NSN White paper November From Voice over IP to Voice over LTE

Transcription

1 NSN White paper November 2013 From Voice over IP to Voice over LTE

2 CONTENTS 1. Introduction 3 2. VoLTE markets 4 3. VoLTE technology VoLTE user experience VoLTE talk time VoLTE service quality VoLTE coverage VoLTE traffic considerations in network design Quality of Service Use of radio resources Signaling capacity IP packet forwarding capacity 8 4. Cellular Voice over IP alternatives OTT VoIP services Best effort SIP VoIP OTT VoIP performance in LTE networks OTT VoIP user experience OTT VoIP user experience VoLTE comparison with VoIP alternatives in LTE 6. Conclusions About Smart Labs Page 2

3 1. Introduction The speed with which operators have adopted Long Term Evolution (LTE) and the rapid growth of LTE subscriptions in advanced markets are testament to the technology s success. In May 2013, the Global mobile Suppliers Association (GSA) reported 175 commercial LTE networks. At the end of 2012, there were already close to 70 million LTE subscriptions across the globe. LTE has become a truly mobile access method for various data applications and services. The first LTE devices were modems such as USB dongles for PCs, yet today, most LTE devices belong to the smartphone category. Voice is naturally a key service for a LTE smartphone user, but in most of today s commercial LTE networks, voice is still based on traditional Circuit Switched (CS) voice in 2G/3G networks. However, this is not only an issue of network readiness, as according to GSA, of the 261 LTE smartphones announced at the end of March 2013, only a few currently support VoLTE. As LTE network coverage continues to expand, the next important step for operators is to deploy a seamless voice over LTE (VoLTE) service. Voice is still a major source of revenue for operators, even in the most advanced mobile broadband markets. This means that operators must carefully plan how their voice and mobile broadband businesses are developed. Over the top (OTT) Voice over IP (VoIP) is an alternative to CS voice for many subscribers, because wide coverage HSPA and LTE networks and operators mobile broadband data services enable mobile use of free OTT VoIP services. However, VoLTE brings many benefits to help operators ensure their voice service remains the most attractive solution for most mobile subscribers. This paper introduces selected VoLTE technology features that affect the user experience and network performance. There are numerous existing white papers that, for example, describe VoLTE architecture, compare deployment alternatives or evaluate power consumption of VoLTE smartphones. This paper differs by describing those VoLTE features that can improve the user experience and network performance compared with OTT VoIP services such as Skype or alternative SIP VoIP solutions. It is recommended that VoLTE smartphones are tested beyond basic VoLTE Inter-Operability Testing (IOT) in order to evaluate VoLTE smartphone performance and VoLTE competitive advantages against alternative solutions. Page 3

4 2. VoLTE markets Mobile broadband (HSPA and LTE) is going mainstream and LTE is the most rapidly adopted mobile technology ever, with the GSA forecasting 234 commercial LTE networks in 83 countries by the end of LTE smartphones are commonly available in many markets, but VoLTE is still emerging. This is because most operators initially offer CS voice for LTE smartphone subscribers using CSFB (Circuit Switched Fallback) in LTE-GSM/WCDMA and LTE-CDMA networks or SVLTE (Simultaneous Voice and LTE) in LTE-CDMA networks. The traditional operator safe havens of voice and messaging have been under attack by OTT service providers over the last few years. Therefore, operators strategies for maintaining profitable business include developing the mobile voice service combined with an enhanced end-user experience. The whole telecommunications industry thus has a strong focus on VoLTE and the evolution of rich communication. LG U+, SKT (Korea) and metropcs (USA) were the first operators to start a commercial VoLTE service in August KT (Korea) was next to launch in October Currently, the Korean VoLTE market is developing the most swiftly, with about 2.8 million VoLTE subscribers in March 2013 and a high monthly growth rate. Furthermore, many operators worldwide have also been testing VoLTE Korea VoLTE subscribers Thousands SKT LGU+ KT Jan Feb Mar 2013 Fig. 1. Korea VoLTE subscribers Source: KCC, April 2013 Korea s position as the leading VoLTE market is also highlighted by the growing selection of VoLTE smartphones available. Currently, these VoLTE smartphones are dedicated for Korean operators and networks, with all these Korean VoLTE devices running Android. Finally, it is worth mentioning that VoLTE is strongly linked to the evolution of RCS (Rich Communication Services), because both are IP Multimedia Subsystem (IMS) based services. In the first phase, RCS provides contacts, chat, file sharing and video sharing services. RCS has been launched by operators in Korea, Europe and the USA. Page 4

