The technologies and business practices that let residential and business customers to make telephone calls using the infrastructure (public or private) of the Internet, instead of the traditional telco network.
For most cable providers, winning with voice-over-IP (VoIP) is capturing residential phone revenues, preferably without ever having to buy entry or exit passage on the public phone network built and serviced by incumbent telcos. Some MSOs, like Time Warner Cable, initially opted to quicken their pace to the VoIP market by partnering with seasoned telephony experts Sprint and MCI, in TWC’s case.
The partnerships were designed to outsource certain aspects of making VoIP into a business. That means customer provisioning, or what it takes to port a new customer onto the cable VoIP network. Also PSTN provisioning, or the physical hand-off of a call from a cable VoIP customer to the public switched telephone network. Standard voice services, like directory assistance, operator assistance, and information. Life critical services, like CALEA and 911. Long distance. Rate center connectivity, or using established links into the many rate centers that populate a metropolitan market (a mid-sized city can have as many as 30).
Outside of the “back office” partnerships, VoIP requires equipment. Ingredient number one: The 1.1 upgrade to the Data Over Cable Services Interface Specification (DOCSIS). Thats the CableLabs 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.
Ingredient number two: 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 usually requires a CMTS upgrade. Doing lifeline VoIP, so that the phone works when the power is out, will probably require a whole new CMTS — the so-called “forklift upgrade.”
Ingredient number 3: A “multimedia terminal adapter,” or “MTA.” 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 does both data and phone, it shaves deployment costs for homes that want both services.
And then the servers. Lots of servers. Cable VoIP essentially duplicates, in software, the way in which calls are made on analog, telco-delivered networks. To do that, you need a lot of software firepower: To look up destination numbers, to issue dial and ring tone commands, to dole out messages (“the number you dialed is busy,” etc.).
Most of the cable VoIP ingredients are laboriously described in the CableLabs PacketCable specification series. Heres a simplified version of how it works:
Customers get a box — the MTA. 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, along the upstream, IP path, to the CMTS. The CMTS recognizes the activity as specific to the 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.
Jane dials. The dialed digits traverse the same IP upstream path, again through the CMTS, 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, via the cable plant, of an incoming call, and instructs it to ring. When the dialed party answers, the call server assures 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 the call 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 the 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.