CableLabs has quietly been working on a SIP trunking project that aims to produce an extension to PacketCable 2.0, give MSOs a way to interoperate with PBX systems, and help cable operators chase after a larger, more lucrative segment of business customers.

Working alongside the SIP Forum and the Internet Engineering Task Force (IETF) , CableLabs has been pursuing that project, dubbed SIP Enterprise Connect, for more than a year, according to Glenn Russell, the director of business services at CableLabs. "It is really part of the extensions to the PacketCable program to support business voice," he says.

The emergence of that extension comes along as cable MSOs look to hunt bigger business game. Although some MSOs are going after mid-sized and large enterprise business customers, most have targeted their early efforts at very small businesses using PacketCable 1.5 -- an architecture that cable's been using to deliver VoIP to residential customers. Thanks to new multiline embedded multimedia terminal adapters (E-MTAs, or voice modems), most of that initial business voice traction has focused on service bundles with 12 lines or less, the average being about three lines per customer.

The addition of SIP trunking will let operators handle bigger jobs, to try to siphon away revenues that usually go to telcos.

Cable's not entirely new to SIP trunking, but the current crop of in-the-field examples is small. Cablevision Systems Corp. (NYSE: CVC), through its Optimum Lightpath enterprise services division and Optimum Business service for small- and mid-sized commercial customers, has already rolled it out. Cox Communications Inc. has been conducting tests. Both companies declined to provide any specific anecdotes of their SIP trunking experiences thus far, but a Cablevision spokeswoman confirms that the MSO is looking to expand on its work with the technology this summer. (See Cablevision Offers SIP Trunking to SMBs.)

Vendors such as Whaleback Systems , meanwhile, are starting to develop products that take advantage of cable's eventual move to SIP trunking. (See Whaleback Swims After MSOs, Carrier Partners .)

However, SIP trunking protocols and equipment interoperability aren't well established. Until they are, cable won't be able to scale its use of the technology, notes one industry source who is involved in commercial cable services with a top-five US MSO.

That's where the CableLabs effort comes into play.

"Everybody agrees that there's a big opportunity there [to do SIP trunking]," says CableLabs' Russell, noting that cable's Docsis 2.0 and newer Docsis 3.0 platforms offer enough capacity to provide business services covering up to 24 lines, and support even more in situations that call for direct fiber. "Everyone also agrees that there's a lack of a standard that everyone could adhere to. SIP's a very flexible protocol, but there's lots of variants to it. We're trying to be very specific on how SIP gets used for these trunks."

To help get that done, CableLabs, a SIP Forum member, is contributing to the emerging SIP Connect 1.1, which promises to support more features than the 1.0 spec. The 1.1 version also intends to tighten up the interoperability issues and other ambiguities that the original 1.0 spec had, notes Russell, who serves on the SIP Forum board. (Cox, Optimum Lightpath, and Cedar Point Communications Inc. , a VoIP vendor that cut its teeth on cable deployments, are also SIP Forum members.)

CableLabs hopes that process will result in an industry-wide spec for interconnecting IP-PBXs and MSOs, and that it will create the necessary technical alignment with the specs work of the SIP Forum and the IETF. That work will touch several components in the technical food chain, including softswitches, PBX gear, telephony application servers, session border controllers, and gateway devices that reside at the customer premises between the PBX and the service provider network.

But it's still too early for Russell to say when he expects that work to be completed, since that's not in the control of CableLabs. However, once SIP Connect 1.1 is complete, publishing it as an extension to PacketCable 2.0 "should be a fairly simple exercise," he says. "We're doing all we can to expedite it."

PacketCable 2.0 borrows heavily from SIP and uses an IP Multimedia Subsystem (IMS) core; that will all remain unchanged. "Think of this [SIP Enterprise Connect] as another application of PacketCable 2.0," Russell says. (See PacketCable 2.0: Back on the Front Burner.)

— Jeff Baumgartner, Site Editor, Light Reading Cable