5 3. VoLTE technology 3.1 VoLTE user experience VoLTE talk time Battery life is a top concern for today s smartphone users. Broadband radios, large touch screen displays and gigahertz multi-core processors consume a lot of energy. Therefore, optimizations that improve battery life are very important for both mobile operators and smartphone vendors. The first VoLTE implementations received much public attention, because the VoLTE talk time seemed to be worse than with traditional CS voice services. However, the early implementations were not optimized for low current consumption. The main components affecting current consumption during voice calls are cellular radio and voice codec components. Usually the display is turned off by a trigger from either a proximity sensor or timer. If a VoLTE audio codec is integrated to the modem processor, VoLTE power consumption can be lower than with OTT VoIP applications, which are running in the application processor. Cellular radio transmission and reception can be optimized significantly to reduce energy consumption. The biggest energy saving is achieved by shutting down the transmission and reception whenever possible. During a voice call, this is possible by using Discontinuous Transmission (DTX) and Discontinuous Reception (DRX). Standard voice is packetized in 20 ms intervals i.e. each voice packet includes 20 ms of voice. Cellular radio resources are divided in the time domain into Transmission Time Intervals (TTI), which in LTE are as short as 1 ms. Because LTE is broadband technology, it is possible to send one voice packet within one TTI. Therefore, a VoLTE smartphone can shut down transmission and reception between voice packets. Further opportunities for DTX/DRX can be achieved using packet aggregation, which means that two voice packets are sent in one TTI every 40 ms. 20 ms TTI 1 ms VoIP packets 40 ms TTI 1 ms Two VoIP packets aggregated UE enb UE enb UE discontinous reception (20 ms DRX cycle) Sleep mode between voice packets saves energy UE discontinous reception (40 ms DRX cycle) Sleep mode between voice packets saves energy Fig. 2. Overview of VoLTE transmission and DTX/DRX. Page 5

6 3.1.2 VoLTE service quality VoLTE supports mobile high definition (HD) voice i.e. the Adaptive Multi Rate Wideband (AMR-WB) codec, which is already used in tens of WCDMA/HSPA networks as well as in a few GSM networks. The wideband codec improves voice quality by transmitting a broader spectrum of human voice frequencies than narrowband codecs. Perceived voice quality depends on the audio codec used and the mouth-to-ear delay, as well as transmission impairments such as jitter, bit errors and packet loss. VoLTE improves performance, particularly during busy hours, by establishing a dedicated Guaranteed Bit Rate (GBR) bearer for voice. The GBR bearer offers a low latency and low jitter connection. Voice service experience is further affected by the call-setup performance and call reliability. LTE can improve mobile voice callsetup time significantly due to the high capacity and low latency LTE radio access available for the signaling connection between the UE and voice service core VoLTE coverage In poor radio conditions, data can be received with errors. This effect is particularly visible at the cell edge due to the limited UE transmission power. In order to improve the reliability of VoLTE packet transmission in the uplink direction, it is possible to use a technique called Transmission Time Interval (TTI) bundling. This relies on sending a few redundant versions of the same set of bits in consecutive TTIs. TTI bundling is estimated to provide 2-4 db uplink coverage improvement for VoLTE. LTE coverage will not be as wide as, for example, GSM for several years. When VoLTE service is deployed, mobility at the border of the LTE coverage area with GSM/WCDMA is solved with Single Radio Voice Call Continuity (SRVCC) technology, which differentiates the VoLTE service experience from alternative VoIP services. Page 6

