In a recent training session on business voice systems that I conducted for a group of cable technicians, one of the participants asked whether Internet protocol (IP) private branch exchanges (PBXs) would connect to a cable company’s network differently than time division multiplexing (TDM)-based PBXs. The answer is "yes," and the explanation is the base for this month’s column on session initiation protocol (SIP) trunking. Trunking SIP trunking is another telephony service that cable operators can provide to business customers. From reading this column over the past couple of months, you know that one of the differences between residential and business customers is that business customers typically require several phone lines. The larger the business, the more likely it will use trunks, rather than individual lines, as the link to the outside world because of the efficiencies and cost savings of an interface that can be shared between several non-simultaneous users. In the traditional world of TDM PBXs, trunking is implemented with T-1 or its broadband cousin, PRI. Although both T-1 and PRI are digital, they are not IP-based and use the public switched telephone network (PSTN) to complete a call. In a universe of TDM switching, using TDM transport makes sense.

SIP trunking, on the other hand, is an IP-based service that uses a data network to complete a call. The shift in technology from TDM-based PBXs and key systems to IP-based customer premises systems drives the need for SIP trunking. IP-based systems are mini-versions of the same technology used in PacketCable networks. Both call management and call content are implemented as packet flows within the system, with conversion to analog voice at customer premises equipment (CPE) end points. When the PSTN is used as the connection for outside calls, both signaling and content must be converted from IP packets to formats understood by TDM at a gateway. Depending on how the service provider establishes the end-to-end path, multiple gateways may be involved, and each one adds cost, inefficiency and possible degradation. Serving business Cable’s business market customers have already begun asking for more efficient solutions than T-1 lines to their businesses, and operators are responding. Kristine Faulkner, Cox VP of product management and development, told me: "A lot of our push is to do trunking over DOCSIS. We are already seeing the market migrate to IP PBXs. Our customers are looking for an IP handoff to avoid the cost of on-premises gateways, and that’s where we are focused."

SIP trunking provides that IP handoff. With a SIP trunk, signaling and content remain as packets, and the service provider’s IP network is used to complete outside calls. Several Internet telephony service providers (ITSPs) offer SIP trunking, but that doesn’t mean the service is generic. Call completion is a complex task, and variables include whether the call is terminating at a location on a company network, at a PSTN line, at an IP PBX, or at a TDM PBX. In addition, multiple service providers may be involved in an end-to-end call. When that happens, several tasks associated with call completion over a data network and the implementation of business rules need to be coordinated across service providers.

For example, phone numbers must be translated to IP addresses and vice-versa, which is a directory service. Also, business rules governing packet flow in a data network need to be agreed upon and consistently implemented, so that end-to-end voice quality of service is preserved. SIP has many options, and when multiple service providers are involved in call completion, protocol mediation at the edge between networks may be required.

What happens at endpoints is also a part of the process of an inter-network IP call. The ITSP has to decide which gateways provide the optimal connection to the PSTN when required. At the business customer’s network edge, firewall and network address translation (NAT) traversal must be done. Adding value By assuming responsibility for these tasks, the cable operator adds value to a SIP trunking offering. In addition to providing a technical solution, the cable operator effectively acts as a buffer for many of the business challenges associated with end-to-end voice over IP (VoIP) calls. If the operator does not manage its own national network, it usually has already negotiated agreements with other carriers to support its residential voice offerings that specify how policy decisions are resolved and how to optimize the cost of a connection to a phone served by the PSTN.

Network security is another attribute that cable operators have to offer as part of a SIP trunking offer. A benefit of SIP trunking is that it shares network connections between voice and data traffic. Unfortunately, this potentially opens another door to network attacks. The problem for a business customer is two-sided.

Incoming packet voice traffic can be used as a tool to attack a business customer’s network, but outgoing traffic can also unwittingly carry viruses and worms to the outside world. Establishing a trusted environment is therefore critical, and using a cable company to provide SIP trunking removes the need for expensive session border controllers (SBCs) and network certification at the customer premises.

Although leading edge cable operators are in the early stages of offering SIP trunking, there are currently no specific CableLabs specs covering it. The good news is that cable has already solved many of the interface problems as part of the PacketCable architecture. Glen Russell, CableLabs director, PacketCable, doesn’t see adding SIP trunking to the PacketCable architecture as a major stumbling block.

"Initially, there will be multiple implementations, and there will eventually be a need for a standard," he said. "Interop testing will take some time, but PacketCable has provided a lot of the necessary foundation in specifying SIP signaling interfaces, and security has always been a major part of cable architecture."

How soon we start seeing SIP trunking as a routine part of cable offerings depends a lot upon the overall pace of cable’s move into business services. Although business customers account for about 20 percent of the telephony lines at Cox, the majority of other MSOs are still at rampup stages of market penetration, with two- to eight-line voice products as the focus of marketing activity.

Justin J. Junkus is president of KnowledgeLink and telephony editor for Communications Technology. Reach him at

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