New promise for more efficient voice capacity

John C. Tanner
12 Jun 2009

With all the excitement over wireless, broadband, 'real' mobile internet access and the netbooks, iPhones and smartphones that will drive data usage in the 4G future, it's all too easy to forget that in the 3.5G present, circuit-switched voice is still the cash cow of mobile. And while W-CDMA has built its reputation partly on efficient voice delivery, it still has to share its capacity with data, and the growing success of HSPA is putting pressure on the voice side of the business.

The solution, according to Nokia Siemens Networks, is to start pushing circuit-switched voice over the HSPA network in a manner that will not only double voice capacity on 3.5G networks, but also help mobile customers save on device battery power.

NSN is pushing a technology called circuit switched call over HSPA (CSoHSPA), which is not, as it sounds at first pass, VoIP, explains NSN's W-CDMA/HSPA business chief Keith Sutton.

'What it does is use the IP bearers in HSPA\'s packet-based transport channels to transport the voice,' Sutton told Telecom Asia. That not only means up to a 100% boost in voice capacity, but also call set-up times being cut in half, he adds.

CSoHSPA isn't proprietary - it's enabled under 3GPP Release 7 (a.k.a. HSPA+) by way of a feature called continuous packet connectivity (CPC), which means it\'s essentially a RAN software upgrade for 3G networks. NSN is the first vendor to actively push the idea, demonstrating the technology last month by making a live CSoHSPA call via Finnish cellco Elisa using prototype Nokia handsets.

An interesting extra benefit of using CPC is that it can make devices more energy-efficient, says Sutton.

'A traditional circuit-switched call occupies the channel even during the dead spots in the conversation,' he explains. 'With CPC, you can enable the phone to be inactive during those times, so it's not transmitting or receiving. The CPC link keeps it in standby mode. That means less battery power is used up during the call.'

QoS essential

However, Sutton says, the real selling point of CSoHSPA is more efficient voice capacity.

'Many 3G operators are running on narrow spectrum bands,' he says. 'It's not a very wide channel, so they have to be as efficient with it as possible. With the growth that will be coming in the next few years, they\'ll need to manage that spectrum more efficiently than ever without sacrificing QoS.'

Meanwhile, the QoS angle - which has always been circuit-switched voice's trump card over VoIP - is also on the minds of LTE players, as LTE is designed to be all-IP. In March, a group of vendors led by German cellco T-Mobile formed an industry group called the VoLGA (Voice over LTE via Generic Access) Forum. Mission: define specs to deliver circuit switched voice and SMS over LTE, using the 3GPP's Generic Access Network (GAN) standard.

The founding VoLGA vendor roster includes Alcatel-Lucent, Ericsson, Huawei, Kineto Wireless (whose Universal Mobile Access technology is now the GAN in question), LG, Motorola, Nortel, Samsung, Starent and ZTE. Genband, Sonus Networks and Genband have since joined. (NSN has not.)

The VoLGA Forum intends to submit its specs to the 3GPP by mid-year, and has already published its Stage 1 specs and Stage 2 draft specs. VoLGA is pitting itself against IP multimedia subsystem (IMS), which has also been proposed as a way to support circuit-switched voice over LTE.

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