Five years ago industry pundits were saying 9,600 baud was as fast as we could get a telephone line to transfer information. Two years ago V.34 modems were introduced that provided 28,800 bits per second (bps) with the possibility of compression to transfer rates in excess of 100,000 bits per second. According to the experts at that time, 28kbps was as fast as a modem could go on an analog line. The last year has seen a whole new generation of modems running at 56kbps and the modem market got another boost. Designed specifically for connecting from home to the Internet or an on-line service, the 56k modems employ digital techniques to squeeze more from a transmission. (There is a difference between "bits per second" and "baud" at speeds higher than 9,600bps, but most users employ the terms interchangeably. The proper term to be used with high-speed modems is bits per second, as it is a more accurate measurement.)

It is true that V.34 is probably as fast as an analog system can transfer information. These types of modems are analog at both ends of a connection. However, the popularity of the Internet and on-line services like CompuServe and America OnLine has changed the market as these systems don’t use analog technology at their end. Instead, they are almost always digital today. For this reason, 56k technology couldn’t have happened a few years ago. The system depends on the digital connections used by most Internet and on-line service providers, and the digital system used in local switching offices. While we often think of our telephone systems as analog, for most locations the only analog portion of the connection is from the telephone jack to the switching office. Everything else is usually digital, at least to companies that offer modem access.

The 56k revolution was started by a new chip developed at Lucent Technologies and Rockwell Semiconductor. They realized the difference between analog-to-analog and analog-to-digital connections is important. In a purely analog modem-to-modem connection (such as a V.34 modem), there are two converters in the telephone company’s circuit. At one end, the home telephone line goes to a switching office where an analog-to-digital converter converts the analog phone tones from the modem to digital signals. At the recipient end’s switching office, a digital-to-analog converter reconverts the signal to analog tones for the recipient modem. With two converters in the line, the line quality (signal to noise ratio) suffers due to quantization noise at each converter. The quantization noise problem limits the modem speed to about V.34 speeds through a formula known as Shannon’s law.

56k modems remove the quantization noise problem by generating digital signals instead of analog tones, so that the converters in the switching office are bypassed or only a single analog-to-digital converter is used, and less noise is introduced. With the lack of noise, higher transfer speeds can be supported. The sole limitation with the 56k modem system is that both ends of the line have to be able to handle digital signals without conversion to analog. You could not use a 56k modem to connect to an older V.34 modem and expect to retain the higher speed, for example.

The 56k systems go even further, in that they are asymmetrical. They are designed to give priority to downloads (server to client) rather than equal priority to transfers both ways. This helps the user, as most clients are requesting information from a server and they get faster performance through the asymmetrical service. The basic design means that transfers from the server to the client run at the theoretical maximum of 56kbps, but transfers from the client to server run at a slower speed (such as 33.6kbps). These rates can be adjusted on the fly by the modems, due to the asymmetrical design.

To take advantage of 56k modem technology a user needs to purchase a new modem or upgrade an older model to provide the new digital signals. At the other end of the connection, the Internet provider or on-line service needs to provide 56k-compatible modems, too. Often, the server end (the Internet or on-line service provider) must install a whole new bank of modems to support 56k, as they are digital units and not compatible with the older analog modems many services still employ. This can be an expensive proposition for some services, but consumer demand will often drive the upgrade if enough 56k modems are bought by the public.

The 56kbps modem market is not without problems, though. There are two distinct types of 56kbps modems on the market, and they are not compatible with each other. The major modem manufacturers have fallen into two different camps and have caused considerable confusion for users. Hayes, often thought of as the grand-father of consumer modems, and Multitech, another popular modem manufacturer, both have adopted the K56flex technology developed by Lucent and Rockwell. A large number of other modem vendors support the K56flex system as well. USRobotics, one of the hottest modem vendors of the last couple of years, developed their own 56k technology called x2 and have been pushing it. The two are completely incompatible. This is causing a lot of problems for service providers who need to buy two different banks of 56k server modems, one for K56flex and another for x2. The consumer market doesn’t seem to be differentiating between the two types, with both selling relatively well.

