Although it hardly seems possible, V.34 modem speeds - 28.8Kbps and, latterly, 33.6Kbps - have been commonplace in the comms industry only for 18 months or so. Just a few years ago, the highest speed modem users could eke out of the PSTN was 14.4Kbps. That was no doubt why BT priced its ISDN-2 service, which offers speeds of 64Kbps, at just under #400 for installation and roughly as much again for an annual subscription.
Despite the fact that these rates are almost eight times what state monopoly Deutsche Telekom charges its ISDN subscribers in Germany, BT has persisted in offering ISDN as a value-added telecoms solution to its customers.
Some experts have suggested Deutsche Telekom and, to a large extent, France Telecom, have been able to offer ISDN at cut-price rates because of their virtual monopoly on inland calls in their respective countries.
But even BT's senior ISDN management dismisses such arguments, claiming that ISDN has required a significant investment in bandwidth on the UK network.
Even this notion is patent nonsense, as most telephone lines no longer connect to the phone exchange. In fact, they terminate at a device known as a line concentrator just a few hundred yards from offices and houses in city areas, and in clusters in the countryside.
These devices concentrate the local loop voice signals on to one or more 2Mbps data channels feeding into the telephone switch. The concentrators also transfer the inbound analogue audio channel into a digital data feed, using a codec.
Codecs can also be found in digital mobile phones. More powerful than a 100MHz Pentium processor, they cost about #100 each. But their functionality outweighs their cost, and BT has splashed out on installing digital concentrators in most urban and city networks across the UK.
Concentrators also allow BT to service a cluster of, say, 200 phone lines with about 40 call channels into the exchange. The savings on local loop connections offset the cost of the concentrators and their associated codec technology.
BT would like to harness its powerful multimegabit network by broadcasting digital multichannel TV signals. As Sky's progressive dominance of the UK TV industry shows, broadcasting is where the money is. But the government won't allow BT to start broadcasting down its digital network, for fear that the company might go even further than Rupert Murdoch has in broadcasting.
So BT has been exploring new technologies, one of which is asymmetric digital subscriber line (ADSL).
ADSL allows data to be moved in one direction at up to 7Mbps, and 640Kbps in the reverse direction. The data flow can be reversed several times a second, which means that ADSL's primary data flow can take place in the direction of the main data stream from the subscriber or the host system to which they are connected.
ADSL, like ISDN, has the great advantage of being compatible with the copper twisted-pair cabling that almost all phone subscribers rely on for the final leg of their links from the switch (or concentrator) to their BT or cable phone socket on the wall. For a payment of about #400, providing the telecoms network supports ADSL, a phone subscriber can plug an ADSL device straight into the phone line.
ADSL has caught on big time among the US telecos. In return for roughly the same investment as ISDN, telcos can install ADSL modems instead. Seven megabits is a lot of bandwidth. Put simply, the networking bandwidth is in the same realm as ATM except, of course, it is available right to the home or office.
Although BT is forbidden to broadcast in the current regulatory environment, what would happen if it narrowcasts signals to subscribers? Narrow-casting involves feeding a unique TV signal to one subscriber and ADSL is the ideal environment for this - hence the phenomenal worldwide interest in the technology.
BT is pilot testing ADSL on its UK network in a selected number of cities.
The trials have proved that the technology works well. BT remains tight-lipped on the subject of its ADSL plans for fear of alerting the cable TV/phone companies to its actions, but sources suggest that it will embrace ADSL as a narrowcast and datacoms medium par excellence over the next six to 12 months.
ADSL offers almost unbelievable benefits to the network user. Seven megabits will support a full bandwidth company Lan, although few organisations could generate Wan traffic at that bandwidth. It also offers the ability to multiplex several dozen voice channels together, as well as a video conferencing link or two. In short, ADSL is a networking nirvana.
BT is not unaware of this, but its primary focus is on narrowcasting.
Once the network is installed, few doubt that BT will capitalise on its broadband links into the home or office. It has a near monopoly in terms of the local loop outside those areas serviced by cable TV and phone companies, and would have to be extremely stupid not to capitalise on its resources.
In the US, ADSL is now being rolled out as a cable modem technology to tens of thousands of homes and offices. It will not be long before the user community realises ADSL's potential, and then things will really start to motor.
ADSL has also started to attract interest from other kinds of companies in the US. Early last month, Ameritech and IBM announced they were teaming up in the Chicago area to test ADSL links to homes and offices. The trial will involve some 200 customers of both firms, concentrated in a part of the Chicago area which has yet to be chosen. IBM says the plan is to offer channels of 1.5Mbps each to subscribers to allows them to access the Internet.
