If Middlebury, Connecticut-based General Datacomm Industries is right then it’s time to abandon the wise old corporate practice of building wide area networks in a mesh configuration. In the run-up to the launch of its inverse multiplexor, the company is building a novel and persuasive sales pitch for backing up or replacing 2Mbps leased lines with primary rate ISDN connections. Inverse multiplexors take the 30Kbps, 64Kbps second channels in a primary rate ISDN connection and present them to end systems as a single 1.9Mbps pipe. General Datacomm’s new RAMP inverse multiplexor has been launched for just under UKP10,000 from the company’s Wokingham, Berkshire base in the UK, and it is unusual in that it uses the UK’s proprietary DASS2 signalling system. There are a number of similar systems about, however, and more interesting than the products are the tariff analyses that the company has put together which show just how competitive primary rate ISDN has become with the leased line service.
Couple of hours a day
The company is making two claims here, and the first is that a substantial proportion of customers for British Telecommunications Plc’s Megastream only use their circuits for a couple of hours a day. While this is debatable for the circuits that are used to connect PABXs, it is in data applications such as CAD/CAM that marketing director Simon Boyle reckons all high speed file transfers laid end-to-end last for next to no time. Couple that with the way that leased line costs increase steadily with distance while primary rate ISDN steps up to the maximum public switched telephone network long-distance rate and then stays constant, the financial incentives become substantial, so much so says Boyle, (almost rubbing his hands) that the equipment costs of inverse multiplexors become negligible. For those that are dubious that Megastreams are used this thinly (and British Telecom says that it does not keep Megastream usage statistics) there is the back-up argument. The mesh topology has become almost ubiquitous among big companies as they attempt to make their wide area networks robust, but Boyle argues that primary rate ISDN is in the position to assume the role of Megastream back-up in the same way that basic rate ISDN is becoming an understudy for Kilostream lines. The figures look incontrovertible and General Datacomm’s graphs suggest that as long as your Megastream doesn’t need to be backed up for more than around a quarter of an hour a day, 89 hours a year, then using ISDN and a star topology makes sense. And why hasn’t anyone done it before? Because, says Boyle, both UK carriers are still intent on marketing ISDN solely as a voice rather than as a data service. This is partly because the latter requires an inverse multiplexor, but there is the sneaking suspicion that British Telecom and Mercury Communications Ltd are keen to retain the valuable revenues coming from the leasing of their high speed digital lines.
