided versions of MSD. Then simply type MSD at the DOS prompt and press Enter. Soon you see the MSD screen.

Choose the Serial Ports option. Notice that you are provided information about what type of serial chip you have in your system, as well as information about what ports are available. If any of the ports are in use (for example, a mouse), that information is provided as well.


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