March 28, 2011, 9:19 PM — Workers in telecom departments may be understandably apprehensive about switching from TDM to SIP, since it could conceivably put them out of work.
But any company that adopts SIP and promptly lays off its telecom team while shifting responsibility for voice to its IT team would be making a big mistake, according to Graham Francis, CIO of The SIP School training and certification program. That's because people working in telecom teams have several skills that IT department workers might not possess, such supporting quality of service for real-time applications such as voice calls.
PRIMER: SIP trunking
"All the telecom guys are going to have to understand how voice and data will mix on the same network," says Francis, whose training sessions focus on areas such as SIP messaging, SIP security, troubleshooting and interoperability. "People with TDM backgrounds need knowledge about data switches, firewalls and proxy servers. They need to understand that on a data network, voice just becomes another application."
While there is still definitely a place for telecom specialists in enterprises that use SIP trunks, they'll have to learn additional skills if they want to make a successful transition from TDM to SIP. In particular, they'll have to learn much more about traditional IT tools such as session border controllers and firewalls, since a SIP trunk is a broadband Internet link that utilizes SIP to connect a company's IP-based PBX to an Internet telephone service provider (ITSP). Instead of terminating the trunk directly at the IP-PBX, for security's sake companies tend to terminate the trunks at a SIP-capable session border control system that acts as a firewall.
Jim Maloff, the principal consultant for Maloff NetResults, says the biggest difference that telecom workers have to get used to when switching to SIP is thinking about phone calls in terms of bandwidth rather than available lines. This means that for large conference calls they'll need to figure out exactly how much bandwidth they'll need beforehand so they can provision it off and give it priority over other traffic on the network.
"If I know that I'm going to have 10 concurrent calls then I should know that I'm going to need 1 meg of bandwidth just for my calls," he explains. "You'll need to do packet prioritization and shaping to makes sure that voice packets get higher priority."
Like Francis, Maloff also thinks that telecom workers can bring much-needed skills and perspective to the job of managing SIP-based voice networks that IT workers will need some time to acquire. However, he also thinks telecom workers should realize that preparing themselves to manage SIP will take a lot of time and effort since there are a lot more variables in a SIP system than a traditional TDM system.
"The hardest thing for people who have been in telephony is now there's so much more they need to think about," he says. "In the past if I'm a telecom manager I never had to pay for local outgoing calls. With SIP trunking it's more like a cellphone model where 'local' has no meaning because you're on the Internet."
Brian Graves, a network engineer at the University of Washington who has a background in managing telephony, says he has had to learn much more about implementing quality of service as his department started designing a SIP core that it plans on rolling out over the entire network over the next few years. The difficulty for many people on the telecom side, he says, is understanding that voice calls no longer get automatic preference for bandwidth or even the total amount of bandwidth they'll need to successfully complete calls on an IP network.
"TDM is a circuit-switched technology, so QoS is essentially built right in because you have a dedicated amount of bandwidth on your call," he says. "An IP network is packet-switched so you don't have natively the same bandwidth guarantee. You have to implement QoS to make sure it isn't delayed by other data."
