Enterprise telephony vendors are clear that IP is the future, but customers haven't universally agreed how to get there. Some companies that made grand commitments to IP telephony are scaling back to hybrid environments, while others are forging ahead with pure-IP convergence plans.News last week that\u00a0Merrill Lynch is migrating from a Cisco-based IP telephony network\u00a0it installed in 2000 to an environment based on a mix of Avaya-based IP and\u00a0TDM\u00a0telephony gear raised eyebrows among industry watchers. A Merrill Lynch spokesman says the company was concerned that having data and voice on one network could leave the firm susceptible to outages, either accidental or the result of a hostile attack.Ironically, it was voice over IP (VoIP) that helped Merrill Lynch recover and keep business running after its operations were affected by the Sept. 11 attacks in New York, says Anita Dulude, technical service director with the integration firm ThruPoint, which was involved in the IP telephony installation at Merrill Lynch's Hopewell, N.J. campus in 2000."It was easy for [Merrill Lynch] to move workers and deploy phones in a short period because data network ports were available," Dulude says. "If it were a TDM solution it would have taken twice as long to do what they did."Dulude says the Merrill Lynch VoIP network was sound when the installation was completed. "They've upgraded [their IP PBXs] numerous times and really put them through the ringer," she says, adding that ThruPoint was not involved in Merrill's decision to go back to a hybrid network.Dulude says most clients are doing the opposite, looking to migrate from TDM completely. "Clients aren't planning on hybrid environments," she says. "With a hybrid environment, you're not taking advantage of convergence. The whole idea is to have one network for supporting voice and data, plus video or anything else that comes along."It will be more expensive in the long run to have separate ports, cabling and switches for different services, she adds.A Merrill Lynch spokesman says the company will still be able to take advantage of convergence by mixing TDM and IP voice, and get the added advantage of having one back up the other.While Merrill Lynch is going hybrid, CarrAmerica Realty is plowing ahead with a pure-IP plan. The realty management company is in the thick of a nationwide rollout of Cisco's Architecture for Voice, Video and Integrated Data IP telephony gear, spreading VoIP to 60 offices nationwide with a total of about 1,000 phones.The system replaces several large Avaya TDM-based PBXs and key systems with two redundant Cisco CallManager IP PBXs that serve remote offices over a nationwide IP WAN. The system is slick, says Barry Krell, vice president and director of engineering at the Washington D.C. firm, but requires a measure of old-school telecom practicality.Krell's remote offices have three or more extra public switched telephone network (PSTN) lines so users can still make calls if there is an IP WAN outage. A feature on the Cisco routers in those offices called Survivable Remote Site Telephony allows the router to become a mini-PBX, providing basic call-control features to IP phones for PSTN calls. Another feature allows the remote offices to access Cisco voice mail servers in Washington, D.C., via dial-up modem lines."You have to realize that it's going to be a mixed environment," Krell says. "It has to be at this point because the [telephony] world is still based on the PSTN."One analyst says projects like CarrAmerica's are the exception rather than the rule. "Migration from TDM to IP is not going to happen in a few years; it's going to be decades," says Ron Gruia, an analyst with Frost and Sullivan.One reason is that there is little impetus now for big PBX shops to swap out to IP. "The installed base of PBXs is much newer than it has ever been in the past," Gruia says. Many firms bought new systems in 1998 and 1999 when the economy was hot and Y2K was around the corner. Now with budgets strapped, firms might sit tight with their PBXs, which can provide a decade of usefulness."My PBX has five-nines reliability," says James Sposito, a telecom manager for Penn State University at Altoona. "Why not IP-enable that PBX instead of installing a [server-based IP PBX]?"He says linking PBXs across Penn State's campuses with IP has been a benefit, but putting IP phones on desks is not in the cards. "My goal is to evolve the [TDM] switch and get the best performance I can out of it. There's no reason for me to go to a total IP network."Still, vendors of IP voice gear soldier on. Synergy Research says the enterprise IP telephony market reached $850 million in the first half of this year, up $265 million from a year ago. And market leader Cisco last week announced the shipment of its 2-millionth IP phone.However, 2 million phones is only about one-third of the TDM phones corporations will buy this year, says Allan Sulkin of PBX research firm TEQConsulting Group. And even by 2009, he projects that IP phone shipments will trail PBX phones shipments by about 3 million units.Another company living in the TDM world but with an eye to a future of pure IP is EDMC. The Pittsburgh firm, which manages secondary education institutions, and professional training and certification schools across the country, has Siemens HiPath hybrid PBXs in its main offices and satellite offices. These PBXs are tied together with an IP WAN.While telephony at EDMC is a mixed environment, "there's an urgency for us to be prepared to have an entirely IP voice system" so we don't have to manage two networks, says Derek Fink, assistant vice president for telecom at EDMC. He says the goal is to have all voice and data on the LAN in all branch offices and running over the same IP backbone across the WAN.The ability to mix TDM and IP is helping the firm as it builds out this blueprint, Fink adds. "We can deploy equipment now while we work to upgrade our Cisco network to support the level of [quality of service] we'll need," for an all-IP network, he says.\u00a0Best and worst of both worldsSome pros and cons of running a hybrid IP\/TDM telephony environment, according to users and industry experts:Pros:\u00a0Cons:\u2022Having traditional voice lines provides a backup in case of IP network failure.\u2022VoIP can be integrated slowly without disruptive changes to staff or equipment.\u2022Companies can squeeze every last minute of life out of older PBX gear.\u2022Maintaining two networks for voice defeats the point of convergence.\u2022Keeping separate staffs for voice and data could lead to responsibility conflicts. \u2022Applications that take advan-tage of full IP convergence could be harder to implement.