December 14, 2000, 1:04 PM — Telecom market analysts predict that service providers will soon bundle Voice over
DSL (VoDSL) with their small business DSL Internet access packages. VoDSL is a hybrid
service designed first and foremost to reduce local access costs.
I have yet to purchase a VoDSL interface or speak with anyone who has one on-site,
but by year's end the situation could be quite different. href="http://www.coppercom.com">CopperCom, href="http://www.tollbridgetech.com/">TollBridge Technologies and others offer
integrated access devices (IAD) that seamlessly connect the small office and
telecommuters' voice and data traffic directly to central office switches and wide area
data networks, respectively.
Sharing the wire
Data and voice over the same copper -- it sounds familiar. But VoIP and VoDSL are
different animals, designed to address different (though complementary) business needs.
With VoIP, analog voice is digitized and packetized at the gateway or in the handset,
and the voice-bearing packets pass through IP routers and switches to the VoIP gateway
nearest their called party's location. Today's VoIP solution providers have built and
are commercializing specialized IP infrastructure for low-latency end-to-end
conditions. Due to changes in standards incorporated in voice/data gateways, including
signaling protocols (e.g., H.323 or href="http://www.cs.columbia.edu/~hgs/sip/sip.html">SIP) and compression
algorithms, VoIP service providers' infrastructures have needed a lot of upgrades,
interoperability testing, and management. Such value-added services as call recording,
multipoint conferencing, call transfer, and caller ID are not on the current services
menu. Nevertheless, offering simple point-to-point VoIP service across countries -- and
particularly internationally -- is so much more economical than using circuit switches
that the price per minute of VoIP is often one half or one quarter the cost of using a
traditional service provider's network.
In contrast with VoIP, voice over DSL is not packetized. It is first digitized, then
compressed by a standard adaptive differential pulse-code modulation (ADPCM) codec in
the IAD and sent as a 32 Kbps digital signal over copper to a voice switch. A customer
with a VoDSL-enabled handset (or an IAD and any analog handset) can get call waiting,
caller ID, and any other service provisioned in central office voice switches.