The Scoop: The Ins and Outs of Voice Over DSL

FRAMINGHAM (03/27/2000) - With voice-over-DSL technology, carriers can provide impressive multiple-channel voice and broadband data services running simultaneously on a regular phone line - but they can't do it with any old DSL.

Vendors that make voice-over-DSL gear rely on symmetric DSL (SDSL), a stable, well-understood flavor of DSL that doesn't interfere with other services running on nearby wires in carrier networks.

Voice-over-DSL vendors don't make gear that supports multiple lines of voice over asymmetric DSL (ADSL). So if the only flavor of DSL available in a particular area is ADSL, carriers that want to provide voice-over-DSL services are out of luck.

There are other issues as well. Competitive local exchange carriers (CLECs) rely on regional Bell operating companies to supply them with phone wires to customer sites. They also rely on RBOCs to share space in their switching offices - for a price - where the CLECs can install their own equipment to connect with customer lines.

If RBOCs put ADSL equipment in remote switching office terminals - small, typically green boxes you may have seen in residential neighborhoods - there is no room for any other kind of DSL gear. So CLECs that need to install their SDSL gear in those little boxes won't have any place to put their equipment and will be unable to sell an SDSL service.

Those issues aside, voice-over-DSL service requires a box at customer sites called an integrated access device (IAD) that has ports for data equipment as well as phones. IADs convert voice and data into ATM cells and send it over SDSL pipes. Vendors are making IADs that support up to 16 voice channels on a single DSL connection.

In the carrier network, the voice traffic is sorted out by a voice gateway that turns ATM voice packets into circuit-switched voice signals traditional phone switches understand.

Some experts predict that by using customer-site routers that combine voice and data onto DSL links, service providers could offer bargain prices by the end of the year - 30% to 40% off the combined phone and Internet access bills customers currently pay.

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