To channel or not to channel?

Voice over DSL has had a rough start. Most involved with the technology blame scalability issues. Now a new approach to carry voice over broadband is in development. It may correct some issues, but it also may cause confusion in the market.

While current VoDSL technologies developed by vendors such as CopperCom, Jetstream Communications and Tollbridge Technologies use an “unchannelized” technology to carry voice traffic, some now are promoting a “channelized” approach to deliver voice traffic. The channelized method transports voice traffic in the physical layer. That way, voice can be carried without the use of higher layer protocols, which also is intended to lower the latency on the access network to the extent that echo cancellation isn't necessary.

Voice traffic can be transported as pulse code modulation (PCM) data from the customer premises through the access network and all the way to the switch, according to Peter LeBlanc, vice president of marketing for Aware, which is developing the technology along with vendors such as ADC and AFC.

“This is a very simple implementation of voice,” LeBlanc said.

Using the channelized method, providers ride on the existing public network and the traffic is packetized into either ATM or IP when it reaches a DSL access multiplexer (DSLAM). As a result, providers don't need to perform ATM backhauling in between co-location sites and switches (see figure).

With unchannelized VoDSL, an integrated access device (IAD) translates analog voice signals into ATM cells or IP packets. That data goes through a DSLAM and over an ATM network to a voice gateway. The gateway then converts the cells or packets to PCM data that the circuit switch can handle.

But a lot of those steps can be avoided, according to LeBlanc.

“The voice-over-ATM guys transport voice in the ATM band,” LeBlanc said. “You need an ATM gateway to do that while you don't with channelized VoDSL.”

Considering VoDSL currently is targeted to small businesses with eight to 12 voice lines, residential users are left out, LeBlanc said. That is largely due to the lack of toll quality and price points that are too high for consumers, he said.

“Channelized is much more cost-efficient,” LeBlanc said.

But others caution the channelized method is really a step backward.

“Channelized [VoDSL] is like the old way of doing voice over T-1,” said Jennifer Stagnaro, CopperCom's chief marketing officer. “They are really just trying to repackage something that has been around forever. You have to dedicate a whole channel. [Unchannelized] VoDSL is dynamic so you aren't nailing up the bandwidth.”

Others argue that the channelized approach is so far behind in terms of deployment that it is really not a factor.

Unchannelized VoDSL has been tested for three years and works with existing DSLAMs and IADs, whereas the channelized approach is far from that stage, according to Steve Gleave, vice president of marketing for Jetstream. “Additionally, they allocate bandwidth for voice regardless of whether there is voice or not,” he said, noting that that isn't very efficient.

So far, the technology has a lot of ground to cover within standards bodies. Vendors are working to present specification recommendations to the DSL Forum and guidelines for the International Telecommunication Union.