Understanding VoLTE: The Voice of Tomorrow.
A generational leap in cellular voice requires a leap of faith.
Published 22/05/19
This is the second in a two-part series examining WiFi Calling and VoLTE, two technologies that fundamentally transform the voice calling experience.
The beauty of cellular voice is that it just works. Whether you're one of those collectors still using the original iPhone on a 2G network or grace a smartwatch boasting 4G connectivity, the communication medium of voice is more ubiquitous than ever before.
It's easy to see why this decades-old communication method remains fundamental to the orchestration of our society. No other technology has emerged to offer a similar mix of backwards-compatibility, straight up simplicity and consistent quality.
But, everything is subject to the test of time, and voice needs to move with the evolution of data networks in order to compete with free OTT alternatives. As revenues from traditional sources dry up, and consumer expectations reach an unprecedented high, is there a viable path for operators?
Enter VoLTE, a vision for the future of voice, hinging on a radically new network architecture based on packets. Ending stagnation in the calling space, VoLTE delivers a multitude of benefits for both operators and their customers, while maintaining the principles that define voice.
It's easy to see why this decades-old communication method remains fundamental to the orchestration of our society. No other technology has emerged to offer a similar mix of backwards-compatibility, straight up simplicity and consistent quality.
But, everything is subject to the test of time, and voice needs to move with the evolution of data networks in order to compete with free OTT alternatives. As revenues from traditional sources dry up, and consumer expectations reach an unprecedented high, is there a viable path for operators?
Enter VoLTE, a vision for the future of voice, hinging on a radically new network architecture based on packets. Ending stagnation in the calling space, VoLTE delivers a multitude of benefits for both operators and their customers, while maintaining the principles that define voice.
The Modern Evolution of Voice
Credit: Apple
VoLTE enhances the fundamental technologies that allow cellular calling to function while maintaining the same concept graced by the industry for decades. With VoLTE, digital voice packets traverse the Internet over IP via the medium of an operator's 4G Radio Access Network (RAN).
The beauty of 4G is derived from the fact that it is a pure packet technology which is service-agnostic. In practice, this means virtually any IP-based activity can be supported by a 4G network, including VoIP.
The utilisation of data networks to facilitate legacy services such as voice is a breakthrough for the industry, moving the dependency of revenues away from archaic 2G and 3G architectures and towards a versatile 4G network. And while that may not seem like a significant change on paper, it is in reality.
The beauty of 4G is derived from the fact that it is a pure packet technology which is service-agnostic. In practice, this means virtually any IP-based activity can be supported by a 4G network, including VoIP. If you're an operator, this promise of liberty is refreshing, especially considering the cost-per-transported bit with 4G is lower than that exhibited by any previous cellular generation.
Market data released by ComReg shows stagnation in the domain of voice traffic, with an increase of just 1% in the past four quarters. That's an important metric and one which indicates that the eventual decline of cellular voice will be prolonged. With its cost-friendly nature, VoLTE allows operators to maintain voice revenues, even in the event of a decline in traffic.
The beauty of 4G is derived from the fact that it is a pure packet technology which is service-agnostic. In practice, this means virtually any IP-based activity can be supported by a 4G network, including VoIP. If you're an operator, this promise of liberty is refreshing, especially considering the cost-per-transported bit with 4G is lower than that exhibited by any previous cellular generation.
Market data released by ComReg shows stagnation in the domain of voice traffic, with an increase of just 1% in the past four quarters. That's an important metric and one which indicates that the eventual decline of cellular voice will be prolonged. With its cost-friendly nature, VoLTE allows operators to maintain voice revenues, even in the event of a decline in traffic.
Credit: ComReg
The streamlining of network assets is perhaps the most advantageous aspect of VoLTE for operators, allowing voice and data to co-existence on the same network. As the adoption of VoLTE progresses, so too does the spectrum refarming process, supporting the operator's goals to enhance 4G and 5G capacity by utilising existing bands.
