Version 2.0

An Industry Whitepaper

Contents

Executive Summary ................................... 1

Introduction to Voice over LTE ..................... 2

The Benefits of VoLTE ............................. 2

Requirements of a VoLTE-Capable PCRF ....... 3

The Anatomy of a VoLTE Call .................... 4

The QoS Class Identifier ....................... 4

The Traffic Flow Template .................... 4

Default and Dedicated Bearers ............... 5

VoLTE Call Flow ................................. 6

Challenges and Opportunities ....................... 9

Signaling Considerations .......................... 9

Verifying Call Quality ............................. 10

Other Quality Considerations ................ 10

Preventing Fraud .................................. 10

Abusing the VoLTE Bearer .................... 10

Abusing a Video Bearer ....................... 11

Universal Policy Control.......................... 11

WiFi Offload..................................... 12

Life after VoLTE ................................... 12

LTE VoIP without IMS .......................... 13

Conclusions ............................................ 14

Related Resources ................................ 14

Invitation to Provide Feedback ................. 14

Executive Summary The Policy and Charging Rules Function (PCRF) is the key element that enables the transition to and reliable operation of VoLTE services. Heavy Reading reports that more than 70 percent of service providers believe they will need to upgrade or replace their existing data PCRF to handle the rigors of VoLTE.

With voice services now sharing the pipe with other data services like web browsing, video and social media, the ability to manage the speed, quality, volume and diameter signaling associated with VoLTE is critical to providing a differentiated experience.

Additionally, although the LTE network provides a framework for signaling application QoS, CSPs need to verify the actual quality experienced by subscribers who have purchased VoLTE services. There is a significant difference between signaling priority and verifying subscriber quality of experience (QoE).

There is also an opportunity for CSPs to consolidate their control plane architecture with a PCRF element that can leverage the LTE QoS architecture to enable such use cases such as non-IMS voice services, gaming, universal fixed and mobile policy control, and WiFi offload. A capable PCRF offers CSPs the opportunity to save costs and deploy additional revenue-generating services that capitalize on the new QoS control capabilities of the LTE network.

This paper first presents a crash course in VoLTE, and then explores the challenges and opportunities associated with this emerging technology.

Voice over LTE: Challenges and Opportunities

VoLTE: Challenges and Opportunities

2

Introduction to Voice over LTE For mobile communications service providers (CSPs), Voice over Long Term Evolution (VoLTE) has emerged as the preferred solution for enabling real time voice traffic in the emerging world of all-IP networks. When this transition is complete, the circuit-switched network for voice communications (in which a dedicated circuit path is reserved for each call) will be replaced by an all-IP network in which flow-based QoS informs network resources of quality requirements to ensure call quality.

The motivation to completely transition to this all-IP approach is strong: until CSPs can support real-time IP voice services that meet the high expectations for call quality and reliability set by traditional circuit-switch networks, they will be burdened with the huge capital and operating expenses of maintaining two separate networks. As a consequence of this commercial driver, VoLTE usage is projected to soar: by the end of 2014, there are expected to be 59.6 million VoLTE subscriptions in place, and it is forecasted that nearly 56 percent of LTE-related cellular subscriptions will be using VoLTE services by the end of 2019.1

The Policy and Charging Rules Function (PCRF) is the key network element that enables the transition to and reliable operation of VoLTE services; to enable VoLTE the PCRF must include a number of conditions, and early generation PCRFs may fall short. For their part, CSPs are aware that additional investment may be necessary: Heavy Reading reports that more than 70 percent of service providers believe they will need to upgrade or replace their existing data PCRF to handle the rigors of VoLTE.

2

With voice services now sharing the data pipe with other data services like web browsing, video streaming, and social media, the ability to manage the speed, quality, volume and diameter signaling associated with VoLTE is critical to providing a positively differentiated experience.

Additionally, although the LTE network provides a framework for signaling application QoS, such signalling isn’t a true guarantee of call quality. To ensure a strong customer experience, CSPs need to verify the actual quality experienced by subscribers who have purchased VoLTE services, so that they can take immediate action if quality is not as high as intended or expected. In short, there is a significant difference between signaling priority and verifying subscriber quality of experience (QoE).

