Is the game up for ISDN?

DSL brings high-bandwidth capabilities to the existing copper wire telephone network. It provides a constant connection to the PSTN (public switched telephone network), giving 24-hour access to the Internet and to email, and allows the simultaneous use of the normal telephone line for voice traffic.

There are many flavours of DSL technology, but three stand out: ADSL (Asymmetric DSL), DSL Lite and HDSL (High bit-rate DSL). Of these, ADSL is currently the front-runner, being aimed at small and medium-sized business, teleworkers and the domestic user. It gives high-speed access to the Internet, as well as the company LAN or intranet.

Simon Brooks, marketing manager for BT's Interactive services network trial in West London, said: "ADSL gives people a step in between ISDN and a virtual private network. The large proportion of the ADSL target market would be those that are using BT Highway now."

With download speeds of up to 8Mbps, ADSL certainly seems attractive. But there are flaws. ADSL is not symmetric and so can only transmit data upstream at speeds of 640Kbps - still 10 times faster than a single ISDN connection.

However, ADSL cannot maintain bandwidth capacity over long distances, unlike ISDN, which uses line drivers to push packets along the wires in both directions. For example, ISDN can keep up its maximum speed of 128Kbps each way over three miles. ADSL, on the other hand, slows down to 16Kbps upstream and 2Mbps downstream over the same distance - although it is rare for the splitters in the local exchange to be that far away from the receivers in the users' premises.

ADSL is also more costly than ISDN for telcos to implement. Even though the line itself doesn't need upgrading, boxes need to be installed at the local exchange. In its trials, BT is charging between £250 and £420 per month per line.

However, Robin Duke-Woolley, principal analyst at Schema, says the price can be attractive, depending on how you look at it: "ADSL is essentially a leased line, so if a small business is using ISDN a lot, then it could be more cost effective to upgrade to ADSL."

ADSL will make its commercial entry into the UK market through Kingston Communications, which is expected to launch products next spring. Meanwhile, BT has begun trials in North London and is also expected to introduce commercial services next year.

One possible solution to the cost issue is DSL Lite. Although it is primarily being targeted at the consumer market to compete with cable modems, it acts as a plug-and-play version of the technology. A technician is not needed to install the line at the consumer end, which reduces the costs, but speed is sacrificed.

Finally, for corporates who need to send large packets of data upstream there is HDSL. This sends data at up to 2Mbps in either direction. However, this technology currently has to run over two twisted pairs, which in many cases will ramp up the price. Indeed, businesses with bandwidth demands that high would be better off looking at the fibre option.

ADSL will be the first of the DSL technologies being offered to businesses. For companies wanting to provide email and Internet access to their employees, it will more than match their needs - even for those in the media industry who have large upstream data requirements.

However, the telcos, wary of cannibalising their existing ISDN and dial-up revenues, are dragging their feet. So it won't be until the end of next year at the earliest before businesses should consider DSL as a serious option.

So is ISDN about to become yesterday's technology? The answer is a tentative 'yes' - but not just yet.