In Ireland, the vast majority of voice calls are facilitated by the 900MHz band, with 2G (or GSM) coverage reaching a point of ubiquity. This low-band spectrum excels for voice because of its inherent ability to penetrate physical obstructions such as walls without incurring a significant loss in signal quality.
However, the utilisation of the 900MHz band isn't merely limited to one cellular generation, and this is where 3G (or UMTS) plays a role. As the pervasiveness of 3G in the 900MHz and 2100MHz bands expanded from pockets of availability to blanket coverage, operators began to exploit their new networks by introducing a feature known as HD Voice.
HD Voice or Wideband audio pushed the cellular calling experience forward by providing more than twice the frequency of audio signals (300Hz to 3400Hz), resulting in radically improved voice quality and a reduction in ambient noise. This was a compelling use case for 3G networks, and it explains why the 900MHz band remains the most widely available in Ireland today.
In Ireland, the vast majority of voice calls are facilitated by the 900MHz band, with 2G (or GSM) coverage reaching a point of ubiquity. This low-band spectrum excels for voice because of its inherent ability to penetrate physical obstructions such as walls without incurring a significant loss in signal quality.
However, the utilisation of the 900MHz band isn't merely limited to one cellular generation, and this is where 3G (or UMTS) plays a role. As the pervasiveness of 3G in the 900MHz and 2100MHz bands expanded from pockets of availability to blanket coverage, operators began to exploit their new networks by introducing a feature known as HD Voice.
HD Voice or Wideband audio pushed the cellular calling experience forward by providing more than twice the frequency of audio signals (300Hz to 3400Hz), resulting in radically improved voice quality and a reduction in ambient noise. This was a compelling use case for 3G networks, and it explains why the 900MHz band remains the most widely available in Ireland today.
Credit: Android Authority
For reference, Ireland consistently ranks as one of the countries with the lowest level of 4G availability in the world, falling behind even many developing nations.
The above descriptions of HD Voice and the 900MHz band serve a purpose here because they help to illustrate how VoLTE will evolve to become the dominant medium for voice traffic. Unfortunately, however, achieving that elusive goal will be a challenge given the poor state of 4G availability on the island of Ireland.
For reference, Ireland consistently ranks as one of the countries with the lowest level of 4G availability in the world, falling behind even many developing nations. This is a profound problem and one which will require exorbitant investment to overcome.
While the 800MHz band has underpinned 4G deployments to date, the imminent liberalisation of spectrum in the 700MHz band represents a glimmer of hope, reducing the site density required to reach a high level of coverage. The latter will be key to support the growth of VoLTE in Ireland, and efforts to enforce appropriate geographic coverage obligations with low-band spectrum licenses will only aid this.
For reference, Ireland consistently ranks as one of the countries with the lowest level of 4G availability in the world, falling behind even many developing nations. This is a profound problem and one which will require exorbitant investment to overcome.
While the 800MHz band has underpinned 4G deployments to date, the imminent liberalisation of spectrum in the 700MHz band represents a glimmer of hope, reducing the site density required to reach a high level of coverage. The latter will be key to support the growth of VoLTE in Ireland, and efforts to enforce appropriate geographic coverage obligations with low-band spectrum licenses will only aid this.
Requirements shaping VoLTE
For VoLTE to be a desirable technology in the eyes of consumers, it has to meet a number of fundamental requirements, all of which have a major bearing on the experience.
The most basic requirement is the capability of VoLTE to facilitate legacy telephony services including voice and messaging. These services must be as, if not more, intuitive to access compared to the mediums in which they are enjoyed today. This is particularly important in the case of emergency calling - a tightly regulated space.
But, additionally, VoLTE should include support for supplementary services such as video telephony, without producing excessive network complexity. This strive for simplicity was integral in the design phase of VoLTE, limiting the required software and hardware development effort.
The most basic requirement is the capability of VoLTE to facilitate legacy telephony services including voice and messaging. These services must be as, if not more, intuitive to access compared to the mediums in which they are enjoyed today. This is particularly important in the case of emergency calling - a tightly regulated space.