7 3.2 VoLTE traffic considerations in network design Quality of Service Traffic between the UE and the network is carried over bearers, which can have different QoS characteristics. When a LTE UE attaches to the network for the first time, it will be assigned default bearers, which will remain as long as the UE is attached. Typically, for an IMS/ VoLTE based network, there is a default bearer for IMS signaling and a default bearer for Internet traffic. Both are non-guaranteed Bit Rate (non-gbr) bearers. IMS signaling bearer has higher priority. When a VoLTE call is setup, a dedicated GBR bearer is established for the voice connection. Thus, QoS differentiation must be taken into account in VoLTE network design and dimensioning. In contrast, OTT VoIP applications always run on the default bearer for Internet access and therefore differentiated QoS cannot be guaranteed Use of radio resources There are two options for scheduling of VoLTE packets over the radio interface. Dynamic scheduling (DS) is designed for data applications. DS enables efficient use of radio resources for bursty traffic, but due to dynamic resource allocations, control channel information must be sent along the data transmission both in uplink and downlink directions. To overcome this, the LTE base station can use an alternative scheduling method called Semi Persistent Scheduling (SPS), which assigns predefined radio resources for the VoLTE user. SPS does however, have some drawbacks. It must use fixed link adaptation and a fixed resource block in the frequency domain, which prevents any advanced link adaptation or scheduling. SPS is also not well suited for mixed voice and data traffic, because fixed SPS allocations limit data scheduling. Another method to reduce the control overhead is packet aggregation, resulting in a 40 ms transmission interval. As a conclusion, in realistic network deployments with mixed VoLTE and data usage, instead of using SPS, the optimal solution is to use packet aggregation with dynamic scheduling. Use of Robust Header Compression (RoHC) with a GBR bearer will reduce the user plane traffic by several bytes over the air interface. For VoIP packets, the size of IP headers (IP/UDP/RTP) is larger than the voice payload itself. RoHC can compress the header size from 40 bytes to two or three bytes between the user device and base station. This high compression ratio not only increases network capacity but also provides coverage improvements for VoLTE users compared to OTT VoIP of up to 3 db due to the lower bandwidth required at the cell edge. Page 7

8 3.2.3 Signaling capacity There is no frequent background keep-alive traffic associated with IMS based services, which is an advantage over OTT VoIP services. OTT VoIP apps (Viber, Skype, etc.) must maintain active sessions with keep-alive messages in order to stay reachable for incoming calls. These frequent keep-alive transactions eventually result in large signaling load in the network. With IMS services, the client device performs periodical re-registrations to the IMS, but the frequency is significantly lower than that of OTT apps. A VoLTE service requires additional signaling for setting up a dedicated GBR bearer to fulfill the QoS requirement, but the expected network impact is low. It is worth noting that smartphone platforms such as ios, Android and Windows Phone tend to have platform specific connections (e.g. to get notifications and automatic software updates), which generate data transactions and signaling load. The impact of VoLTE on signaling load is therefore assumed to be negligible IP packet forwarding capacity Generally, trends in mobile broadband traffic growth indicate that voice will play a minor role in total traffic volume, with video content and browsing being the main contributors to mobile data volumes. Although VoLTE and OTT VoIP are not significant services when the throughput capacity of network (bit/s capacity) is dimensioned, they can have a relatively high effect on the dimensioning of IP packet forwarding elements such as S/PGW (packet/s capacity). Page 8

9 4. Cellular Voice over IP alternatives OTT communication services are challenging traditional mobile voice and messaging services. Therefore, operators must take OTT communication services into account in their strategies. Operators can compete with OTT services, but can also collaborate with OTT service providers. In both cases, operators must understand how the user experience with OTT services compares with the operator communication services, including VoLTE. Operators should also understand how the growing adoption and use of OTT communication apps affects the mobile network. 4.1 OTT VoIP services OTT refers to services provided independently over the mobile operators Internet access services. Skype is probably the most well known OTT VoIP service. This and many other VoIP services are today commonly available from application stores for all major smartphone platforms. Many OTT applications that support VoIP also include rich communication features, such as instant messaging, file sharing, presence and video calls. Table. 1. Examples of OTT VoIP applications SKYPE VIBER KAKAOTALK FACETIME TANGO TuMe Voice call X X X (X) X X Group call X X Video call X X X Group video call X Chat X X X X X Group chat X X X X File sharing All Files Multimedia Multimedia Multimedia, Dropbox Smartphones, tablets Other devices PSTN interworking ios, Android, Windows Phone, Blackberry, Symbian PCs, TVs, ipod touch, PlayStation Vita, Skype handsets, Xbox One Premium services available ios, Android, Windows Phone, Blackberry, Symbian, Bada ios, Android, Windows Phone, Blackberry, Bada ios PCs ipod touch ipod touch, Mac PCs Viber client places a regular call, if VoIP is not available ios, Android, Windows Phone PCs, ipod touch ios, Android Page 9

