US Robotics, Motorola and Hayes have followed Boca Research by saying they will deliver products based on the proposed standard and throw their combined weight into making the speed an International Telecommunications Union (ITU) standard. However, experts familiar with the specification said the speeds will only be available where the destination end is digital. Rockwell said last week that it would show devices based on its chipsets working at the 56,000bps speed during November's Comdex trade show in Las Vegas. The first 56,000bps devices are expected to be available by the end of the year. On a normal analogue modem connection, both ends of the call send audio tones. These are converted to a digital equivalent by the exchanges, which then send the data to each other. These are then converted back to the audio tones that the modems understand. For a modem to be approved, it can only send tones of a limited amplitude and frequency range -- these limitations, plus filtering in the exchange, set an uppermost speed of around 33,000bps, the current limit of the most advanced modems. With ISDN, on the other hand, both ends are digital, there are no such limits and the data can be sent at the full speed of the telephone network -- 64,000bps. The new Rockwell standard is half digital. One end has to have an ISDN connection; the other must be analogue. Normally, under these conditions, the ISDN end sends digital data that simulates what would be sent if an analogue phone were present, thus keeping compatibility with the distant end. With the new standard, the ISDN end sends codes that are much larger than would be possible from an approved analogue modem; the exchange at the remote end converts these to analogue signals with a greater range than normal and the remote modem can thus extract extra information. Because there are still filters and some voice processing at the analogue end of the link, the full 64,000bps isn't obtainable but, in practice, 56,000bps is. However, the remote modem can only send back data that corresponds to the analogue approval limits; Rockwell claims that 28800bps is possible here, but that remains to be seen. Bill Pechey, chief engineering officer for Europe at Hayes, said that technology is ideal for connection to Internet Service Providers who have an incoming ISDN line, as most of the larger providers do. "Its a natural match for Internet downloads, delivering close to ISDN speeds for transmitted data, although there are no advantages for telephony or videoconferencing as its asymmetric," he said. "You'll do well to get 28,800bps at the send end. There are a lot of unknowns as yet. I wouldn't like to say you'll get 28,800bps everywhere so there may be modems that train to 19,200 or even slower speeds. Also, if there is any alteration of the digital signal between the exchanges -- as there normally is on international calls -- the system won't work, and both ends will drop down to V.34 speeds." Pechey said he didn't expect the faster modems to threaten burgeoning ISDN sales. "There's an overlap of applications there but [56,000bps modems] are not as good as ISDN, and until standardisation is agreed there'll be limits on its take-up. Also, unlike previous analogue modems, there will be no way to use the higher speeds on a point-to-point link: two people with the new modems will only be able to talk to each other at the same speed as V.34." Ascend Communications, a supplier of digital modems to most Internet ISPs, has said that it will support the Rockwell standard and expects to have product ready by the end of the year. Existing installations, as found in Demon and other UK ISPs, will be firmware upgradable -- no physical changes will have to be made before they can offer 56,000bps transmission across the ordinary telephone network.