With the adoption of VoLTE, there is also an opportunity for CSPs to consolidate their control plane architecture with a PCRF element that can leverage the LTE QoS mechanisms to protect the quality of applications beyond VoLTE. Such a capable PCRF offers CSPs the opportunity to save costs and deploy additional revenue-generating services that capitalize on the new QoS control capabilities of the LTE network.

For instance, in the same manner that VoLTE sets QoS for the CSP’s voice services, a CSP could use the same PCRF to enable QoS for non-IMS (IP Multimedia Subsystem) applications (e.g., third-party voice services, gaming, video and audio streaming, etc.), to apply universal policy control across all access networks (i.e., not just LTE), and to conserve radio resources by enabling WiFi offload.

The Benefits of VoLTE For both subscribers and network operators, VoLTE offers major benefits. An independent research study by Signals Research Group3

1 ABI research

analyzed VoLTE performance in a commercially active VoLTE network

2 See this report. 3 The SRG study can be accessed here. Alcatel-Lucent summarizes the report findings here.

VoLTE: Challenges and Opportunities

3

with visibility of radio access, core and IMS, including the primary VoLTE functionality. The report evaluated call setup time, reliability, quality, the network resource requirements, and the impact on device battery life. The study produced the following insights:

• VoLTE call quality greatly exceeded that of 3G circuit-switched voice and was measurably higher than the HD voice service offered by Skype4

• With network loading (i.e., lots of competing traffic), and in particular with background applications running on the mobile phone and transferring data with the network, the VoLTE results were considerably better than Skype

• VoLTE call setup time was nearly twice as fast as 3G Circuit Switched Fallback (CSFB) call setup • VoLTE used substantially fewer network resources than Skype voice, which in turn resulted in

longer estimated device battery life for the subscriber and a more efficient network for CSPs • When leaving LTE coverage, VoLTE calls were successfully handed over to 3G circuit-switched

voice, ensuring calls continued without interruption Ultimately, then, subscribers benefit from a high quality of experience and improved device battery life, while operators enjoy greater delivery efficiency and happier subscribers.

Requirements of a VoLTE-Capable PCRF The table below summarizes the must-have features of a VoLTE-capable PCRF:

Table 1 - Requirements of a VoLTE-Capable PCRF

Function Explanation of Features

Voice Services

• Premium call quality with dedicated-bearer set up and teardown with dynamic quality of service (QoS) that offers a competitive quality advantage over OTT voice services

• Differentiated VoLTE services: meter voice and data separately to create differentiated service plans; enable flexible QoS to support regional, subscriber or device-specific policies

• HD Voice

Emergency Services and Location Information

• Allow IMS emergency call priority and data plan override; high QoS by default when invoked

• Ensure access to reliable emergency call support; 3GPP standards for network-provided subscriber location information (NPLI)

VoLTE Roaming

• Global roaming coverage for VoLTE services with guaranteed QoS • Local breakout support: supplying policy rules and charging parameters to

outbound / inbound roamers with guaranteed VoLTE QoS • Identify subscriber location and apply appropriate QoS / charging parameters

Scalability and Reliability

• Support for the always-on nature of the VoLTE dedicated bearer and increased number of concurrent voice sessions

• Handle the ten-fold increase in diameter signaling generated by VoLTE call flows • Manage increased performance requirements as more VoLTE services and devices

are on-boarded • Reduce signaling with policy decision point capability in the data plane • Circuit-switched fall back (CSFB) in case the LTE network is not available

Regulatory, Standards, and Device Requirements

• Support Rx call flows in real time to ensure highest available media codec on a per-device basis

• 3GPP standards-based solution; includes comprehensive 3GPP R7 through R12 support

• Modify and customize VoLTE policy rules per device: reject improper calls, adjust

4 When considering quality, the absolute rock bottom minimum for an CSP-provided voice service is the quality delivered by over-the-top alternatives

VoLTE: Challenges and Opportunities

4

QoS parameters when needed • Dual stack IPv4 and IPv6 support with intelligent NAT integration: deploy a future-

proof solution with support for multiple IP address allocation and a broad range of VoLTE devices

• Fraud prevention: CSPs need to guarantee the proper utilization of dedicated bearers and minimize exposure to revenue leakage and fraud

• Lawful intercept: support regulatory requirements for identification and filtering of targeted voice traffic

The Anatomy of a VoLTE Call VoLTE is based on two separately introduced 3GPP standards: IP Multimedia Subsystems (IMS), first introduced in 3GPP UMTS Release 5; and LTE, which was first introduced in 3GPP UMTS Release 8. IMS does not depend on the existence of LTE nor does LTE rely upon IMS, but VoLTE can be thought of as a process that couples IMS and LTE to create an environment capable of supporting high quality voice traffic in a shared packet data network.

