Getting Ready for VoLTE
Michael Keeley is a Director of Product Management at Spirent Communications’ wireless test equipment division. He has led various teams involved in wireless network emulation and automated systems used for testing mobile devices. Prior to joining Spirent in 2000, Mike worked for Lucent Technologies. He earned his BSEE and MEng from Cornell University and an MBA from New York University’s Stern School of Business.
MWJ: Can you give us an update on commercial deployment of VoLTE?
MK: A couple of years ago we heard some aggressive-sounding plans for massive commercial VoLTE rollouts this year. In fact, MetroPCS has launched VoLTE here in the US so far, as well as two operators in Korea.
Lately, larger US operators in the US have publicly pointed out that we can’t rush large-scale rollouts of the technology. The rest of the wireless industry sees a lot of wisdom in ensuring that this technology is fully-baked when we deliver it to the public… there is a lot riding on it.
MWJ: Once VoLTE goes live, how will subscribers transition to it?
MK: For most operators, it will take years before LTE service coverage attains the level of current 3G coverage. In the first transition phase, all voice traffic has been handled by the legacy circuit-switched networks and data traffic by the LTE packet-switched networks, where available. There are two fallback mechanisms that a VoLTE-ready device can use when LTE service is unavailable.
The first, Circuit Switched Fallback (CSFB), uses a single radio that switches between the LTE network and the legacy circuit-switched connection. The fallback mechanism is not immediate… call setup times are significant and noticeable to the subscriber. Call continuity can also be an issue, as can interruptions in packet-switched services when the radio transitions, since data service will also transition to 3G during a voice call.
The second interim solution is called Simultaneous Voice and LTE (SVLTE), and requires two radios in the device: one for LTE and the other to connect to a legacy circuit-switched network. Issues with this approach include manufacturing cost, managing interference between the radios, maintaining power output limits and the impact on battery life.
A second transition phase is the introduction of VoLTE and a technique called Single Radio Voice Call Continuity (SRVCC). Like CSFB, SRVCC uses a single radio but manages call continuity as the user moves in and out of areas with LTE coverage. With SRVCC, signaling is complex and there will still be a discernible disruption in audio as the device transitions between networks. The complexity of SRVCC means that it is unlikely to see commercial deployment before 2013 and is likely one factor in the decision of major operators not to rush VoLTE rollouts.
MWJ:Conceptually, VoLTE is VoIP on a wireless data connection. But how is it different?
MK:The main difference between VoLTE and OTT VoIP services is that VoLTE includes a Quality-of-Service (QoS) component. VoIP services are usually pretty good, but nothing is guaranteed; IP packets are delivered on a “best-effort” basis. Operators recognize that providing “carrier-grade” voice service is critical and QoS can’t be a function of factors outside their control.
This sounds like a minor difference, but delivering specific QoS levels involves not only the IMS subsystem but every step in the service chain, including devices and the network core. It is a more significant step than many people realize. The concept of dedicated bearers is a key differentiator for VoLTE since it enables certain types of traffic, such as VoIP traffic, to be isolated from all other traffic. OTT VoIP traffic, since it’s delivered on a “best effort” basis over non-dedicated or default bearers, is subject to unforeseeable latency or dropped packets.
MWJ: Is there anything else important needed in the network to support VoLTE-capable UEs?
MK: Absolutely. VoLTE devices require new RAN-related functionality in order to deliver the carrier-grade voice service that VoLTE promises. There are four critical ones. The first is Semi-Persistent Scheduling, making use of reasonably predictable patterns in resource blocks to reduce granting control channel overhead. Second is Transmission Time Interval Bundling, a technique for reducing latency at the cell edge by preemptively including multiple copies of the transmitted data (coded in different ways) without waiting for a request for re-transmission. Third is Discontinuous Reception which takes advantage of the predictability of VoLTE traffic, effectively turning off the UE’s receiver between reception periods to save power. Finally, Robust Header Compression or RoHC compresses multiple layers of headers to help avoid the channel inefficiencies that can result when overhead data (mainly headers at various layers of the protocol stack) occupies more traffic space than payload data.
MWJ:And what is Spirent doing to help VoLTE come to market?
MK: In addition to the VoLTE testing functionality on the automated test systems for which Spirent is best known, we also offer products specifically for the device R&D lab.
Our CS8 Device Tester is a multi-RAT network, including IMS and full-featured Evolved Packet Core, in a bench-top format. It lets the design engineer quickly and easily isolate and resolve issues early in the design cycle, preventing patch-fixes that are much more costly to address later in the product’s lifespan.
A key goal of our recently-released set of software tools has been to strike the right balance between highly flexible control and ease of use. This means that making changes to network configurations and test setups is quick and painless. We’ve been able to save some of our customers weeks of development time compared with complex script-based solutions.