10 4.2 Best effort SIP VoIP While commercial VoLTE is still emerging, operators can consider offering VoIP services based on IMS infrastructure and SIP VoIP clients over best effort mobile broadband access. Such a pre-volte service could be offered over LTE as well as over HSPA and Wi-Fi. The Pre- VoLTE phase is an opportunity to develop the SIP VoIP technology and the VoIP business, particularly in markets with limited LTE coverage or in markets missing standard VoLTE smartphones. The availability of downloadable and configurable SIP compliant VoIP clients is limited to major smartphone platforms and application markets. For example, Bria and Acrobits are SIP VoIP clients, which are available for both ios and Android platforms. 4.3 OTT VoIP performance in LTE networks This chapter includes some highlights of OTT VoIP performance based on Smart Labs measurements and analysis OTT VoIP user experience Voice quality is a very important criterion in providing a satisfactory voice service. CS mobile voice has not been excellent, due, for example, to quite a narrow voice spectrum. Wideband codecs improve the quality and therefore wideband audio is used by some OTT VoIP services. If we compare the voice quality in excellent radio conditions in an empty cell by measuring the MOS (mean opinion score), some OTT VoIP applications provide better speech quality and some provide worse quality than CS voice. Conversation quality depends on the mouth-to-ear delay. Latency of mobile CS voice depends on network implementation details, including physical distances between network elements. Typical CS voice latency in commercial networks is ms, which is good for smooth conversation. OTT VoIP clients must have adequate jitter buffers to manage the best effort quality in today s mobile broadband networks. Furthermore, OTT VoIP services must have a connectivity solution for the varying Network Address Translation (NAT) and firewall environments of mobile networks. Therefore, OTT VoIP calls may have to be routed via gateway nodes on the public Internet, which increases the mouth-to-ear delay. Measurements show that OTT VoIP services pose challenges in offering satisfactory conversation quality, even in a very low latency LTE network. Page 10

11 NAT traversal can be managed by OTT services, but the complex NAT traversal solutions can increase call setup times. Measurements and analysis show that OTT VoIP call establishment time varies a great deal, in particular due to the NAT traversal protocols. Battery life is also an important customer experience factor for operators, because network settings affect smartphone current consumption and users can detect the difference between mobile service providers. With VoIP services, the smartphone current consumption can be measured during a call and during periods of user inactivity. Because an average user can spend some tens of minutes in voice calls daily, it is more important to compare the current consumption caused by background activity of a VoIP application. Measurements show that the background activity of different OTT VoIP applications varies significantly, with a noticeable effect on the UE battery life time. Current consumption during a OTT VoIP call depends mainly on the hardware components of the smartphone, as in practice, HSPA or LTE radio must be continuously in a high power connected state (Cell_DCH or RRC connected respectively). On the other hand, VoLTE is specified to support a connected state DRX during the call. Therefore, VoLTE is expected to have lower current consumption during the call than any OTT VoIP service OTT VoIP network impacts Traffic characteristics during a VoIP call depends on the voice packetization interval and silence suppression capability, as well as other protocols that may be active while voice media is transmitted. Average packet inter-arrival time (or packets per second rate) is important for dimensioning high capacity packet forwarding elements such as packet core gateway elements. OTT VoIP traffic can be analyzed even though the content can be encrypted. Measurements show that OTT VoIP apps can generate similar traffic to VoLTE with a 20 ms packetization interval. However, in some cases even a doubled packet rate is generated. Page 11

