Skip to main content

Anatomy of a Circuit Switch-Out

Pioneering cable-telephone provider Cox Communications Inc. is spelling out the engineering details of its migration from circuit-switched to voice-over-Internet protocol telephony, the approach the big cable-phone newcomers are taking.

In a nutshell, Cox is adding softswitch VoIP capability to its current time-division multiplexing (TDM) switches in order to gain the operational and cost efficiencies associated with VoIP.

Nortel Networks Inc., Cox’s circuit-switched supplier, is upgrading those switches with VoIP capabilities, and bringing media gateways from Nuera Communications Inc. into the operation.

The VoIP conversion is a mixed approach that adds VoIP technology to Cox’s existing circuit switched launches alongside greenfield VoIP launches.

“Essentially, we cap circuit-switch connects and grow with VoIP,” Cox vice president of telephone and data engineering Jay Rolls said last week, accompanying Cox’s release of a white paper with the telephony details.


Cox and Nortel have begun upgrading 10 Nortel circuit switches to the hybrid-switch model in five of the MSO’s 12 circuit-switched markets.

Atlanta-based Cox didn’t announce a timetable for the other seven markets, but it’s believed those transitions will occur over the next few years.

In the white paper, Cox said it expects more than half of its new phone subscribers to be of the VoIP variety by year’s end.

Cox operates 24 circuit switches in 12 markets: Orange County and San Diego, Calif.; Phoenix and Tucson, Ariz.; Omaha, Neb.; Hartford, Conn.; Hampton Roads and Fairfax County, Va.; New Orleans; Oklahoma City; Wichita, Kan.; and Rhode Island.

It has launched standalone VoIP service in Roanoke, Va.; Tulsa; Baton Rouge and Lafayette, La.; and in five cities in West Texas. Those cities are all served by a soft switch in Atlanta.

The MSO already has a national VoIP Internet backbone, which transports 13% of its long-distance calls, as well as cable-modem traffic.

Cox and Nortel are in the process of “evolving” the majority of Cox’s circuit switches to hybrid TDM/VoIP switches, allowing the MSO to keep TDM lines in place until a full migration is warranted.

“It’s a pretty typical model for circuit-to-packet migration,” said Nortel vice president of carrier packet networks marketing Jim Dondero.

“We can share some of the computer power and the processing power,” Rolls said, “but we put a VoIP front end on the switch.”


Cox had one to five circuit switches in any of its 12 original phone markets. Phoenix, for instance, is at the high end of the scale with five switches, Rolls said.

As part of the evolution, Cox and Nortel will be adding new phone lines to the VoIP side of the same circuit switch processor. “Long-term, well do this with most of the switches,” Rolls said.

Nortel also is providing Cox Nuera’s BTX media gateways, as part of a Nortel-Nuera reseller agreement.

In addition to the hardware upgrade, Nortel is upgrading the core software to a VoIP-enabled load, and reconfiguring the operating system to support IP telephony.

In the conversion, Cox does not have to reproduce interconnection and E911 facilities, rate-center connections or back-office infrastructure.

VoIP’s distributed architecture enables Cox to separate call control and signaling from the actual call-switching fabric and bearer path, the MSO said, to improve scalability and allow carriers to distribute parts of the network for efficiency and geographic reach.

That allows Cox to locate media gateways and media-terminal adapters remotely from call-center and signaling resources.

“The combination of increased call-server scale, greater geographical reach and distributed gateways enables Cox to enter markets at low initial line sizes, since existing softswitches can be shared across a large number of sites,” Cox said in the paper.

The deployed Nortel switches can scale to 150,000 lines, though the company is deploying new software which will push that number to 180,000 lines, Rolls said.


While Cox has launched telephony to 70% of its footprint, a few sizable pockets — such as Las Vegas, Santa Barbara, Calif., and Cleveland — remain to be served. Those markets would be greenfield IP-telephony markets, Rolls said.

Whether calls would go through the Atlanta softswitch or through switches in nearby markets — such as Phoenix for Las Vegas or Orange County, Calif., for Santa Barbara — is an open question.

Adding on VoIP capability wasn’t part of the early circuit-switch design. But Nortel first starting thinking about switch hybrids in the mid-to-late 1990s, Dordero said.

“I can remember some of those early conversations,” he said. “We had a lot of internal debate. What’s the business case? It was very clear there was going to be a migration and we embraced it early.”

Nortel also announced VoIP deals with Adelphia Communications Corp. and Rogers Communications Inc.

Adelphia will deploy Nortel’s CS 2000-Compact soft switch, which uses session initiation protocol for packet connectivity. It will also use the vendor’s Global Services arm to plan, build, operate and maintain the VoIP network.

Nortel will act as network integrator with the cable-modem termination system, the multimedia terminal adapter and the provisioning and interconnection components.

The interesting part of the Adelphia rollout is that it won’t involve any media gateways, said Nortel Global Cable Solutions director of cable marketing Elaine Smiles. Rather, Adelphia plans to directly connect calls to an interexchange carrier.

That will eliminate not only the cost of the media gateways but the small time delays with the TDM-to-TDM hop.

“They are using packet interconnection,” she said. Phone calls would travel from the home, through Adelphia’s cable modem termination system to the IXC. The IXC would then send the call to the public switched network termination point. “They are the first to do this to the best of our knowledge,” she said.

Adelphia will use PacketCable on the telephone line side of the CMTS, and SIP for interconnection between the carriers.

Nortel said Canadian MSO Rogers Communications Inc. selected it as its primary softswitch vendor for a digital phone rollout that will commence in mid-2005. Nortel is providing its Compact softswitch and Nuera’s ORCA BTX-4k media gateway.