The IMS network is the master controller for VoIP calls on an LTE network in the sense that it is IMS that recognizes the need for special network conditions required to support voice traffic. The LTE network takes instruction from the IMS network using the Session Initiation Protocol (SIP)5

The QoS Class Identifier

to establish call connections with appropriate QoS. With VoLTE, IMS directs LTE to establish the desired QoS environment and initiates the voice call. IMS also notifies LTE when the call has completed, and directs LTE to tear down the special voice environment.

The QoS Class Identifier (QCI) specifies the level of acceptable latency for different types of traffic, as shown by Figure 1, and is specified in a Traffic Flow Template (TFT) triggered by the PCRF element.

Figure 1 – LTE QCI Values6

The Traffic Flow Template

Speaking in UMTS terms, in 3GPP a TFT is a classifier that matches on fields on the inner-IP of a GTP-U tunnel (or dedicated bearer). Whether using the static-TFT model or the dynamic Gx-signaled TFT model, the same sequence occurs: a dedicated bearer (secondary PDP context) is created, and specific application traffic is forced to match it.

5 Defined in RFC 3261. 6 See a full breakdown here.

VoLTE: Challenges and Opportunities

5

Figure 2: TFT mapping to PDP context on 3G (dedicated bearer analogous to secondary PDP context)

The TFT is where CSPs create definitions for how specific application traffic, including VoLTE, will be treated based on pre-set policy conditions.

Default and Dedicated Bearers A bearer is a network mechanism that enables the same IP Cellular Access Network (CAN) session to discriminate both quality and charging for different applications. When an LTE device attaches to the network for the first time, it is assigned a “default bearer” which remains as long as the device is attached. Devices can have more than one default bearer, but each default bearer has a separate, unique IP address. The default bearer does not provide Guaranteed Bit Rate (GBR), and non-GBR QCI values 5 to 9 can be specified.

A dedicated bearer is essentially a dedicated tunnel for one or more specific applications (e.g., VoIP, video, gaming, etc). A dedicated bearer does not require a separate IP address, and uses the IP address associated with the previously-established default bearer. The TFT is used to specify quality settings for a specific traffic application carried on a dedicated bearer, which can be GBR or non-GBR depending on the QCI value chosen to support a specific use case.

Figure 3 – Default and Dedicated LTE Bearers7

7

Ibid.

VoLTE: Challenges and Opportunities

6

In the all-IP network, the dedicated bearer determines QoS for a particular application. In the case of VoLTE, the hard-wired certainty of the circuit-switched network is replaced by the QoS guarantee of a dedicated bearer with a QCI value of 1. With VoLTE there are also typically two default bearers – one is used for Session Initiated Protocol (SIP) messages related to the IMS network (QCI value of 5), and the other default bearer, established upon connection, is used for all other smartphone traffic (video, chat, email, browsing, etc.) on the LTE network. The SIP signaling component requires its own default bearer (with associated unique IP address) because the IMS network is separate from the LTE network and comes with its own APN. This also helps structure the moving parts of the IMS-LTE coupling process that enables VoLTE calls.

Figure 4 – VoLTE architecture – two networks

There are three main examples of how a VoLTE call can take place. First, there is the situation where a caller calls a callee in another network, in which case both networks handle half the signaling required to apply QoS. When a caller calls a callee within the same network, signaling for both sides of the call is handled by a single network. The third example is where the network uses the circuit switched fall back to establish the call.

VoLTE Call Flow When a subscriber turns on their VoLTE-enabled device (e.g., smartphone), it connects to the LTE network infrastructure and is assigned two default EPS (Evolved Packet Switch) bearers – one for SIP signaling with a non-GBR QCI value of 5, and another for the LTE network with a non-GBR QCI value from 5-9. This dual-bearer approach allows a VoLTE smartphone to communicate in the languages of both LTE and IMS (SIP).