But, additionally, VoLTE should include support for supplementary services such as video telephony, without producing excessive network complexity. This strive for simplicity was integral in the design phase of VoLTE, limiting the required software and hardware development effort.
Credit: Nokia Bell Labs
Quality is an inherent measure that hangs over voice, and it's one that consumers will notice throughout the life cycle of VoLTE. Needless to say, fulfilling an expectation to enhance voice quality is difficult, and that technical challenge is amplified by the nature of a packet-switched system, which can sustain a severe degradation in quality as it passes outside an operator's sphere of influence.
VoLTE must build on the strong reputation of quality exhibited by HD Voice in the 3G era, and this means supporting mobility in the form of inter-RAT (IRAT) operations and seamless international roaming across operators. Inter-Radio Access Technology mobility is critical, especially in Ireland, permitting handover between different radio technologies such as 4G LTE to 3G and 2G.
VoLTE must build on the strong reputation of quality exhibited by HD Voice in the 3G era, and this means supporting mobility in the form of inter-RAT (IRAT) operations and seamless international roaming across operators. Inter-Radio Access Technology mobility is critical, especially in Ireland, permitting handover between different radio technologies such as 4G LTE to 3G and 2G.
It's not just our phones that can access the medium of voice anymore, thanks to the growth of eSIM technology, virtually any device boasts calling functionality.
The rate of energy consumption is an inconspicuous aspect of the cellular voice experience, but remember, an increase in battery consumption often impedes the adoption of new technologies. With the evolution of cellular co-existing alongside and supporting an IoT revolution, the number of connected devices is exploding, and most of those devices are dependent on small batteries - both in terms of physical size and capacity.
It's not just our phones that can access the medium of voice anymore, thanks to the growth of eSIM technology, virtually any device boasts calling functionality. By maintaining low energy consumption, VoLTE can meet the requirements of simple IoT devices, and enhance the experience for smartphone users.
It's not just our phones that can access the medium of voice anymore, thanks to the growth of eSIM technology, virtually any device boasts calling functionality. By maintaining low energy consumption, VoLTE can meet the requirements of simple IoT devices, and enhance the experience for smartphone users.
Of course, there are VoLTE requirements and benefits that apply to operators too. The aforementioned concept of network simplification, with 4G supporting data and voice simultaneously, is perhaps the most notable. Efficiency, however, is another realm into which VoLTE touches, and this will be applicable to operators of any scale.
An efficient VoLTE implementation is one that allows a greater volume of traffic to be handled per node and per megahertz of spectrum. The importance of this cannot be underestimated in a world where spectrum is an operator's most valuable asset, and also the shortest in supply.
Ultimately, VoLTE should work to enhance overall network performance by exploiting the versatility of 4G, and delivering on goals of efficiency and simplicity. The ability to deploy the technology atop existing equipment is paramount, slicing costs and ensuring longevity is a core component.
An efficient VoLTE implementation is one that allows a greater volume of traffic to be handled per node and per megahertz of spectrum. The importance of this cannot be underestimated in a world where spectrum is an operator's most valuable asset, and also the shortest in supply.
Ultimately, VoLTE should work to enhance overall network performance by exploiting the versatility of 4G, and delivering on goals of efficiency and simplicity. The ability to deploy the technology atop existing equipment is paramount, slicing costs and ensuring longevity is a core component.
The Path to True VoLTE
Credit: ComReg
There is diversity in the implementation process of VoLTE for operators, with options ranging from multi-radio solutions to those which tunnel legacy signalling through 4G and even SIP-based VoIP solutions. Each of the methods to enable VoLTE is based on the development of OTT VoIP solutions such as Skype, FaceTime and Whatsapp, all of which depend on the availability of competent fixed broadband.
