Voice Over LTE – What the Fuss is All About

US cellular carriers continued the aggressive deployment of LTE in all major metro areas in the year 2014 to attract more subscribers (and also the right to brag who has the biggest LTE network in the country). We all enjoy the higher data rates. But, there is more LTE. One of the most exciting features of recently announced Apple iPhone 6 and iPhone 6 Plus is the support of Voice over LTE (VoLTE). VoLTE provides HD call quality using all IP telephony.

As you know the new cellular networks are all about providing higher data speeds and running new applications for smart mobile phones. VoLTE can be thought of yet another voice app for the operator’s data network. The new protocol brings faster, better voice calls and even video chat tied to your cell phone service and number. One key advantage is that it may send Voice-minutes billing to go away for good in the future since every call is a data call.

In a way, VoLTE, is similar to VoIP (Voice over Internet Protocol), which Internet voice applications such as Skype and Vonage use to support voice calls over a broadband connection. HD Voice more than doubles the audio frequency range for voice calls versus legacy narrowband technologies giving you clear sound.

Let’s discuss some interesting issues about VoLTE and its deployment.

Long Term Evolution (LTE) – 3G Path to 4G, and then to 5G

Long-Term Evolution (LTE), commonly marketed as 4G LTE, is a standard for wireless cellular communication of high speed data for mobile phones and data terminals. 4G LTE utilizes OFDMA as the multiple access schemes that enable deployment of highly scalable and high data rate cellular networks.

LTE based on the GSM/EDGE and UMTS/HSPA network technologies was designed to increase the network capacity, reduce network latency and improve data transfer speeds. To improve the speed it uses sophisticated new radio interface together with many core network improvements. The standard was developed by the 3GPP (3rd Generation Partnership Project -a cellular standards development body) and is specified in its Release 8 document series, with minor enhancements described in Release 9.

Peak download rates specified in the specifications are up to 299.6 Mbit/s and upload rates up to 75.4 Mbit/s depending on the user equipment category (with 4×4 antennas using 20 MHz of spectrum). Five different terminal classes have been defined from a voice centric class up to a high end terminal that supports the peak data rates. All terminals will be able to process 20 MHz bandwidth.

LTE is the natural upgrade path for carriers with both GSM/UMTS networks and CDMA2000 networks. The different LTE frequencies and bands used in different countries will mean that only multi-band phones will be able to use LTE in all countries where it is supported.

Enhanced Voice Quality & Core Networking

Originally LTE was seen as a completely IP cellular system just for carrying data, and operators would be able to carry voice either by reverting to 2G / 3G systems or by using VoIP in one form or another. However, it would have caused network fragmentation and incompatibility issues not allowing all phones to communicate with each other.

Moreover, SMS remains a very active revenue stream for most operators. While revenue from voice calls and SMS is falling, it was as necessary to have a viable and standardized scheme to provide the voice and SMS services to protect this revenue. Hence, VoLTE scheme was devised as a standardized system for transferring traffic for voice over LTE.

One major problem impact the wider deployment of VoLTE. 3G networks have yet not gone away. To ensure compatibility, 3GPP requires at least AMR-NB codec (narrow band), but the recommended speech codec for VoLTE is Adaptive Multi-Rate Wideband (AMR-W), also known as HD Voice. This codec is mandated in 3GPP networks that support 16 kHz sampling.

LTE uses the SIP based IP Multimedia Core Network Subsystem (IMS) architectural framework. IMS allows for ubiquitous multimedia access using a common IP interface. This greatly simplifies network management and provides better developer accessibility.

Using this framework, connections over different protocols can be bridged together. With the IMS framework, VoLTE is able to inter-operate with circuit switched voice networks without having any dependency on or requirement for them. Additionally, the common IP interface of IMS will allow VoLTE to inter-operate with other voice networking technologies even after legacy circuit switched networks are finally phased out.

Voice over LTE Deployment Challenges

A small country in Europe or Asia can roll the LTE in the entire country simultaneously. However, it is logistically complex and too expensive to launch a new network in the entire US.

Therefore, LTE networks have to co-exist with their 2G-3G zones. A user while talking on VoLTE to someone may go outside his LTE area. It requires that a call needs to be able to fall back onto legacy networks.

The VoLTE standard accomplishes this with a clever technique called Single Radio Voice Call Continuity (SR-VCC). With SR-VCC, when an LTE tower determines that a user might leave the coverage area during an active connection, it leverages the IMS framework to initiate a second connection over the legacy standards. Both connections are maintained until the LTE signal is lost, the LTE tower determines the user will not leave the coverage area, or the voice call ends.

This scheme has two requirements. First, SR-VCC requires the network provider to have the IMS framework in place. Secondly, mobile devices must not only be able to connect to both the LTE and the legacy networks, but be able to connect to both networks at the same time.

This creates a difficult design issue for the designer of the phones who are attempting to reduce the cost and pack even more hardware in a decreasing physical space. They now essentially need two antennas for both 3G and 4G simultaneous connections. Some manufacturers such as Apple make use of single antenna and toggle it as needed.

It is important to note that in HD Voice calling; only connections between capable devices are at the higher quality and connections in which either device is not capable are at the lower quality for both devices.

Another approach to deploy LTE is Circuit-switched fallback (CSFB). Basically an operator uses LTE for a data transfer and any call made still goes through the standard circuit-switched domains. The benefit of this approach is that a cellular operator need not upgrade its entire network to expensive packet switched IMS infrastructure; He has to mainly upgrade just MSC.

Video over LTE (ViLTE)

Video applications remain the key drivers of data consumption in the cellular networks.ViLTE uses the same IMS network with applicable Application Server (AS) and SIP signaling. Video is encoded in standard H.264 codec which delivers superior quality compared to low bit rate codec used in 3G networks.

Over the Top (OTT) Alternatives for Voice and Video & Emergence of WebRTC

For getting the HD quality audio and video calls, consumers don’t have to rely on your service providers to provide all the solutions. As a matter of fact, they do have significant competition in this area.

There are many third party applications like Skype, Vibe, Whatsup, Telegram etc. that can provide users alternative means of communications. More interesting apps like will likely continue to emerge in the future.

As a result service providers face decreasing revenue from voice and SMS services. However, these very same apps also contribute to the increasing data usage and expensive monthly data plans, so net gain for the cellular operators in the data revenue has been good so far.

Emerging WebRTC technology will also impact how audio and video services will evolve in the future beyond 2015.

That hopefully will be the topic of my future article.

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