By limiting the data speeds, IBM claims that standard voice calls can also be carried on the same circuit, at the same time as the data channel.
Ameritech says it is using prototype ADSL modems from several manufacturers in the Chicago trials. The price of these modems is about $1,500, although officials say that the production price will fall to about $700.
Other trials announced so far include one launched in February by GTE in Texas; a joint effort by GTE, Microsoft, and the University of Washington announced in August; and a trial by Bell Canada which started in two Canadian communities in September.
As part of the Chicago trials, IBM will provide Internet and intranet services, as well as various network and datacoms resources through its IBM Global Network operation.
Although it's still early days in the Chicago trial, IBM says it plans to use the trial as a means of showing what ADSL has to offer the home and business user, and, of course, that its existing network infrastructure can handle the high data speeds involved.
The GTE/Microsoft trial in Redmond, Washington, meanwhile, started in August of this year and will run until February 1997. The Redmond ADSL trials held in conjunction with the University of Washington centre on 40 Microsoft and GTE employees using ADSL technology to test high-speed access to the Internet and private data networks using Microsoft Windows NT-based servers and GTE's first generation of ADSL modems.
Phase two of the test will increase ADSL coverage to more Microsoft and GTE employees, along with the University of Washington and up to 60 businesses in the Redmond area. This phase, which starts at the end of this year, will include different types of ADSL modems, and will also incorporate other Microsoft products like email, news services, conferencing and electronic commerce.
GTE claims that, as a result of the trials, ADSL will be adopted on a wide scale across several regions in the US. As a result, GTE predicts that an ADSL modem will fall in price to about $250 by the end of 1997.
So where does ATM fit in with ADSL? As far back as April 1994, Newbridge Networks announced that it was developing a hybrid version of ATM which would work over ADSL modems. Back then, of course, ADSL modems cost well into five figures and were only found in the laboratory. Two-and-a-half years down the road, ADSL is being used as a communications medium in the real world, and several vendors of ADSL technology already support ATM as an option on their units.
But it's not all wine and roses for ADSL, as several networking companies have discovered that ADSL can't be provisioned on copper circuits equipped with load coils or bridge taps. Furthermore, copper cables more than 400 yards long cannot support the full 7Mbps data bandwidth. This is one of the reasons why several of the US telcos have limited their data speeds across ADSL links to 1.5Mbps, since cables of up to four miles long can easily accommodate such bandwidth.
ADSL is very distance-sensitive. With a copper loop of about 12,000ft, ADSL claims to support data streaming at up to 7Mbps. But at more than 18,000ft, even 1.5Mbps ADSL signals can break down.
According to Ameritech, despite the fact that at least 80 per cent of businesses and residences in highly populated areas of the US fall within these thresholds, much of the wiring is old, cracked and wet. While the copper local loop was designed to support an audio bandwidth of about 6KHz, ADSL makes use of the full radio spectrum up to 1.1MHz.
One investor, Bellcore, claims that ADSL has been designed to accommodate difficult telecoms media, stepping down automatically to slower speeds until the signal gets through. David Waring, director of broadband local access and premises networks with Bellcore's New Jersey operation, says line quality should not be a significant problem if the telcos limit ADSL usage to a maximum cable run of 18,000ft - significantly greater than most runs BT employs on its UK network.
A possible solution for far-flung telephone users is for the telco to install a remote device known as a digital loop carrier (DLC). Copper lines can connect to the DLC, which in turn can connect to a switch via a digital circuit. As long as the subscriber is within 18,000ft of the DLC, even the most remote of country satellite exchange links can support ADSL to at least 1.5Mbps.
Perhaps more importantly, the ADSL modem manufacturers are already wise to the problem of distance limitations and are already working on a second generation of ADSL modems known as rate-adaptive ADSL, or RADSL. These devices, which are expected to ship in the second quarter of 1997, automatically check what sort of ADSL speeds a copper cable can support and negotiate a link accordingly.
According to Bellcore, RADSL modems can support step-down speeds as low as 640Kbps in both directions at distances of up to 30,000ft - way above the normal limits for standard telecoms operation - yet still supporting data rates 10 times that of ISDN.
Against this backdrop in the US, BT may face something of an uphill struggle to continue marketing ISDN at its current price much beyond the end of 1997. But before then, networking resellers can expect some sexy new ADSL product launches from the computing and networking majors, which are also working feverishly behind the scenes on testing the new technology.
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