The press releases and mud is flying between the proponents of each 56k camp. USRobotics believes their x2 technology to be superior to the K56flex system, but K56flex has the widest number of adoptees (as well as likely standards support from the ITU). Both internal and external versions of the 56k modems are now on the market, and Hayes was the first to introduce a PC Card version of their K56flex system.

To their credit, most modem manufacturers have developed upgrades to existing modem models that are relatively inexpensive. A typical late-model V.34 modem can be upgraded by either a software upload or a new hardware chip for $60 to $100 depending on the manufacturer and model. Those late-model modems with flash PROMs are the easiest to upgrade, and can be done over the Web in some cases. Some manufacturers (including Hayes) have an interesting policy: they will upgrade any modem, regardless of manufacturer and speed. This is not really an upgrade but a trade-in policy, where they accept old modems as a credit against new 56k modems.

Testing, Testing…

If you believe the manufacturers, the new generation of modems provide not only 56kbps, but with compression techniques such as V.42 bis (a fairly common compression technique) speeds to 115.2kbps can be reached. This is the same as a bonded ISDN connection over an analog line! While these numbers may be theoretically possible, the practical speeds are much slower. In a series of tests using a private digital switching office, we tested V.34 analog modems and both K56flex and x2 modems to observe actual throughput. The test involved connecting through a digital switch to a modem bank attached to a UNIX-based World Wide Web server, and using a pre-programmed script on a Windows 95 client to send and receive download requests.

The V.34 modems performed as you might expect, with a peak throughput of 28.1kbps and an average throughput of about 26kbps (not far off their limit of 28.8kbps). The USRobotics x2 modems provided a peak throughput of 51kbps and an average throughput of 41kbps. The Hayes K56flex modems provided a peak throughput of 52kbps and an average throughput of 42kbps. The difference in performance between x2 and K56flex, at least in our tests, is not significant. Connecting 56k modems to each other over a typical telephone system slows the transfer rates down considerably. We connected through a telephone line to our local switching office, then back to a second analog number (which puts the two digital converters in-line) and got an average transfer rate of 28kbps, not much different than standard V.34 modems.

It is worth noting that the average throughput of both types of 56k modems is much lower than the advertised 56kbps. The numbers can be expected to be even lower when connecting over a distance to an Internet or on-line service provider (our tests were conducted in an isolated office network). On the other hand, 40kbps is a heck of a lot faster than 26kbps, especially when downloading files and Web material. It’s a noticeable improvement, and while not as good as ISDN is much better than the older V.34 systems offer.


The shelves at local mass marketers are already showing a variety of 56k modems. Users are asking VARs about the technology and whether it’s worth the upgrade price. What do you tell them? The answer is actually quite simple. Since the number of Canadian on-line service and Internet providers that currently support 56k modems is extremely small, users should wait until their favorite service offers support. There is no advantage to buying 56k modems for regular analog calls.

When the users’ Internet providers announce upgrades, then a modem that is compatible with the upgraded modems can be purchased. On-line services like CompuServe and America OnLine have both announced plans to roll out 56k modems in the future, but both say Canadian hubs will not be in the first upgrade phases. Internet providers are a little different, in that some of the smaller and more flexible ISPs have already started purchasing 56k modems. Larger ISPs like UUNET have adopted a policy of gauging demand before they upgrade.

Since ISPs and on-line services will end up having to install both K56flex and x2 modems to satisfy all users, the customer gets to choose their technology. Which is better? The industry momentum is likely going to be behind K56flex, but there is little difference in real performance terms between K56flex and x2. While this may not shape up to be another Beta vs VHS format fight, it is unlikely that both 56k technologies will survive in the long run. Since the average modem life is only a couple of years, this may not be an issue for most users. Short advice: invest in whichever is the best buy when the services support them.

Modem standards and speeds

K56flex and x2 57,600bps maximum, not standardized

V.FC (V.Fast Class) 28,800bps

V.34 / V.Fast) 28,800bps

V.32terbo 19,200bps

V.32bis 14,400bps

V.32 9,600bps

V.22bis 2,400bps

212A 1,200bps

V.21 300bps

103 300bps

Data Compression Systems:

V.42bis 4:1 compression to 115,200bps on a 28,800bps line

MNP5 2:1 compression algorithm used in older modems