Voice Alternatives for DSL 

Many competitive local exchange carriers entered the competition for the local loop as alternative leased facilities and DSL service providers. Those that have succeeded in weathering the early years of competition as data service providers on second phone lines are now poised to venture into the voice market, the historical cash cow of the incumbent LECs. How large a market are we talking? Like most technology market analyses, the growth graph is a predictable hockey stick-slow today, meteoric in less than 5 years-but even setting aside my personal skepticism of such graphs, it's clear that voice is a huge and tappable market.

The path from data to voice-and-data, and perhaps ultimately, broadband integrated services access provider, can follow many routes. You have many technologies to consider, and what you choose may be influenced by what you currently offer, the competencies your organization holds, and now and future business relationships you have with other data and voice service providers, who may be partners today, and competitors tomorrow-or vice-versa.

Flavors of "Voice Over…"
Let's begin by considering Asymmetric DSL. ADSL modems use Frequency Division Multiplexing (FDM) or Echo Cancellation to divide the available bandwidth in the 4 KHz - 2.2 MHz spectrum of a telephone line into multiple channels. Separate bands are used for upstream and downstream data. Data are packetized into IP, segmented into ATM cells, then transported first over a PVC to a DSLAM then on through an access line into a public Internet or private network. ADSL also splits off a 4 kHz region for POTS at the DC end of the band. 

The POTS channel is typically split off to a Class 5 telephone switch in a voice network, so it is a perfect play for ILECs, especially given the number of primary lines they've installed and operate. In most deployments, customers connect a conventional telephone handset into a familiar and standard RJ11 phone jack, and can receive basic and enhanced telephone feature support. ADSL is also a good reverse play strategy for voice CLECs who have installed voice lines and wish to operate data services. Covad and Rhythms offer ADSL nationwide, and Internet provider turned CLEC, HarvardNet, offers ADSL in New England, and North American DataCom will soon offer ADSL in the Southeast. 

The added revenue opportunity with a single voice channel over DSL is interesting, but the real revenue windfall for voice over DSL comes from providing many voice lines over a single copper pair, alongside a fat Internet pipe, the traditional dominion of TDM mux equipment (channelized-T1). Delivering four to twenty-four voice lines over each copper pair to a business while still providing a fat pipe to the Internet makes Voice over ATM (over DSL) an attractive alternative for remote offices, home and small businesses. 

Among many requirements, compression of voice PVCs is essential to leave room for an Internet data PVC that satisfies a tolerable definition of "fat". The ability to support compressed and uncompressed voice traffic (the latter for fax and modem) is also critical for business customers. And to provide toll quality voice, ATM Quality of Service must either be implemented into new DSLAMs, or retrofitted into existing equipment, so that voice calls aren't adversely affected by bursty data traffic.

CLECs with ATM-based DSL deployments can meet these requirements. Several DSL flavors, notably Symmetric DSL (SDSL), are sold today as inexpensive alternatives to T1 private lines. Those SDSL services that transport data over ATM PVCs use a data-specific adaptation layer, AAL5. With a different adaptation layer (AAL2), ATM can be used to carry voice and video services, simultaneously and in addition to transporting AAL5-encapsulated data. The same technique can also be applied over ADSL when data are transported over ATM PVCs, but the number of voice PVCs that can be supported is more constrained than SDSL due to the upstream bandwidth. 

In the ATM-based voice scenario, data over ATM PVCs follow the same path as previously explained. PVCs carrying voice are switched through a voice gateway, where the ATM PVC is terminated, and where voice traffic cells are converted to a digital voice signal and sent over a standard (e.g., Bellcore GR-303-CORE) digital trunk interface to a Class 5 switch. A Voice over DSL deployment is also attractive for CLECs who have left the business of IP to Internet Service Providers, and have strong strategic alliances with ISPs. Of course, ISPs and CLECs who operate DSL and IP networks may also compete for voice, using Voice over IP. 