12 Another traffic characteristic is the average bit rate (kbit/s) during a VoIP call. This is affected by the codec and IP overhead. Measurements indicate that OTT VoIP applications can generate two to three times the bit rate of 12.2 kbit/s AMR codec VoIP traffic. RTP/UDP/IPv4 overhead is 40 bytes per voice packet, which for example with kbit/s AMR-WB voice results in around 29 kbit/s IP traffic in one direction. The actual average bit rate with AMR can be significantly less due to silence suppression. The gain of silence suppression can be estimated assuming 60% voice activity. Currently, some OTT VoIP applications support silence suppression, but not all of them yet do so. One of the key concerns of operators is the signaling load generated by smartphones. Part of the load is caused by the mobile operating system services, but a significant proportion is generated by online applications. OTT communication services are particularly challenging, because always-on reachability requires a persistent connection to the network. OTT VoIP applications are a common reason for frequent data transactions. These transactions include, for example, keepalive messages for the persistent connections, but there is also other application specific activity. The exact behavior of the OTT apps varies depending on the OS platform and the application version. Measurements show that some OTT VoIP apps generate frequent data transactions, which increase the signaling load in mobile networks. Page 12

13 5. VoLTE comparison with VoIP alternatives in LTE The following table summarizes the expected key differences between VoLTE and alternative VoIP services. Table. 2. Comparison of VoLTE and VoIP alternatives User Experience VoLTE SIP VoIP OTT VoIP Voice quality-audio Wideband codec Wideband codec Wideband codec Voice quality latency Minimal Minimal. Potentially longer jitter buffer. Large variance due to traffic routing, jitter buffers & different packetization Voice quality loaded cell GBR bearer Best effort Best effort Voice coverage Optimized voice coverage with TTI bundling Lower voice quality at cell edge Lower voice quality at cell edge Call setup time Minimal Minimal Large variance due to NAT traversal Battery life talk time 20 and 40 ms DRX No DRX No DRX Battery life standby time Minimal activity in background Minimal activity in background Network impact Frequent keep-alive and presence activity Signaling call setup SIP call control. Policy control. Dedicated bearer establishment SIP call control. Policy control Proprietary call control over IP. Optional policy control Signaling load background activity Minimal activity. Negligible impact on smartphone signaling load Minimal activity. Negligible impact on smartphone signaling load Frequent background activity increases smartphone signaling load Traffic packet per second 20 ms packetization 20 ms packetization Varies Traffic bit per second Coding rate + IP overhead. ROHC over radio Coding rate + IP overhead Coding rate + IP overhead Page 13

14 6. Conclusions Operators need a voice evolution strategy, which must include the right timing for the introduction of VoLTE, as well as a defined position on OTT VoIP and alternative SIP VoIP. Strategic and product decisions such as the selection of a device portfolio can be improved by having exact information about device and service performance. VoLTE is a standardized service and correct functional interworking will be checked in IOT testing between device and network vendors. However, the scope of IOT testing does not fully cover user experience and measurements and analysis of network effects. In particular, the comparison to OTT and other alternatives is missing. Operators must understand the user experience differences between VoIP alternatives and how the network should be designed, dimensioned and configured for VoIP services. For example, VoLTE talk time optimization is not straightforward and it is not enough to simply check the device capabilities for VoLTE and DRX. Specialized test methods are needed to verify the current consumption of VoLTE with different network configurations and in different radio conditions. Other special testing and analysis methods are also needed to verify the user experience and network performance with different devices and VoIP services. Page 14

15 7. About Smart Labs Nokia Solutions and Networks (NSN) opened its first Smart Lab in 2009, and has since established labs across North America, Europe and Asia. By engaging with all the players in the mobile ecosystem application developers, smartphone manufactures and mobile operators the labs bring the industry together to improve network performance. Smart Labs support mobile network operators to find the ideal balance between the best user experience and the lowest impact on network performance. The Smart Labs help clients implement technology improvements with our two-phased project process. Each phase includes self-funding performance guarantee, and delivers meaningful, measurable benefits. The Smart Labs pioneered consulting projects to capitalize on the mobile eco system. Our library of templates goes beyond conventional industry tests and tools to identify increases in network performance and user experience. Page 15

16 Nokia Solutions and Networks P.O. Box 1 FI Finland Visiting address: Karaportti 3, ESPOO, Finland Switchboard Product code C WP EN 2013 Nokia Solutions and Networks. All rights reserved. Public NSN is a trademark of Nokia Solutions and Networks. Nokia is a registered trademark of Nokia Corporation. Other product names mentioned in this document may be trademarks of their respective owners, and they are mentioned for identification purposes only.