Simultaneous voice and 4G LTE (SVLTE) represents one of the most primitive means to implement VoIP, surfacing issues in the realm of battery life and mobility between varying 4G and 3G coverage footprints. This method utilises a dual-radio solution whereby mobile devices connect simultaneously to both 4G and legacy technology (3G/2G). The purpose of the legacy network is to support voice and messaging, with 4G facilitating data traffic during calls.
The above method ensures 4G and the legacy network can operate independent of one another, eliminating the need for an upgrade to support SVLTE. However, there are significant drawbacks with SVLTE, and these have stifled advancement in the field. An increase in cost at the device level is one of the most severe implications, resulting from the requirement for independent base-band and Radio frequency (RF) chains.
Simultaneous voice and 4G LTE (SVLTE) represents one of the most primitive means to implement VoIP, surfacing issues in the realm of battery life and mobility between varying 4G and 3G coverage footprints. This method utilises a dual-radio solution whereby mobile devices connect simultaneously to both 4G and legacy technology (3G/2G). The purpose of the legacy network is to support voice and messaging, with 4G facilitating data traffic during calls.
The above method ensures 4G and the legacy network can operate independent of one another, eliminating the need for an upgrade to support SVLTE. However, there are significant drawbacks with SVLTE, and these have stifled advancement in the field. An increase in cost at the device level is one of the most severe implications, resulting from the requirement for independent base-band and Radio frequency (RF) chains.
Circuit-switched fallback (CSFB) is an extremely important solution in the Irish market, where it has been deployed uniformly across each operator to tackle the 4G voice challenge. This method prompts mobile devices to temporarily shift from 4G to a legacy RAT in order to facilitate the initiation or reception of a voice call. Upon completion, the device automatically reverts to 4G for data.
Irish operators have adopted CSFB because it leverages their existing infrastructure and supports different phases of network evolution. The latter is critical in the context of roaming. Moreover, the solution also simplifies the operation of SMS services, allowing devices to remain on a 4G network by tunnelling messages via the SGs network-to-network interface (NNI). This can also be achieved with SMS-over-IMS (IP Multimedia Subsystem), where an NNI is not required.
Unfortunately, to the dismay of Irish operators, CSFB is not a perfect solution. For one, it does not enable simultaneous voice and 4G data access, a key component of VoLTE. This means devices are forced to access 3G (UMTS) for data during voice calls, something that will become an issue as more and more spectrum is transitioned to 4G and 5G.
Adding to the above, CSFB negatively impacts call setup time by forcing devices to perform cell reselection and synchronisation before initiation of the call. CSFB can also cause devices to remain on the legacy network after the termination of a voice call because of packet data transfer, temporarily prohibiting consumers from accessing the benefits of 4G by increasing time spent on 2G and 3G.
Irish operators have adopted CSFB because it leverages their existing infrastructure and supports different phases of network evolution. The latter is critical in the context of roaming. Moreover, the solution also simplifies the operation of SMS services, allowing devices to remain on a 4G network by tunnelling messages via the SGs network-to-network interface (NNI). This can also be achieved with SMS-over-IMS (IP Multimedia Subsystem), where an NNI is not required.
Unfortunately, to the dismay of Irish operators, CSFB is not a perfect solution. For one, it does not enable simultaneous voice and 4G data access, a key component of VoLTE. This means devices are forced to access 3G (UMTS) for data during voice calls, something that will become an issue as more and more spectrum is transitioned to 4G and 5G.
Adding to the above, CSFB negatively impacts call setup time by forcing devices to perform cell reselection and synchronisation before initiation of the call. CSFB can also cause devices to remain on the legacy network after the termination of a voice call because of packet data transfer, temporarily prohibiting consumers from accessing the benefits of 4G by increasing time spent on 2G and 3G.
Credit: The 3G4G Blog
Voice over LTE via Generic Access (VoLGA) was another solution designed to enable 4G users to receive voice and SMS services as they transitioned between 2G, 3G and 4G access networks. VoLGA was positioned as a cost-effective and low-risk strategy to bring voice and SMS, an operator's primary source of revenue, onto their 4G networks.
