Cable VoIP, Part 2: Here's How it Works

For most cable providers, winning with voice-over-IP (VoIP) means capturing residential phone revenue, preferably without ever having to buy entry or exit passage on the public phone network built and serviced by incumbent telcos.

Getting the win means different things to different MSOs; this discussion will focus on calls that move from home to destination via Internet protocol (IP). But you should know that some MSOs want to convert IP phone traffic back to analog at the headend, to pass calls through the pricey phone switches they already own.

Like any other new service, VoIP requires some new equipment. Ingredient No. 1: the 1.1 upgrade to the Data Over Cable Service Interface Specification (DOCSIS). That's the Cable Television Laboratories Inc. cable-modem specification that offers "quality of service," or QoS (pronounced as the letters: Q-oh-S). In short, QoS offers a way to "stripe" voice packets to stand out as high-priority, isochronous traffic that requires low latency.

Low latency
means few or no transit-related glitches during a call. Symptomatically, transit delays are those awkward pauses, common to satellite phones. They create an urge to buttress each sentence with "over," as in, "Bad connection. Over."

is tech-speak for data traffic that requires round-trip synchronicity more than its needs a wide swath of bandwidth.

Ingredient No. 2: a VoIP-ready cable-modem termination system, or CMTS. Anyone offering cable-modem service already owns and operates CMTS gear, because it is the necessary "other end" of each cable modem. The CMTS is usually located at the headend.

Adding VoIP to the data-services mix will almost certainly require a CMTS upgrade. A lifeline VoIP service, in which the phone works when the power is out, will probably require a whole new CMTS-the so-called "forklift upgrade." (Next-generation CMTS providers, like RiverDelta Networks, Broadband Access Systems Inc./ADC Telecommunications Inc. and others, love this lifeline idea. It gives them an inroad to try and unseat CMTS market-leader Cisco Systems Inc.)

Ingredient No. 3: A "multimedia terminal adapter," or MTA. (AT&T Broadband, Broadcom Corp. and Motorola Inc. also call this a "broadband telephony interface," or BTI. It's the same thing.) This is the box that straps to the side of the house, or gets installed inside.

Picture a cable modem with RJ-11 jacks on it-and because it handles both data and phone, it shaves deployment costs for homes that want both services.

Then there are the servers. Lots of servers. Via software, cable VoIP essentially duplicates the way in which calls are made on today's analog, telco-delivered network. To do that, you need a lot of software firepower: to look up destination numbers, to issue dial and ring tone commands and to dole out messages ("the number you dialed is busy," etc.).

Most of the cable VoIP ingredients are laboriously described in CableLabs' PacketCable specification series.

Here's a simplified version of how it works: Customers get a box-the MTA or BTI, depending on who describes it. The phones in the house are linked into it.

Say customer Jane decides to make a call: She picks up the phone. Unbeknownst to her, an off-hook indicator zips through the MTA to the CMTS, along the upstream IP path. The CMTS recognizes the activity as specific to Jane's MTA. It alerts a call-management server (CMS) that she needs to make a call. The server acknowledges by giving Jane a dial tone.

(An aside on the possible future of dial tone: In June of 1999, AT&T's Shannon Labs demonstrated an alternative to dial tone, called "AT&T Annie"-a pleasant-sounding she-bot that cheerfully patched through voice commands, like "call Dan," or "check voice mail." Silences were covered by the sound of a heartbeat, to indicate that the call was live. It made me wonder if dial tone eventually gets replaced by audio brand-advancement.)

Jane dials. The dialed digits traverse the same IP upstream path, again through the CMTS and again to the call-management server. The server consults a built-in lookup table to see what it knows about the dialed number: Is it within this cable system? Is it outside the system?

If within the cable system, the call server alerts the destination MTA of an incoming call, via the cable plant, and instructs it to ring. When the dialed party answers, the call server assures that the dialer and dialee are hooked up, then retreats. Before it leaves, it tells the MTAs to let it know when the call ends.

If the dialed call is intended for a destination outside of its zone, the call server hands it to a signaling gateway for completion. The gateway is a device that moves calls over the public phone network-and ultimately over a managed, private cable backbone-to their destination.

Jane completes the call, and hangs up. The CMS notes the duration of the call, and hands the usage information to a records server, which in turn links to a billing system. And so on, for every VoIP call. It has to work as well for one as it does for millions, so scale matters, too.