Getting to the vaunted five nines

Architecting five nines Putting plans into practice Servers at the core Inside the box Five rules for five nines Architecting VoIP survivability

If you had to choose, what could you live without: dial tone or e-mail? That's not a choice network executives want to make. But forced into it, most probably would pick a failed e-mail system as the lesser of two evils. They know a phone network gone dead on their watch is the quickest route to the unemployment line.

That fear has been the pall hanging over VoIP for years. "The network is down" is not an acceptable explanation when it comes to phones. For that reason, many companies have been reluctant to bet their telecom infrastructure on commodity servers, IP WANs and phones plugged into Ethernet switches.

But as companies evolve to the new data center model of computing, the benefits of replacing disparate PBX and key telephone system hardware throughout a corporation with a centralized cluster of IP PBXs are getting harder to ignore. Hosted and managed from the glass house, voice can be treated just like any other application. Plus, these days, reliability doesn't have to be an issue, experts and experienced users say.

Achieving Ma-Bell-like reliability with VoIP simply means building a network with redundant call-processing hardware and gateways, providing ubiquitous power backup, and implementing best practices in security, patch management and virus protection.

Architecting five nines

First, understand your bandwidth requirements, says Ray Ortega, voice and video infrastructure consultant with ThruPoint, a New York integrator that has installed IP voice and data networks for many large companies. IP PBXs, network gear and IP phones all can be up and running, but poorly engineered bandwidth can lead to congestion and make the VoIP network as useless as if an IP PBX or router had crashed.

Ensuring that doesn't happen starts by selecting the right codec, or compression method, for encoding and decoding packetized voice. The ITU-standard G.711 codec, which compresses VoIP to 65K bit/sec, makes sense on LANs, while the G.729 codec, with 9K bit/sec compression, is suited for lower-bandwidth T-1 or broadband shared WAN links, Ortega says. Some vendors promote the use of other ITU codecs - such as G.722, which supports higher-frequency voice - but the G.711 and G.729 are the most widely deployed, he adds.

"It comes down to determining what quality a customer wants," Ortega says.

Redundancy of switches, routers and call processors should be the next consideration in your VoIP blueprint.

"We try to split the load across the two active servers," Ortega says of the converged networks ThruPoint has architected for companies such as Deutsche Bank, Merrill Lynch and Morgan Stanley. Load-balanced IP PBXs, available from vendors such as Avaya and Cisco, can run in one data center or in separate data centers, in case a primary site is cut off. When choosing the latter, Ortega adds, you must take network latency into account. WAN links must be measured for delay and jitter; delay greater than 100 millisec could cause a problem with voice quality.