Channelized DSL targets multiservice

There are essentially three ways to transport voice traffic over a copper phone line using digital subscriber line (DSL) bandwidth: voice-over-Internet Protocol (IP), voice-over-asynchronous transfer mode (ATM) and channelized voice-over-DSL.

Voice-over-ATM puts digitized voice and emulated signaling information into ATM cells, which are transported over the phone line and through the network over a dedicated ATM private virtual connection.

Similarly, voice-over-IP places digital voice and emulated signaling into IP packets, and then transports them across the phone line along with other data.

The third method, channelized voice-over-DSL (CVoDSL), is an enhancement of digital subscriber line technology that transports voice traffic over DSL bandwidth not by packetizing, but rather by electronically dedicating physical-layer "channels" of DSL bandwidth over the phone line for each derived phone call.

On the customer end, the caller uses a standard phone plugged into a CVoDSL-enabled DSL modem, which digitizes each derived phone signal. When using pulse-code modulation encoding with no compression, each voice channel consumes 64 kbits/second of bandwidth in each direction without overhead.

The real impact-and the key to understanding CVoDSL and its application-lies in the architecture in which it best fits. CVoDSL is ideally suited for next-generation multiservice access platform (MSAP) equipment.

A multiservice access platform is designed for flexibility; it terminates not only data traffic, but also POTS voice, and has advanced higher-layer capabilities and intelligence. An MSAP terminates customer phone lines just like a digital subscriber line access multiplexer (DSLAM). But unlike a DSLAM, it handles both POTS (plain old telephone service) phone calls as well as data over a digital loop carrier (DSL) by filtering the bandwidth and sending the POTS and DSL frequencies to their respective line cards.

The POTS portion of the MSAP will digitize the analog voice signals and then pass them off to a time-domain multiplexed backplane, which then aggregates the signals and transports them to a Class 5 circuit switch.

A channelized voice call arrives at the MSAP from a dedicated port on the DSL chip set, so the MSAP has the flexibility to do one of several things with it: It can pass the signal onto the voice backplane with the rest of the digitized POTS traffic for transport to the Class 5 switch. But with integrated voice-over-ATM (VoATM) or VoIP voice packetization functionality, the MSAP can alternatively packetize the CVoDSL signals into ATM or IP, then hand them to the data backplane for transport to the data network.

CVoDSL is complementary to VoATM and VoIP in two ways. First, it is intended to enable next-generation MSAP equipment with derived voice capability; it is not necessarily intended for enabling deployed DSLAMs. Second, it can be used with VoATM and VoIP by enabling an MSAP to uniformly migrate both POTS and derived voice over DSL to a next-generation network.

Today, the "network cloud" is a fitting representation of the nebulous future of the telecommunications network. Next-generation networks based on soft-switch platforms promise great bandwidth and operational efficiencies as well as lucrative end-user features. But the transition of a 100-year-old voice network to one run largely by software could take decades, and will require a flexible access architecture.