The call flow to enable VoLTE is shown below.

VoLTE: Challenges and Opportunities

7

Figure 5 – VoLTE architecture and call flow

1. The mobile subscriber indicates on their LTE-enabled smartphone the desire to make a VoIP call. 2. LTE identifies a PDN Gateway (P-GW) that offers a connection to the IMS network. 3. LTE establishes a Default bearer for SIP from the subscriber to the selected P-GW.

The default EPS bearer is established with a QoS Class Identifier (QCI) value of 5 (the QCI value required for SIP signaling).

4. The smartphone sends a SIP “Invite” message toward the IMS network. Contained in the SIP message is a Session Description Protocol (SDP) that carries the QoS requirement.

Note that although SIP messages are carried through the LTE network, the LTE network is unaware of the content of the message (nor the need for special QoS treatment at this stage).

5. The IMS network extracts the required QoS setting from the SIP message. 6. If a charging policy applies, then the IMS network sends an initial diameter Credit Control Request (CCR) to

the OCS over the Ro interface and an initial amount of credit is reserved anticipating the need to precisely meter flow data during the call.

7. The QoS requirement is sent from the IMS network through the Rx interface (using the Diameter protocol) to the PCRF.

8. The PCRF creates actionable charging and QoS rules and forwards these across the Gx interface to the Policy and Charging Enforcement (PCEF) that lives with the P-GW in the LTE network.

9. The P-GW now sends a request to establish a separate “dedicated bearer” (with a QCI value of 1) to the smartphone.

10. After the smartphone confirms that LTE can support the new dedicated bearer, it sends a SIP “UPDATE” message to the IMS network.

11. The IMS network completes the setup process and establishes the call. 12. Bidirectional VoIP call packets flow inside the LTE network (to the P-GW) and smartphone.

VoLTE: Challenges and Opportunities

8

13. For charging, the IMS network requests credit from the OCS throughout the call (e.g., every 10 seconds). If credit does not exist, a 402 (payment required) message is sent back to the smartphone and the call is cancelled. If credits expire during the call, it is terminated.

14. When the call terminates, the smartphone sends a SIP “BYE” message to the IMS network. 15. The IMS network sends a diameter CCR termination request to the OCS, which ends the charging metering and

triggers actions to collect IMS billing records. 16. The IMS network notifies the PCRF of call termination. 17. The PCRF tells the PCEF to close out the LTE billing, and instructs the P-GW to tear down the dedicated

bearer established for the VoIP call.

VoLTE: Challenges and Opportunities

9

Challenges and Opportunities This section explores the many challenges presented by the requirements of VoLTE implementations, and the opportunities that will be presented as CSPs introduce VoLTE-capable systems.

Signaling Considerations A key challenge specific to VoLTE is the over 10-fold increase of signaling load on the control plane and PCRF element, which must specify QoS for every single voice call passing through the LTE network. When one considers the addition of non-voice application services such as streaming video and online gaming, the signaling load increases further.

A detailed report from Oracle shows that global LTE Diameter signaling traffic will grow at a 78 percent compound annual growth rate (CAGR) from 2013 to 2018, expanding from 12 million messages per second (MPS) to nearly 216 million MPS. In the same report, Oracle predicts LTE Diameter signaling traffic will increase at a 140 percent compound annual growth rate (CAGR), from 1.2 million messages per second (MPS) in 2012 to nearly 99 million MPS by 2017.

As mentioned earlier, the majority of CSPs report that their legacy PCRF implementations are inadequate. In most cases, the CSPs have assessed their systems either to be not compliant with the latest 3GPP standards specifying VoLTE or simply incapable of handling the transaction rates required to manage a VoLTE offering.

One way to significantly reduce signaling at the front end is with data plane decision-making: the PCRF signals an overall service plan identifier to a data plane PCEF, which hosts TFTs that manage the actual QoS required for thousands or even millions of VoLTE- and QCI-related flows per second.

