Voice Over DSL: The Next Generation Business Phone Line

by Brian McConnell

Imagine being able to order a single circuit which provides your office with multiple voice phone lines plus high-speed access to the internet. Such a beast will be coming to your market sooner than you might think, thanks to Digital Subscriber Line (DSL) technology.

DSL is currently being promoted as the next big thing for high-speed internet service. What few people know is that DSL is also going to revolutionize how basic telephone service (dialtone) is delivered to both home and business users. The reason is simple, using DSL you can deliver many simultaneous phone calls over a single physical circuit.

Voice over DSL technology provides phone companies with a cost-effective way to deliver more service over existing facilities. Rather than provisioning individual circuits for each phone line, many calls can be multiplexed onto a single DSL line, which doubles as a high-speed data circuit for internet access.

DSL Line SpeedCalls without CompressionCalls with ADPCM CompressionCalls with Strong Compression
384 Kbps 4-6 8-12 up to 40
768 Kbps 8-12 16-24 up to 80
1100 Kbps 16-18 32-36 up to 110
1500 Kbps up to 24 up to 48 up to 150

Table 1 - Call carrying capacity of DSL lines based on speed and type of compression used.

The table above illustrates how effective voice over DSL can be in conserving the use of copper pairs (circuits). A single 384 Kbps DSL line can deliver up to 12 simultaneous calls while also providing high-speed data service without compromising audio quality. (ADPCM compression doubles the capacity of a circuit without noticeably degrading audio quality.) This is more than adequate for a typical small office (up to 20 or 30 people). By upgrading the speed of the DSL line, or by installing additional DSL lines, the user can easily add additional data or voice capacity.

Voice over DSL: How does it work?
Voice over DSL is a "back room" technology, meaning it will be largely invisible to its users. The telephone carriers will provide end users with a black box which converts one or more DSL lines into 10BaseT Ethernet (data) and Plain Old Telephone Service (POTS) (analog voice) ports. Therefore, your existing office equipment (LAN/Ethernet hubs, phone system, etc.) can be easily connected to the service.

Voice over DSL equipment combines voice and data traffic in a manner similar to multiplexers for T1 and Frame Relay circuits. Voice calls are digitized and compressed. This information is interleaved with ordinary data traffic. The reverse process is carried out on the other end of the circuit. The difference between voice over DSL and other technologies is cost. A typical 384 Kbps DSL line costs less than $200 per month. A T1 or Frame Relay line outfitted for high-speed data will cost upward of $1,000 to $2,000 per month, a big difference. DSL is also easier to install because you can adjust the data rate (the closer you are to a telephone company central office, the faster the line can run).

One of the vendors pioneering this space is Jetstream Communications (http://www.jetstream.com). Some of our readers may remember Jetstream's FrontDesk product (this was an innovative ISDN-based phone system for small/home offices). Jetstream now provides voice over DSL hardware to local and long-distance phone companies.

How will voice over DSL affect you?
The primary benefit this technology offers is cost. Competitive local phone companies can now deliver more service to customers using fewer copper wires. Since they must lease these wires from the incumbent local phone company (i.e., Baby Bell), this has a direct cost benefit for carriers, and hence their subscribers. This will also make it much easier to deliver business grade phone service to locations that do not have extensive wiring in place.

For example, let's suppose you move into an older building which was never wired for a lot of phones (a warehouse for example). You have something like 4 analog phone lines running into the facility, not nearly enough for your intended use of the space. Each of those 4 analog lines can be reconfigured as a DSL line. Assume the building is relatively far from the central office, so you can't get the highest speed, and have to settle for 384 K per line. By converting all 4 analog lines to 384 K DSL lines, you would have a total of 1.5 megabits of data carrying capacity, enough to carry 24 simultaneous phone calls while still leaving plenty of room for high-speed internet access. Plus, you can do this without rewiring the building, an important consideration both for you, and for the carrier providing the service.

The real benefit for users is choice, and therefore price. This technology will enable more companies to offer competitive local dialtone. This means more choice for buyers, and ultimately lower prices for local communications services.