The breakthrough with VoLGA, one of the first true VoLTE solutions, was the ability to support legacy services without the requirement to leave 4G. Importantly, this solution leveraged established technological principles such as Unlicensed mobile access (UMA) and functioned by hiding the access network from the core network, negating the need to change legacy systems.
Despite being a momentous stepping stone in the path towards a packet-based voice network, VoLGA failed to gain a critical mass of support from the industry, in part because of its over-reliance on legacy calling systems. As VoLGA developed, SIP emerged as a new and more advanced calling mechanism.
The breakthrough with VoLGA, one of the first true VoLTE solutions, was the ability to support legacy services without the requirement to leave 4G. Importantly, this solution leveraged established technological principles such as Unlicensed mobile access (UMA) and functioned by hiding the access network from the core network, negating the need to change legacy systems.
Despite being a momentous stepping stone in the path towards a packet-based voice network, VoLGA failed to gain a critical mass of support from the industry, in part because of its over-reliance on legacy calling systems. As VoLGA developed, SIP emerged as a new and more advanced calling mechanism.
Credit: ExploreGate
"One Voice" (now VoLTE) is the voice and data over 4G solution that has prevailed, and it will bring forth a new era of cellular calling in Ireland upon launch. This technology, which has been spearheaded by a group of operators and vendors, and subsequently refined in a profile by GSMA (IR.92), is a pure VoIP solution for VoLTE.
Incorporation of IMS is the fundamental building block of VoLTE, enabling it to play a role as a rich media solution, but initially, this also posed as the most profound barrier to operator adoption. You see, IMS is traditionally frowned upon by operators because it introduces unprecedented complexity and increases OpEx.
However, VoLTE overcomes the above qualms by utilising a watered down version of IMS MMTEL (Multimedia Telephony). As IMS is the foundation of VoLTE, it's important to understand that it is an architectural framework for delivering Internet Protocol (IP) multimedia services. When implemented in the core network, IMS enables a multitude of services to operate concurrently.
The watered-down version of IMS mentioned above involves a simplification of inter-connectivity, focusing on the elements needed for VoLTE. And coming back to the SVLTE solution detailed earlier, VoLTE succeeds in achieving the same goal without incurring the downfall of an increase in energy consumption.
Incorporation of IMS is the fundamental building block of VoLTE, enabling it to play a role as a rich media solution, but initially, this also posed as the most profound barrier to operator adoption. You see, IMS is traditionally frowned upon by operators because it introduces unprecedented complexity and increases OpEx.
However, VoLTE overcomes the above qualms by utilising a watered down version of IMS MMTEL (Multimedia Telephony). As IMS is the foundation of VoLTE, it's important to understand that it is an architectural framework for delivering Internet Protocol (IP) multimedia services. When implemented in the core network, IMS enables a multitude of services to operate concurrently.
The watered-down version of IMS mentioned above involves a simplification of inter-connectivity, focusing on the elements needed for VoLTE. And coming back to the SVLTE solution detailed earlier, VoLTE succeeds in achieving the same goal without incurring the downfall of an increase in energy consumption.
Credit: The 3G4G Blog
VoLTE achieves the above feat with Single Radio Voice Call Continuity (SRVCC), facilitating a seamless transition back to circuit-switched legacy networks. The real advancement with VoLTE and SRVCC is that it supports the aforementioned function while requiring only one transmitting radio in mobile devices, maintaining consistent battery life.
Every digital voice system requires an audio codec, and VoLTE is no different. The adaptive multi-rate (AMR) codec specified by 3GPP applies to VoLTE, just as it did to 2G (GSM) and 3G (UMTS). Codec continuity throughout cellular generations is fundamental to enable interoperability with legacy systems, forgoing the need for transcoders.
With AMR-wideband (WB), VoLTE enhances speech quality in a similar method outlined earlier with 3G (UMTS) and HD Voice. The enhancement is enabled by a wider speech bandwidth of 50-7000Hz compared to narrowband speech codecs optimised for plain old telephone service (POTS) wireline quality of 300-400Hz.
