IMS Taxes Mobile Voice
One of the main tenets of upgrading mobile core networks to support an IP multimedia subsystem (IMS) is that operators will then be able to offer converged voice, video, and data services in IP form over a number of different fixed and mobile access networks, from cellular and wireless LAN, to cable/DSL and WiMax.
This is the Holy Grail of convergence, giving operators the ability to bundle voice and multimedia services and deliver them to customers wherever they may happen to be. Moving to IMS will enable cellular carriers to get a slice of this triple-play action, offering a blend of multimedia services over the air.
But packetizing voice calls may not be the most efficient use of bandwidth on a cellular network. In fact, a VOIP call could use up to four times the bandwidth required for a traditional cellular voice call.
Or -- to put it in a broader context -- one million VOIP calls could take up the bandwidth that would support four million cellular calls.
Steve Shaw, director of marketing at startup Kineto Wireless Inc., lays out how an IMS tax would be levied:
"For all VOIP services (SIP, IMS, H.323, UMA) there is overhead associated with IP transport," says Shaw. "It will certainly not be more efficient from a pure bandwidth per call perspective to use IP versus the highly optimized GSM voice.
"In today’s GSM voice network, the bearer (voice path) traffic uses about 12 kbit/s. With that same codec, it’s fair to assume the 'IP tax' will [increase] call data rates to about 45 kbit/s. As a mobile operator, the benefit of carrying a voice call over the radio access network in IP must be carefully weighed against the cost of using [four] times the bandwidth per call session."
Kevin McCracken, director of product marketing at startup NewStep Networks agrees that moving to VOIP would slap cellular operators with an IMS tax, although he didn't say how much more bandwidth such services would require.
"Those networks aren't designed for voice-over-IP," he says.
Rival startup Persona Software Inc. is a little more cagey about the concept of an IMS tax. "It's not something that we've seen," says David Schwartz, director of marketing at Persona. "I'm not saying it doesn't exist -- we just haven't looked for it."
Persona argues that any IMS tax will be made irrelevant by the bigger pipes offered by the high-speed 3G networks that operators will put in place as they move to upgrade their core networks to IMS.
NewStep's McCracken concurs, noting that as operators move to networks using CDMA EV-DO (evolution, data only) and UMTS HSDPA (high-speed downlink packet access) they will start to have enough horsepower to offer converged services.
"That's the big drawback of today's cellular networks: You can't combine voice, video, and data," says McCracken.
Kineto's Shaw is more circumspect about when operators will start runing VOIP over cellular access networks, noting that many operators today are deploying 3G to increase the voice capacity of their networks.
"They use the additional spectrum in 3G to provide more capacity for standard GSM calls, and not voice over IP," says Shaw. "The operators are deploying high-speed data services over those same networks, but because of the inherent optimization of the GSM voice calling network, and the relatively precious commodity of licensed spectrum, mobile operators are not rushing into providing voice service over IP in the 3G access network."
— Dan Jones, Site Editor, Unstrung