March 28, 1997

How modems achieve 56K-bps throughput
By Michael Caton, PC Week Labs

  With x2 and K56Flex technologies both vying to become the next high-speed modem standard, buyers are befuddled about which camp to support now that K56Flex backer 3Com Corp. has acquired x2 developer U.S. Robotics Corp.

While the standards issue is moving toward resolution, both groups are facing potential changes to their technologies based on the International Telecommunications Union's preliminary discussions. Furthermore, product delays are hampering deployment of K56Flex technology.

The x2 and K56Flex modem technologies take advantage of the fact that the telecommunications infrastructure is now primarily a digital network--it's no longer dominated by analog phone lines and their 35K-bps transfer speed limitation.

In a V.32 modem connection over an analog line, the modems at both ends perform digital-to-analog and analog-to-digital conversions to travel between PCs over the PSTN (Public Switched Telephone Network). The conversion alters the signal, creating a reconstructed or corrupted signal different from the original. This difference, called quantization noise, limits transfer rates to 35K bps.

Reduced noise, increased speed

However, the quantization noise only occurs during an analog-to-digital conversion, so eliminating this conversion reduces line noise and increases speed. This is the premise behind the new 56K-bps technologies.

The PSTN is primarily digital, but both x2 and K56Flex assume that there will be a lone analog segment to handle--the local loop from a residence to the PSTN central office.

A server at an Internet service provider or corporation must connect to the PSTN using a digital switch linked to a digital line such as a T-1.

Using the digital switch and modem, data moving downstream from a server to a client can theoretically reach transmission speeds up to 64K bps at 8 bits per sample and 8,000 samples per second. However, a number of problems still remain in the system, such as "robbed bit signaling," which uses some of the bits per sample to indicate the status of a call rather than to transmit data, reducing the effective data rate to 56K bps. Federal Communications Commission restrictions have further limited the data rate to 53K bps.

But in the client-to-server direction of the connection, upstream data rates are limited to V.32 speeds (or less than 35K bps) because the modems perform analog-to-digital conversion that introduces line noise. The poor performance of the upstream connection was evident in PC Week Labs' tests of two U.S. Robotics x2 modems (see "56K modems: Buyer beware"). K56Flex vendors anticipate improving the upstream connection, a necessity for the technology to be used in applications such as videoconferencing that require symmetrical data rates.

PBXes also serve as another potential barrier to high-speed data rates. Some PBXes perform an analog-to-digital conversion before data reaches the central office, interfering with the signal enough that a 56K-bps modem bumps its transmission speed down to V.32 speeds.

U.S. Robotics and K56Flex vendor Lucent Technologies Inc. are combating this by selling the software upgrade feature in their modems as a form of buyer protection. However, there are no guarantees, and vendors may not be able to deliver a standards-compatible upgrade as quickly as users may need.

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