Long after most major carriers finally launch LTE services, service providers will still have to support subscribers on 2G/3G legacy networks.
"It's going to be tough for them to completely and rapidly get away from 3G or even 2.5G as well just because there [are] always going to be [customers] out there that have it," said Bill Rubino, principal analyst at ACG Research.
"It's going to be a tough call for them to say we're no longer going to support 3G…. It really depends on the success they have with the proliferation of 4G clients."
Carriers cannot abruptly shut off service for subscribers on 2G/3G legacy networks, and overlaying 4G systems and platforms will add cost and complexity to network infrastructure and back-end system upgrades.
Supporting voice traffic over LTE is one example of the complications that legacy networks pose.
Many mobile carriers are still struggling with how to manage voice handoff between LTE and 2G/3G legacy networks when LTE subscribers wander beyond the edge of the 4G network.
The One Voice Initiative, which uses IMS architecture to carry voice over LTE, has won widespread approval from service providers, vendors and industry bodies. But carriers are still struggling with how to implement packet-switched VoIP traffic alongside circuit-based 2G/3G legacy networks.
"The technology for VoIP over 4G to work is not the stumbling block. It absolutely does work.… The real issue is legacy infrastructure," said Randy Fuller, director of business at Tekelec, a broadband data management vendor, speaking on a recent panel at 4G World in Chicago.
"How quickly people will flip over to doing voice over their LTE networks depends on how much legacy infrastructure they have and the cost of switching that service over - the cost of the handsets, the amount of coverage in the network, the requirement that … it can fall back to a circuit network if you're not in LTE [range] or sufficient data coverage," Fuller added.