Voice provider Focal Communications recently used a CopperCom gateway to demo how voice calls can be handled from (NorthPoint's) DSL circuits at SuperComm. ICG Communications and Covad Communications recently announced expansion of a San Francisco Bay area VoDSL trial using Jetstream's Voice over Broadband technology, an example of how a data CLEC and a circuit CLEC can cooperate to compete in the voice over market. Internet and Telephone services provider Advanced TelCom Group and TriVergent have also selected JetStream for their VoDSL deployment.

Voice over IP (VoIP, a.k.a., Internet telephony) uses the Real-Time Protocol (RTP) and runs over the User Datagram Protocol (UDP), over IP. The RTP header identifies packets as containing a voice sampling in a particular encoding format. A timestamp and sequence number are used to reassemble a synchronous voice stream from a stream of RTP packets. UDP port numbers are used to multiplex and distinguish multiple call streams between IP endpoints. Compression and call control is facilitated by the ITU's H.323 and related standards for multimedia transmission, or possibly the Session Initiation Protocol and related protocols, developed by the Internet community for simpler client implementation (e.g., for IP phones).

The deployment architecture for VoIP is similar to Voice over ATM (over DSL). An IP-based internet access device capable of supporting compressed voice, data, fax, and modem traffic at a customers premises multiplexes IP voice and data streams onto an IP network, over any broadband access circuit, including any DSL access. Voice traffic is routed to a voice gateway, where, the IP-packetized voice traffic is converted to a digital voice signal and sent to a Class 5 switch, over a standard (e.g., Bellcore GR-303-CORE) digital trunk interface. As is the case for ATM-based deployment, Quality of Service across the entire IP infrastructure between voice users is critical to maintain toll-quality voice. CLECs that support IP and intend or already support differentiated services using IP QOS methods (MPLS, Diffserv) will find VoIP attractive. 

ISP-CLECs are also likely to have the engineering talent capable and cheeky enough to traffic engineer IP networks to be as secure, bandwidth and queuing efficient, jitter-free, scalable reliable and available as the PSTN, if for no other reason than to debunk contrary claims by Voice over ATM proponents. Mpower Communications will deploy Tollbridge equipment for voice over SDSL in ten cities this summer. ICG has installed Cisco packet-based switches in its national network for VoIP. Internet Service provider CRL Networks (San Francisco) has also launched a VoIP initiative.

Issues independent of which voice over you choose…
The question of "Whose voice switch?" calls through the PSTN applies to all these scenarios. The answer depends on what you are today-circuit or data based CLEC, or ISP-how deeply you intend to penetrate the market, and the partnerships you can muster. "Whose voice switch?" also affects whether your network architecture is centralized (e.g., your many DSLAMs trunking into one or a small set of voice gateways, yours or a partner's) or de-centralized (e.g., DSLAMs collocated with voice gateways and switches, e.g., in a Central Office). 

Voice gateway and IAD functionality-the ability to support fax, modem, toll quality voice with telephone features public and PBX phone users expect-is a tougher selection process with VoDSL or VoIP than a pure ADSL play. The operational environment includes more signaling and management interfaces in what is likely to be one of multiple vendors, each having its own management from the start, a situation that begs for a supervisory OSS to rein them all in. As was the case for ISDN, lifeline support is an issue, as is billing. But profitability per deployed copper pair can be many multiples over pure data DSL.

Conclusions
The technologies for "voice over" are maturing as the market is materializing. Products are available for all alternatives mentioned, and some, even at this early juncture, are receiving a great deal of attention, as mentioned above. But there's no "one size fits all" solution for CLECs for voice over. You will make concessions and absorb a greater deployment burden than a pure data-or voice-play irrespective of which technology you choose. But the payoff looks too promising to ignore.  

David Piscitello is president of Core Competence, Inc., a network consulting firm and founder of The Internet Security Conference.