56-Kbps Modem Problems

Although Robert L. Hummel's conclusions were sound, his "How Fast Is a 56-Kbps Modem?" (June ) contained a number of problems and errors:

1. "Informal" testing (to use the term in the article) over a single phone line is not a fair general test of 56K technology, because impairments vary widely on different phone lines, and even on different connections from the same phone line, even when that line passes the (imperfect) x2 LineTest. Many people are seeing consistently high speeds (typically in the high 40s); others are not able to make 56K connections at all. Furthermore, K56flex modems might have produced different results.
2. x2 upload speeds are actually limited to 31.2 Kbps (not 33.6 Kbps) because the 3429 symbol rate is not used (currently, at least) on x2 connections.
3. x2 modems do not "drop down to 33.3 Kbps and then renegotiate back up"; they directly speed-shift up or down (in small steps) from the initial connect speed as line conditions warrant, just as V.34 modems do.

John Navas
Publisher of the Navas 28800-56K Modem FAQ
http://www.aimnet.com/~jnavas/modem/faq.html

I disagree that my informal testing was unfair to 56K technology. U.S. Robotics' documentation clearly implies that if my phone line passes the USR LineTest diagnostic, which it did, it will support x2. I conducted supplemental tests at several locations to incorporate both long and short local loops, different telco central offices, and a variety of long-distance carriers. I also surveyed other x2 users, who reported generally mediocre perfor mance. And, as the article states, K56flex modems were not yet available for testing.

Regarding x2's upload-speed limits, I was referring to the theoretical capability of the technology. The USRFAQ states that "x2 downloads (receives data) at up to 56 Kbps. It uploads (sends data) at up to 33.6 Kbps."

As for the issue of "dropping down," a USR product manager says we both make valid points: x2 servers initially connect at 33.3 Kbps and then, within a couple of seconds, establish the "initial connect speed"; it's part of the protocol negotiation. x2 clients report only this second, usually higher, speed. There's no dropping down from the initial connect speed to 33.3 Kbps. But the negotiation starts at 33.3 Kbps. --Robert L. Hummel