56Kbps Modems

At one time, the V.34 annex speed of 33,600bps (33.6Kbps) was regarded as the absolute speed limit for asynchronous modem usage. However, starting in 1996, modem manufacturers began to produce modems that support speeds of up to 56,000bps. These so-called "56K" or "56Kbps" modems are now universal, although the methods for breaking the 33.6Kbps barrier have changed several times.

To understand how this additional speed was achieved, you must consider the basic principle of modem technology—that is, the digital-to-analog conversion. As you've learned, a traditional modem converts data from digital to analog form so it can travel over the Public Switched Telephone Network (PSTN). At the destination system, another modem converts the analog data back to its digital form.

This conversion from digital to analog and back causes some speed loss. Even though the phone line is physically capable of carrying data at 56Kbps or more, the effective maximum speed because of the conversions is about 33.6Kbps. An AT&T engineer named Claude Shannon came up with a law (Shannon's Law) stating that the maximum possible error-free data communications rate over an all-analog PSTN is approximately 35Kbps, depending on the noise present.

However, because many parts of the United States's urban telephone system is digital—being converted to analog only when signals reach the telephone company's central office (or central switch)—it's possible to "break" Shannon's Law and achieve faster download rates. You can, in some cases, omit the initial digital-to-analog conversion and send a purely digital signal over the PSTN to the recipient's CO.

Thus, only one digital-to-analog conversion is necessary, instead of two or more. The result is that you theoretically can increase the speed of the data transmission, in one direction only, beyond the 35Kbps specified by Shannon's Law—to nearly the 56Kbps speed supported by the telephone network.

Prior to the new ITU V.92 standard, the transmission in the other direction was still limited to the V.34 annex maximum of 33.6Kbps. However, both the modem and the ISP must have support for the ITU V.92 standard to overcome this limitation for uploading speeds.

56Kbps Limitations

Thus, 56Kbps modems can increase data transfer speeds beyond the limits of V.34 modems, but they are subject to certain limitations. Unlike standard modem technologies, you can't buy two 56Kbps modems, install them on two computers, and achieve 56Kbps speeds. One side of the connection must use a special digital modem that connects directly to the PSTN without a digital-to-analog conversion.

Therefore 56Kbps modems can be used at maximum speeds only to connect to ISPs or other hosting services that have invested in the necessary infrastructure to support the connection. Because the ISP has the digital connection to the PSTN, its downstream transmissions to your computer are accelerated. If both sides of the connection support standards pre-dating V.92, your communications back to the ISP are not accelerated.

On a practical level, this means you can surf the Web and download files more quickly, but if you host a Web server on your PC, your users will realize no speed gain because the upstream traffic is not accelerated unless you and your ISP both use V.92-compliant modems.

If you connect to another regular modem, your connection is made at standard V.34 annex rates (33.6Kbps or less). Also, only one digital-to-analog conversion can be in the downstream connection from the ISP to your computer. This is dictated by the nature of the physical connection to your local telephone carrier.

If additional conversions are involved in your connection, 56Kbps technology will not work for you; 33.6Kbps will be your maximum possible speed. With the way the telephone system has had to grow to accommodate new exchanges and devices, even neighbors down the street from each other might have different results when using a 56Kbps modem.

Early 56Kbps Standards

To achieve a high-speed connection, both modems and your ISP (or other hosting service to which you connect) must support the same 56Kbps technology. The first 56Kbps chipsets were introduced in late 1996:

  • U.S. Robotics's x2 used Texas Instruments (TI) chipsets.

  • Rockwell's K56flex was supported by Zoom and other modem makers.

These rival methods for achieving performance up to 56Kbps were incompatible with each other and were replaced in 1998 by the ITU's V.90 standard.

Unfortunately, the 56Kbps name is rather misleading, in regards to actual transmission speeds. Although all 56Kbps modems theoretically are capable of this performance on top-quality telephone lines, the power requirements for telephone lines specified in the FCC's Part 68 regulation limit the top speed of these modems to 53Kbps. The FCC has been considering lifting this speed limitation since the fall of 1998, but it still applies to modems as of early 2003.


V.90 was introduced on February 5, 1998, and was ratified by the ITU-T on September 15, 1998. Its ratification ended the K56flex/x2 standards "war": Shortly thereafter, most modem manufacturers announced upgrade options for users of x2 and K56flex modems to enable these products to become V.90 compliant.

Some modem vendors offer upgrades for K56flex and x2 modems to the V.90 standard. If you purchased your modem before the V.90 standard became official, see your modem vendor's Web site for information about upgrading to V.90.

ITU V.92 and V.44

56Kbps protocols, such as the early proprietary x2 and K56flex and the ITU V.90 standard, increased the download speed from its previous maximum of 33.6Kbps to 56Kbps. However, upload speeds, which affect how quickly you can send email, page requests, and file transfers, were not affected by the development of 56Kbps technologies.

Upload speeds with any of these 56Kbps technologies are limited to a maximum of 33.6Kbps. This causes severe speed lags for both pure dial-up users and those who depend on analog modems for upstream traffic, such as users of one-way broadband solutions—for example, one-way (Telco Return) cable modems, One-way DirecWAY, and one-way (Telco Return) fixed-base wireless Internet services.

Other shortcomings of existing 56Kbps technology include the amount of time it takes the user's modem to negotiate its connection with the remote modem and the lack of uniform support for call-waiting features. In mid-2000, the ITU unveiled a multifaceted solution to the problem of slow connections and uploads: the V.92 and V.44 protocols (V.92 was previously referred to as V.90 Plus).

V.92, as the name implies, is a successor to the V.90 protocol, and any modem that supports V.92 also supports V.90. V.92 doesn't increase the download speed beyond the 56Kbps barrier, but offers these major features:

  • QuickConnect. QuickConnect cuts the amount of time needed to make a connection by storing telephone line characteristics and using the stored information whenever the same phone line is used again.

  • Modem-on-Hold. The Modem-on-Hold feature allows the user to pick up incoming calls and talk for a longer amount of time than the few seconds allowed by current proprietary call-waiting modems.

  • PCM Upstream. PCM Upstream breaks the 33.6Kbps upload barrier, boosting upload speed to a maximum of 48Kbps. Unfortunately, because of power issues, enabling PCM Upstream can reduce your downstream (download) speed by 1.3Kbps–2.7Kbps or more.