How 56k modems [don't] work

How 56k Modems Work (and don't work) (Updated 1-May-99)

Today's global telephone network is largely digital. Typically, the analog portion is between your phone jack and the telephone company's central office (CO). At the CO, your line is separated into a "send" and "receive" signal, and converted from analog to digital. The digital signal representing what the other end is sending is simultaneously converted from digital to analog to your 2-wire line. The digital signal representing a voice (or modem) call is encoded to a 64kbps data stream. With 56k modems, the server modem is totally digital, and puts out a 64kbps digital datastream to the channel assigned to your call. A 56k modem then samples the analog signal synchronized to the 64kbps data rate of the telephone network. If all 8000 samples per second (8000x8bits=64kbps) could be accurately converted, your modem would receive data at 64kbps.

With today's telephone network and modem technology, 56kbps is the highest rate that can be recovered from the 64kbps signal. If things were this simple, there would be no problems. The problem is that equipment used in the telephone network intentionally alters the 64kbps datastream. Presently, both x2 and Flex technology are based upon the detection of, and compensation for, the changes to the datastream introduced by the telephone network. When a call is routed over facilities for which the modem has proper programming (firmware), 56k works. When the call is routed over facilities for which the modem doesn't have proper firmware, 56k can fail miserably - from not connecting at all, to connecting at a "false" high speed which drops drastically, to connecting at a speed that doesn't matter, because it has improperly identified the impairments and will encounter such a high error rate as to make the connection unusable.

These impairments include PADs and robbed-bit-signaling. In addition, telephone company mis-configuration or marginal digital trunks can introduce both intermittent or constant impairments no modem can be programmed for.

This situation makes present 56k modems unreliable for a certain sub-group of owners. Since conditions on your telephone facilities are subject to change, the owners experiencing 56k-killing impairments is in constant flux.

When I originally wrote this page, the promised solution was detailed by John Powell, a 3Com x2/V.90 engineer in his message to me after he viewed my website: an on-the-fly "total kill" of impairments during the modem's initial handshaking. The promise has been elusive: while the digital impairment learning sequence introduced with the V.90 handshake has helped, there hasn't been a "total kill" - it appears that certain impairments - particularly unusual RBS links - are not detected and compensated for in any vendor's V.90 implementation.