The 56K Technology

The communication capabilities (speed) of a modem are limited by the computer’s ability to communicate to the modem, the relative line quality of the Public Switched Telephone Network (PSTN) and the capabilities of the modem. any one of these factors may cause the modem to drop below the 56K capability to a fallback V.34 bis mode.

There are two principle competing technologies for 56K modems, x2 by US Robotics and K56flex designed by a collaboration between Rockwell and Lucent Technologies. While similar in nature, both technologies remain incompatible pending ITU’s adoption of a 56K standard.

K56flex Modem Technology is supported by more than a 1,000 modem manufacturers, Internet Service Providers and computer manufacturers. The advantage the K56flex has over the x2 standard is this broad industry base of support and the technical fact K56flex can reach the highest sustainable Internet connection rates over existing Public Switched Telephone Network (PSTN). This slight speed advantage (about 3 K/s) is due to the close configuration of existing installed phone lines at the telephone companies and the limitation on power levels which can cause cross talk. Because of the different technology employed by K56flex, the 56K rate is obtainable under certain conditions where as the under current conditions, the x2 technology may only sustain 53K rat. This is not a limit of the x2 but a limit on power levels set by FCC regulation forcing most ISP’s and modems to a current maximum data transfer rate of 53K

The limitation on both 56K technologies, x2 and K56flex, is that the current technology only supports 56K one way only, 56K downstream from the ISP and v.34 (33.6K) upstream back to the ISP. By allowing faster downstream speeds from the Internet service provider (ISP) to the user, K56flex technology significantly speeds the delivery of information. Graphics-intensive web pages, audio and video, and files will download at near-ISDN speeds and at almost twice the rate of the prevailing 28.8K standard. K56flex technology operates over standard telephone lines there is no need for users to upgrade to ISDN. ISDN (Integrated Services Digital Network) requires additional upfront installation fees and requires by law a monthly rate for each of the two lines requires by law a monthly rate for each of the two lines required for ISDN to function. This doubles the Basic service rate for a connection. ISDN also requires the telephone companies to make changes in their equipment at the central office to support ISDN.

The K56flex technology provides the user with an economical way of higher speed access to ISP or on-line service providers (OSP) that have digitally terminated modems. A majority of such service providers have central site modems that are connected directly to the digital portion of the public switched telephone network (PSTN). This technology allows high-speed access to such service providers from regular analog telephone lines without any modifications to the PSTN.

The PSTN, upon which the K56flex technology operates, has been traditionally referred to as an analog phone network. However, over the past few decades, the PSTN has become almost entirely a digital network. In most cases, the only portion of the network that remains analog is the relatively short connection from a residence to the central office (the analog local loop)

The Limitations of Today's V.34 Modems

Today’s V.34 modems operate as though the entire network and both ends are analog, although they are not. Each time V.34 modems send signals to the PSTN and receive signals from the PSTN, an analog-to-digital and digital-to-analog conversion, respectively, occurs. The net result of these conversions is a corrupted or reconstructed signal that differs from the original. The difference between the original and the reconstructed signal is called quantization noise. It is this constant conversion, and ultimately the noise produced that restricts network speed to 35K bits/s.

Quantization noise affects only analog-digital conversion, not digital-to-analog. K56flex technology was designed around this premise of less conversions = less noise = faster speeds.

K56 Flex Solution

Lucent Technologies and Rockwell took a distinctly different approach to the PSTN, and K56flex solution was the result. Instead of viewing the PSTN as an analog network, K56flex technology sees it for what it truly is; a mostly digital network working at 64Kbits/s data rate. The key to the new higher-speed technologies is the elimination of the analog loop at the ISP. Since most ISPs are digitally terminated, transmissions don’t have to be converted back to analog. This eliminates one analog loop, lowers noise levels, and allows the higher transmission rate. The K56flex solution achieves fast downstream speeds because digital signals are sent and received with very little noise. The upstream direction remains slower because a conversion must still be made in the client end.

Public Switched Telephone Network (PSTN) Limitations

In view of the above discussions, it should not be assumed that it is possible to achieve the full 64 Kbits/s data rate available within the network. The following are several issues involved that prevent achieving this capacity.

Network-Imposed Limitations

Older signaling systems use the least significant bit of some of the voice samples in the network for other purposes. Furthermore, not all 8-bit code words are allowed on the network. consequently, the modem must be aware of these network limitations and provide compensation to achieve the highest data rate possible on a given connection.

Synchronization

Just like any digital communication system, the client modem at the analog end of the connection must be synchronized with the network D/A clock to be able to decode the PCM code words transmitted by the digital end. However, the network clock is not available on the analog side of the connection; hence, the modem must provide means to acquire the network clock.

Channel Bandwidth

Central office channel banks have filters that limit the channel bandwidth to 4kHz. Since this new technology uses digital signaling at 8K samples per second over the band-limited analog channel, significant inter symbol interference (ISI) is present in the received signals at both ends. The modem must provide synchronization to overcome this severe ISI.

Compatibility

The K56flex technology is compatible with previous communication standards like ITU V.34, which means that whenever high-speed communication is not possible, the connection can be established via V.34. For example, a fallback to V.34 can occur if either the client or server side do not support the K56flex communication protocol or if a combination of network and subscriber loop conditions prevent the utilization of the new modem.

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