The 56K Modem Question- Over the last few years MODEM speeds have steadily increased. With the advent of new technology, Internet users now enjoy much faster download times than when the World Wide Web was first created.

There was a time when most people thought the maximum speed of MODEMs would never be above 28.8K or 33.6K. To get higher speeds, you'd need a different type MODEM that could not work over standard telephone lines.

Recent breakthroughs in 56K technology have brought us the capability to download data from the Internet at seemingly breakneck speeds.

With the devaluation of the Asian market, prices for this new technology are well within the reach of the average user. While it only makes good sense to get the fastest speed possible when dealing with the Internet, there are a few things you should consider before rushing out and buying a 56K MODEM.

The Basics - Before we can understand 56K MODEM technology, we must first look at how analog MODEMs work. The job of the MODEM is to convert digital signals to analog waveforms on one end and analog waveforms to digital signals at the other end. Early MODEMs treated transmission lines as though they were purely analog lines. To understand what that means, consider the following:

  • Information inside your computer is in digital format: all of the data is stored as 1s and 0s.

  • Normal phone lines are analog: they transmit data such as your voice as a series of peaks and valleys.

  • Your "MODEM" is a modulator/demodulator: it modulates (converts) outgoing data from digital to analog and demodulates (converts) incoming data from analog to digital.

  • In summary, your computer uses digital signals and your telephone calls use analog waveforms. See the diagram below.

The Public Switched Telephone Network (PSTN), designed for voice only transmission, uses analog loop signaling with a 3khz bandwidth. This 3khz bandwidth would only allow a transmission speed of around 300khz (the maximum frequency of the human voice is 3000khz). However, through the signaling techniques employed in the v.34 protocol, MODEMs manufactured prior to 1998 could handle transmission speeds of 28.8K or 33.6 khz.

Problems such as the signal-to-noise level of the telephone line being too high can seriously degrade a MODEM's ability to understand received data. For this reason, the quality of an analog telephone line is the limiting factor on speed of data transmission. This still applies to 56K technologies.

Is There a Standard? - Prior To February 1998, no standard existed for 56K MODEMs. However, there were two possible proprietary protocols available for 56K transmission.

  1. U.S. Robotics used the x2 MODEM protocol that is very similar to V.90 technology.

  2. Rockwell and Lucent used the K56flex protocol, which sends a digital stream instead of using analog modulation techniques.


If you bought company one's proprietary 56K protocol, you would not be able to call a MODEM using company two's proprietary 56K technology and get the full speed. You'll still be able to connect 28.8 or 33.6K, though, just as you do now, so you won't be totally sunk.

As an example, PA.net uses a MODEM employing the K56 Flex protocol. You may not be able to connect at 56K into AOL. PA.net does, however, support the V.90 protocol

In February 1998, the International Telegraph Union (ITU) approved a draft 56K standard, which implemented the V.90 protocol. Although full ratification of the V.90 standard occurred in September 1998, over 100 MODEM manufacturers and over 300 Internet service providers still support the k56 protocol. Some MODEM manufacturers are emulating the V.90 protocol through software.

How Does a 56K MODEM work?- For a 56Kbps MODEM to work correctly, you must be transmitting over a 100% digital network. In other words, "a 56K MODEM requires a digital connection from the MODEM being dialed to the local loop of the dialing MODEM." Thus, only a one-way digital to analog conversion is allowed.

The V.90 protocol uses one bi-directional channel to and from the MODEM at the local loop. It does not use packet switching techniques. Quantization noise limits v.34 communications to about 35 Kbps. However, this noise only affects analog to digital conversions. The key to V.90 is there cannot be analog to digital conversions between the distant MODEM and the PSTN. The process works something like this:

The host server you are connecting to establishes an equivalent to a digital connection.Using a variation of 256 PCM modulation, the host server converts the digital signal to the corresponding discreet analog voltages. These voltages are sent to the receiving MODEM over the analog loop. Data bits are sent as binary numbers, eight bits at a time, over the PSTN. The receiving MODEM reconstructs the PCM codes from the analog signals received.

The three conditions needed for full 56 Kbps transmission under the V.90 standard are:

1. A digital connection at one end - - Once the user's analog call is converted to digital, it must stay digital until it reaches a digital MODEM over a T1, PRI, or BRI trunk terminated on the trunk side.

2. V.90 support at both ends - Both ends must be using MODEMs that fully support V.90-signaling techniques. If one end uses a proprietary 56K MODEM protocol, you will not be able to connect above 33.6 Kbps unless your MODEM uses the same protocol.

3. One Analog-to-Digital Conversion - There can be only one analog-to-digital conversion along the entire path between the two MODEMs. If the line is a channelized T1, it must be connected as trunk-side service. With line-side service, there is usually an additional analog-to-digital conversion.

Before You Buy

Make sure your telephone line supports 56K - There's no guarantee that a 56K MODEM will work on your telephone line. As we said, this technology won't work if there is more than one analog/digital conversion between your home/office and your Internet service provider.

U.S. Robotics has a test line that allows you to dial-in with your existing V.34 MODEM and find out if your phone line supports x2. The telephone number to call can be found at the following Web page.:

If your telephone line passes that test, you may be able to use 56K technology. However, it isn't guaranteed. The telephone number you dial for the test is a long distance line. The test line may be routed through different circuits than a call to your local ISP. The test line may indicate you are ok when you really are not.

The only real sure way to test is to borrow a 56K MODEM from someone. Install that MODEM in your computer and try to dial your local ISP.

Make sure your ISP supports 56K - Contact your local ISP and ask them if they support 56K MODEM speeds. Ask them to recommend the brand and model of MODEM to buy. In this way, you will be sure that the MODEM will work properly. A large number of Internet service providers have stated they will support 56K technology. The problem is that only a small percentage of these providers actually provide access.

In summary, 56K technology can go a long way to increase your access speed to the Internet. Just make sure you check with your local provider before you rush out and buy a new MODEM.

Additional Sources:
The 3com Web site
Network Computing Online
MODEM Central