Figure 6 – Control and Data Plane PDP reduces signaling and service latency

VoLTE: Challenges and Opportunities

10

Verifying Call Quality To ensure a positive subscriber quality of experience for VoLTE calls, CSPs require a feedback mechanism that measures the actual (i.e., delivered) quality. With these measurements in place, CSPs can then compare the delivered quality to the QCI value and corresponding quality expectation (see Figure 1, “Packet Delay”). If delivered quality is falling short, then this feedback mechanism allows CSPs to diagnose and correct the issue.

Beyond simply having latency and mean opinion scores (MOS) in place for VoIP, CSPs can also benefit from insight that extends into device-level segmentation. This way, issues that are isolated to particular operating systems, manufacturers, or models, can be quickly identified and addressed.

Other Quality Considerations CSPs can also benefit from having a clear view of the effect VoLTE has on default bearer quality when a large number of calls are being processed. VoLTE calls take precedence with a guaranteed bit rate, but how this affects overall capacity in the access network is something CSPs need to understand for capacity planning. In other words, what happens to the rest of the traffic when VoLTE calls consume (or at least reserve) considerable bandwidth and network resources?

Preventing Fraud There is a real potential for fraud by users who know how to mimic the QCI framework; by doing so, these users can potentially request a specific treatment of data unintended or authorized by the CSP. Figure 7 demonstrates a possible scenario where the QCI-signaled dedicated VoLTE bearer and/or video bearer are manipulated to bypass charging and override the CSP’s intended quality for a specific data type.

Figure 7 - Possible results of fraudulent bearer manipulation

Abusing the VoLTE Bearer A malicious user could establish a dedicated “VoLTE bearer” to transport non-VoLTE traffic by manipulating the applications or operating systems on two devices. The two devices would

VoLTE: Challenges and Opportunities

11

theoretically be able to transmit any data to each other. Since VoLTE bearers are typically (or at least often) zero-rated, then this user would be experiencing free data and the CSP would lose revenue. Additionally, the high guaranteed bitrate for the VoLTE bearer would also let the malicious user experience reserved bandwidth and protected quality for traffic that does not warrant such treatment.8

The same feat could be accomplished even if one of the devices is a VoLTE soft client running on a PC. It is possible for guaranteed delivery to be applied to traffic not authorized by the CSP to receive such specialized treatment.

As a potential solution to this type of bearer fraud, a PCEF or TDF could be used to verify the content of each bearer to make sure it matches up with what is expected.

Abusing a Video Bearer A similar situation could exist with a QCI-signaled video bearer arranged to transport non-video traffic. A video bearer allows much higher bandwidth rates and, depending on the charging plan, the CSP network could unintentionally zero-rate the data.

Universal Policy Control Converged network operators (i.e., those with multiple access technologies) face a number of challenges relating to implementing network-wide policy control. It is not uncommon for these operators to have a different policy control solution in each access type; however, in addition to the onerous training and maintenance overhead, this fractured approach means a single network-wide policy (for instance, to enable quotas across multiple access types, or to zero-rate sponsored data) must be defined in many different locations – a process that is both operationally intensive and prone to error.

As noted previously, many CSPs are planning on upgrading their PCRF infrastructure to support LTE. During this same investment cycle, if they choose the right PCRF vendor then they can dramatically simplify their overall policy control implementation.

Figure 8 shows a network with three access technologies: cable, WiFi, and LTE. Typically, such a network would have at least two separate policy control solutions: a PCRF for the LTE network (and likely for the WiFi network), and a PCMM (PacketCable Multimedia) policy controller for the cable network. Both of these systems fulfill the same functions: making and enforcing policy decisions.

However, Figure 8 only depicts a single policy controller: a PCRF that ‘speaks’ both the LTE and PCMM standards. This ‘universal’ policy controller dramatically simplifies policy control in this network. As added benefits, this universal policy controller contributes to reduced control plane complexity and a smaller solution footprint.

8 A VoLTE bearer will typically be zero-rated at the PGW via Gy, since charging is based on the Ro interface between the PCRF and IMS network, and the VoLTE bearer it specifies GBR for the traffic.

VoLTE: Challenges and Opportunities

12

Figure 8 – Converged network with LTE, Cable, and WiFi access

WiFi Offload The rapid growth of data-hungry smartphones and tablet computers has created the need for what we today refer to as seamless WiFi offload. By offloading mobile network traffic onto a WiFi network, CSPs can prevent or relieve congestion and free up valuable radio network resources.

