Voice-on-DSL Debate Warms5/16/1999 8:00 PM Eastern
Multiple-line, packet-voice service over DSL links has
emerged as a hot button in the small-business and home-office markets, driving the ADSL
Forum to begin an initiative aimed at creating a universal approach to accommodating
"We're seeing the industry go through a period of
rampant innovation as vendors, carriers and service providers try to take advantage of
this opportunity," said Bill Rodey, vice president of sales and marketing at Westell
Technologies Inc. and vice chairman and treasurer of the ADSL Forum.
"What we hope to do at the forum is to come up with
the road map and rules of the road that will support interoperability and massive scaling
of these services," Rodey added.
The economics of voice service delivered over the data
stream of the DSL (digital subscriber line) platform are proving to be one of the most
compelling aspects of DSL service, said Mike Borsetti, director of product management at
NorthPoint Communications Inc., a CLEC (competitive local-exchange carrier) devoted to
installing DSL facilities across the country.
There are about 8 million businesses in the United States
with fewer than 100 employees each that are spending a total of $520 per month on average
for phone connections, typically with four to 16 lines per business, Borsetti said.
"Using our tariff structure, you can deliver eight lines of service for $165 per
month," he noted.
The cost of customer-premises equipment accommodating the
aggregation and conversion of premises phone signals into the DSL modem adds another $28
or so per month, based on amortization of equipment costs over three years, Borsetti
The NorthPoint connection -- which is an SDSL (symmetric)
versus ADSL (asymmetric) link, typically operating at 1.5 megabits per second each way --
has enough bandwidth to support high-speed-data access along with the voice lines, further
adding to the cost advantages, he noted.
Networks like the ones NorthPoint is installing in 68
metropolitan service areas this year use leased copper loop from the incumbent carriers to
transport DSL signals from the premises to the CLEC's DSLAMs (DSL-access multiplexers),
which are installed in the incumbent's central offices.
All of these DSLAMs are linked into a single CLEC ATM
(asynchronous transfer mode) switch, where data signals are handed off to local service
Using the "TR303" digital-loop-carrier interface
developed by Bellcore (Bell Communications Research, now Telcordia Technologies Inc.), the
CLEC switch can also become an interface to the public switched telephone network,
allowing service providers to deliver all of the features available from Class 5 PSTN
switches to end-users, Borsetti said.
"With the introduction of this gateway, which we're
calling a virtual DLC, you can add phones on the CPE -- whether they're black phones, key
systems, PBXs [private-branch exchanges] or whatever -- and split the data from the voice
signals at the other end," he said.
But there's a major issue to be resolved before the
DSL-based telecommunications sector can proceed with a universal approach to accommodating
Brodey said the ADSL Forum is confident that the
voice-over-DSL requirements can be met using the Internet-protocol-telephony
infrastructure taking shape under the H.323 standards umbrella, but others aren't so sure.
"I have heretical news here," said Martin Taylor,
chief technology officer of Silicon Valley start-up CopperCom, in a talk at the recent
Voice on the Net conference in Las Vegas. "For now, voice over IP isn't ready for
CopperCom -- which developed the virtual DLC switch
NorthPoint is experimenting with -- found that service providers reported it would be far
better to do voice over DSL in the ATM format, which is the cell-based transport system
used in the vast majority of DSL applications.
"That conclusion was reached by us with much surprise,
but quite conclusively," Taylor said.
There's no knowing exactly what the technical consensus
will be until a special team within the ADSL Forum reports its findings at a forthcoming
meeting in Vienna, Austria.
At that point, the forum hopes to identify which key issues
remain to be worked out before a consensus approach to voice over DSL is finalized.
Meanwhile, carriers, vendors and service providers will
forge ahead with their own solutions in recognition that the market is too hot to be
ignored while standards are being formulated.