With the latest launches of Long Term Evolution (LTE) smartphones, operators have implemented another way to deliver LTE voice services in the network, called circuit-switched fallback (CS fallback).

To date, only CDMA operators like Verizon Wireless and MetroPCS Inc. (NYSE: PCS) have supported voice calls on LTE smartphones by having more than one radio active in the device -- one for the packet-based LTE data service and another for circuit-switched CDMA voice service.

Now, GSM operators like AT&T Inc. (NYSE: T), Rogers Communications Inc. (Toronto: RCI) and soon Telia Company can also support voice on their LTE smartphones. The LTE devices will have multiple radios to support LTE, UMTS and even GSM, but with CS fallback only one radio can be active at any time. So, the device is forced off the LTE network and onto the 2G or 3G network for voice calls.

AT&T launched LTE devices earlier this month, while Rogers activated its first LTE devices at the end of October. Both operators have launched CS fallback, according to Gabriel Brown, senior analyst at Heavy Reading. (See AT&T Readies Its First LTE Phones .)

In addition, TeliaSonera announced at the end of last week its first LTE tablet, the Samsung Electronics Co. Ltd. (Korea: SEC) Galaxy Tab 8.9 LTE, which will go on sale before Christmas, as well as a Samsung LTE smartphone that will launch early next year and support CS fallback for voice services.

"We will not sell the smartphones until the CS fallback is done. And this isn't an issue," said Tommy Ljunggren, VP of system development for Nordics and Baltics at TeliaSonera. "That is what Europe has been waiting for -- it's crucial to have CS fallback."

According to Heavy Reading's Brown, a majority of operators are launching CS fallback because it's a "time-to-market issue" before voice over LTE (VoLTE) will be commercially available, which Brown says will be in the first quarter of 2013.

Until then, operators can opt for CS fallback, which has been viewed as an interim solution.

"The question becomes, Is there such a thing as an interim solution for CS fallback or will it continue to run in parallel with VoLTE?" says Brown.

TeliaSonera's Ljunggren says operators will need both CS fallback and VoLTE. "You can't live with only one of them," he said. This is because CS fallback will be needed to support users as they roam outside of 4G coverage areas. "It's the most natural way to go forward right now."

For now, though, there are more practical issues about CS fallback for operators to contend with, such as reducing voice call and data session setup times.

"Roughly speaking, the first CS fallback implementations add a second or two of extra call setup time," says Brown. "But there are enhancements coming next year in the network and terminal that will get that down to about 500 milliseconds [of additional call setup time]."

There is also another optimization technique called network-controlled packet-switched handover, which will work in parallel with CS fallback, that will reduce the data session setup times down to a few hundred milliseconds, according to Brown. This will be available next year.

— Michelle Donegan, European Editor, Light Reading Mobile