VoLTE in LTE Migration Strategies

By | November 7, 2011

Voice over LTEAt a recent conference I attended, a strategy executive at Deutsche Telekom stated that they would turn off the 3G network before turning off 2G GSM as the migration to LTE evolves. Sounds bizarre? Not really, it makes a lot of technical sense. One reason to come to this decision is the nature of the LTE architecture and the way LTE handles voice traffic. As a full-packet network, LTE is fundamentally different from the full circuit-switched GSM and hybrid circuit and packet-switched 3G networks which include today’s data workhorse HSPA+. In LTE, voice is just another application, albeit one with specific parameters and requirements. Therefore, voice is packetized and classified according to a certain Quality of Service level (QoS) to maintain important parameters such as latency and jitter. This is essentially what Voice over LTE (VoLTE) is.

When operators will roll out VoLTE remains an open question, but is one of the factors that play out in network migration decisions. Circuit-switched networks are good at providing low-jitter and latency trunks which allow greater compression at the codec resulting in relatively low bit rates transmitted over the air. Meanwhile, packet-switched voice would have to trade back some gain in compression for shorter latency resulting in higher data volume transported over limited air-interface resources.

How operators will roll out VoLTE appears to have been settled. The industry will implement an intermediate ‘Circuit Switched Fall Back,’ or CSFB which means that an LTE handset will revert to 2G or 3G network to connect a voice call. This will surely buy operators some time to fully test out the VoLTE technology which is evolving to maturity.

Today’s LTE networks are used mainly to provide data services through USB dongles. But as the technology further matures and becomes widely integrated into handhelds, the roll of VoLTE and CSFB are one of the factors that will be considered by operators in making migration decisions. LTE offers higher spectral efficiency than 3G so it makes sense to transition 3G networks to LTE while using GSM for voice services through CSFB.  The universality of GSM makes it a good fall-back solution.

But not all decisions are based on purely technical merits. Millions of mobile data devices today rely on 3G and transitioning to LTE without continued support for 3G services is not very practical. The bifurcation of wireless technologies and frequency bands are not making life easy for both operators and handheld vendors. How to proceed is a very interesting and complex subject which I will be watching out for. The future will tell if indeed T-Mobile and other Tier 1 operators will aim to turn off the 3G network before the 2G network. But it’s safe to bet that this decision will take many years to unfold ensuring a long period of co-existence of three technologies.