In terms of IP versions, VoLTE functions in a dual-stack configuration to support both IPv4 and IPv6. When and if the IMS application profile assigns an IPv6 address, the mobile device must prefer that address and specifically use it throughout the P-CSCF discovery phase. VoLTE tackles the issue of overhead from the IP header by implementing header compression with Robust Header Compression (RoHC), a protocol designed for data and voice packet headers.
Every digital voice system requires an audio codec, and VoLTE is no different. The adaptive multi-rate (AMR) codec specified by 3GPP applies to VoLTE, just as it did to 2G (GSM) and 3G (UMTS). Codec continuity throughout cellular generations is fundamental to enable interoperability with legacy systems, forgoing the need for transcoders.
With AMR-wideband (WB), VoLTE enhances speech quality in a similar method outlined earlier with 3G (UMTS) and HD Voice. The enhancement is enabled by a wider speech bandwidth of 50-7000Hz compared to narrowband speech codecs optimised for plain old telephone service (POTS) wireline quality of 300-400Hz.
In terms of IP versions, VoLTE functions in a dual-stack configuration to support both IPv4 and IPv6. When and if the IMS application profile assigns an IPv6 address, the mobile device must prefer that address and specifically use it throughout the P-CSCF discovery phase. VoLTE tackles the issue of overhead from the IP header by implementing header compression with Robust Header Compression (RoHC), a protocol designed for data and voice packet headers.
Conclusion: Smashing the Glass Ceiling
VoLTE is the voice of tomorrow, a voice supported by the continued growth in pervasiveness of 4G in Ireland and across the world. The migration of voice and SMS from legacy networks will be accelerated in the presence of spectrum refarming, an intricate process intended to enhance the quality of 4G and soon 5G by terminating 2G and 3G services in low-band spectrum.
With a clear and future-proof IMS Profile for VoLTE set by GSMA, adoption of the technology should not be thwarted by a sense of ambivalence. If you're an operator, there are unwelcome challenges in the quest to enable VoLTE, most of which concern functional, performance and quality aspects. Further, thorough testing and verification of VoLTE-enabled devices will be critical to ensure a frictionless rollout.
With a clear and future-proof IMS Profile for VoLTE set by GSMA, adoption of the technology should not be thwarted by a sense of ambivalence. If you're an operator, there are unwelcome challenges in the quest to enable VoLTE, most of which concern functional, performance and quality aspects. Further, thorough testing and verification of VoLTE-enabled devices will be critical to ensure a frictionless rollout.
Most importantly, however, VoLTE smashes the packet network glass ceiling by demonstrating the inherent ability of 4G to facilitate virtually every form of traffic, from voice to data.
But, even with these challenges, the pursuit of VoLTE is a fruitful endeavour - for both consumers and operators. It is a true generational leap for the stagnant voice experience, introducing benefits such as rapid call establishment time and enhanced quality and battery life.
On the operator side, VoLTE enables the continued exploitation of voice as a cash cow. Not only does the technology allow for more efficient use of spectrum assets, but VoLTE also simplifies network architecture, eradicating the barriers that stand in the way of other solutions such as RCS and video over 4G.
Most importantly, however, VoLTE smashes the packet network glass ceiling by demonstrating the inherent ability of 4G to facilitate virtually every form of traffic, from voice to data.
On the operator side, VoLTE enables the continued exploitation of voice as a cash cow. Not only does the technology allow for more efficient use of spectrum assets, but VoLTE also simplifies network architecture, eradicating the barriers that stand in the way of other solutions such as RCS and video over 4G.
Most importantly, however, VoLTE smashes the packet network glass ceiling by demonstrating the inherent ability of 4G to facilitate virtually every form of traffic, from voice to data.
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Your Technical Guide to WiFi Calling in IrelandTo turn the VoIP tide, operators must strive to maintain calling's best feature - simplicity.
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