To enable WiFi offload, mobile operators with WiFi networks have introduced SIM authentication (EAP-SIM/AKA) to automatically log users onto the WiFi network when coverage is detected.

The universal policy control depicted in Figure 8 also simplifies WiFi offload use cases and ensures uniform application of network-wide management and charging policies.

Life after VoLTE There are many opportunities to further monetize VoLTE services, and to leverage the LTE QCI framework (provided the overall network policy control solution has the right capabilities), including:

• Bundled offers: premium voice with data services • Time and volume offers: charging per minute/second/byte • Prioritized gaming, video, etc. • QoS for other VoIP services (e.g., Push to Talk over Cellular) • Location-based offers: differentiating voice charging based on congestion levels • Tiered services: quality of experience based on different packages and plans • Dynamic offers: real-time value offerings promoted on a dynamic basis • Roaming offers: tailored offerings based on partnerships

VoLTE: Challenges and Opportunities

13

CSPs can also use the LTE QCI framework to efficiently signal QoS to arbitrary and over-the-top applications such as video streaming and online gaming. Subscribers could pay additional revenue to CSPs for prioritization of their gaming traffic, signaled using a TFT that specifies a QCI of 3 for all gaming flows.

In effect, the QCI signaling framework can be used to specify QoS for any traffic flow where end-to-end state can be maintained by an overall network policy control solution.

LTE VoIP without IMS CSPs are free to leverage network policy control and QCI signaling to launch VoIP services over LTE without needing the IMS aspect, thereby saving much in terms of cost and complexity.

How would QoS be achieved in this environment? While it does not work in the current PCRF framework, an intelligent PCRF coupled with data plane PCEF that supports PDP could provision a service to set up a VoIP call where the QCI parameter is signaled directly from the PCRF to the PCEF instead of through the IMS network.9

Furthermore, the concept can be extended to any non-IMS service such as gaming and video streaming services, provided a policy control platform (e.g., PCEF or TDF) that supports PDP exists in the data path to identify traffic.

This means no Rx and no Ro, and it could be a much more cost-effective means to provide a VoIP service.

9 This is not supported by PCRF standards today, but can be supported through traditional DPI-based network policy control if the PCEF can negotiate the required QoS.

VoLTE: Challenges and Opportunities

14

Conclusions With voice services now sharing the pipe with other data services like web browsing, video and social media, the ability to manage the speed, quality, volume and diameter signaling associated with VoLTE is critical to providing a differentiated experience.

In addition, although the LTE network provides a framework for signaling application QoS, CSPs need to verify the actual quality experienced by subscribers who have purchased VoLTE services. There is a significant difference between signaling priority and verifying subscriber quality of experience (QoE).

CSPs can use the necessity of upgrading to VoLTE-capable PCRFs as an opportunity to truly revolutionize their policy control solutions. For instance, by investing in a universal policy controller with close integration to an intelligent PCEF, CSPs can:

• leverage the LTE QoS architecture to enable use cases including quality-protection for non-IMS voice services, video, and gaming services

• simplify policy control across all access technologies, by having a single system that can make decisions and enforce policies across multiple standards

• preserve radio resources by implementing seamless WiFi offload

Related Resources In addition to the documents and resources cited throughout the footnotes, please consider reading these other Sandvine papers.

• For additional information about policy control and LTE, refer to the Sandvine whitepapers Quality of Service in LTE and Network Policy Control and the Migration to LTE

• Detailed information about Sandvine’s Service Delivery Engine acting as a universal PCRF to enable VoLTE services, see the technology showcase VoLTE and the Service Delivery Engine

Invitation to Provide Feedback Thank you for taking the time to read this whitepaper. We hope that you found it useful, and that it contributed to a greater understanding of VoLTE technology.

If you have any feedback at all, then please get in touch with us at whitepapers@sandvine.com.

Copyright ©2015 Sandvine Incorporated ULC. Sandvine and the Sandvine logo are registered trademarks of Sandvine Incorporated ULC. All rights reserved.

European Offices Sandvine Limited Basingstoke, UK Phone: +44 0 1256 698021 Email: sales@sandvine.co.uk

Headquarters Sandvine Incorporated ULC Waterloo, Ontario Canada Phone: +1 519 880 2600 Email: sales@sandvine.com