nsn_from_voice_over_ip_to_voice_over_lte_white_paper.pdf

7/25/2019 nsn_from_voice_over_ip_to_voice_over_lte_white_paper.pdf

1/16

From Voice over IPto Voice over LTE

NSN White paperNovember 2013


2/16

nsn.comPage 2

CONTENTS

1. Introduction 3

2. VoLTE markets 4

3. VoLTE technology 5

3.1 VoLTE user experience 5

3.1.1 VoLTE talk time 5

3.1.2 VoLTE service quality 6

3.1.2 VoLTE coverage 6 3.2 VoLTE trac considerations in

network design 7

3.2.1 Quality of Service 7

3.2.2 Use of radio resources 7

3.2.3 Signaling capacity 8

3.2.4 IP packet forwarding capacity 8

4. Cellular Voice over IP alternatives 9

4.1 OTT VoIP services 9 4.2 Best eort SIP VoIP 10

4.3 OTT VoIP performance in LTE networks 10

4.3.1 OTT VoIP user experience 10

4.3.2 OTT VoIP user experience 11

5. VoLTE comparison with VoIPalternatives in LTE

13

6. Conclusions 14

7. About Smart Labs 15


3/16

nsn.comPage 3

1. IntroductionThe speed with which operators have adopted Long Term Evolution(LTE) and the rapid growth of LTE subscriptions in advanced marketsare testament to the technologys success. In May 2013, the Globalmobile Suppliers Association (GSA) reported 175 commercial LTEnetworks. At the end of 2012, there were already close to 70 millionLTE subscriptions across the globe.

LTE has become a truly mobile access method for various dataapplications and services. The rst LTE devices were modems suchas USB dongles for PCs, yet today, most LTE devices belong tothe smartphone category. Voice is naturally a key service for a LTEsmartphone user, but in most of todays commercial LTE networks,voice is still based on traditional Circuit Switched (CS) voice in 2G/3Gnetworks. However, this is not only an issue of network readiness, asaccording to GSA, of the 261 LTE smartphones announced at the endof March 2013, only a few currently support VoLTE. As LTE networkcoverage continues to expand, the next important step for operatorsis to deploy a seamless voice over LTE (VoLTE) service.

Voice is still a major source of revenue for operators, even in the mostadvanced mobile broadband markets. This means that operators mustcarefully plan how their voice and mobile broadband businesses are

developed. Over the top (OTT) Voice over IP (VoIP) is an alternative toCS voice for many subscribers, because wide coverage HSPA andLTE networks and operators mobile broadband data services enablemobile use of free OTT VoIP services. However, VoLTE brings manybenets to help operators ensure their voice service remains the mostattractive solution for most mobile subscribers.

This paper introduces selected VoLTE technology features thataect the user experience and network performance. There arenumerous existing white papers that, for example, describe VoLTEarchitecture, compare deployment alternatives or evaluate powerconsumption of VoLTE smartphones. This paper diers by describing

those VoLTE features that can improve the user experience andnetwork performance compared with OTT VoIP services such asSkype or alternative SIP VoIP solutions. It is recommended that VoLTEsmartphones are tested beyond basic VoLTE Inter-Operability Testing(IOT) in order to evaluate VoLTE smartphone performance and VoLTEcompetitive advantages against alternative solutions.


4/16

nsn.comPage 4

2. VoLTE marketsMobile broadband (HSPA and LTE) is going mainstream and LTE isthe most rapidly adopted mobile technology ever, with the GSAforecasting 234 commercial LTE networks in 83 countries by the endof 2013. LTE smartphones are commonly available in many markets,but VoLTE is still emerging. This is because most operators initiallyoer CS voice for LTE smartphone subscribers using CSFB (CircuitSwitched Fallback) in LTE-GSM/WCDMA and LTE-CDMA networksor SVLTE (Simultaneous Voice and LTE) in LTE-CDMA networks.The traditional operator safe havens of voice and messaging havebeen under attack by OTT service providers over the last few years.

Therefore, operators strategies for maintaining protable businessinclude developing the mobile voice service combined with anenhanced end-user experience. The whole telecommunicationsindustry thus has a strong focus on VoLTE and the evolution of richcommunication.

LG U+, SKT (Korea) and metroPCS (USA) were the rst operators tostart a commercial VoLTE service in August 2012. KT (Korea) was nextto launch in October 2012. Currently, the Korean VoLTE market isdeveloping the most swiftly, with about 2.8 million VoLTE subscribersin March 2013 and a high monthly growth rate. Furthermore, manyoperators worldwide have also been testing VoLTE.

Fig. 1. Korea VoLTE subscribersSource: KCC, April 2013

0

200

400

600

800

1000

1200

1400

1600

Jan Feb Mar 2013

Korea VoLTE subscribers

SKT

LGU+

KT

Thousands

Koreas position as the leading VoLTE market is also highlighted by thegrowing selection of VoLTE smartphones available. Currently, theseVoLTE 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 IPMultimedia Subsystem (IMS) based services. In the rst phase, RCSprovides contacts, chat, le sharing and video sharing services. RCShas been launched by operators in Korea, Europe and the USA.


5/16

nsn.comPage 5

20 ms

UE eNB

TTI

1 ms

40 ms

UE eNB

TTI

1 msVoIPpackets

Two VoIPpacketsaggregated

UE discontinous

reception(20 ms DRX cycle)

Sleep mode

between voicepackets saves energy

UE discontinous

reception(40 ms DRX cycle)

Sleep mode

between voicepackets saves energy

Fig. 2. Overview of VoLTE transmission and DTX/DRX.

3. VoLTE technology

3.1 VoLTE user experience

3.1.1 VoLTE talk time

Battery life is a top concern for todays smartphone users. Broadbandradios, large touch screen displays and gigahertz multi-coreprocessors consume a lot of energy. Therefore, optimizations thatimprove battery life are very important for both mobile operatorsand smartphone vendors. The rst VoLTE implementations received

much public attention, because the VoLTE talk time seemed to beworse than with traditional CS voice services. However, the earlyimplementations were not optimized for low current consumption.

The main components aecting current consumption during voicecalls are cellular radio and voice codec components. Usually the displayis turned o by a trigger from either a proximity sensor or timer. If aVoLTE audio codec is integrated to the modem processor, VoLTE powerconsumption can be lower than with OTT VoIP applications, which arerunning in the application processor.

Cellular radio transmission and reception can be optimizedsignicantly to reduce energy consumption. The biggest energysaving is achieved by shutting down the transmission and receptionwhenever possible. During a voice call, this is possible by usingDiscontinuous Transmission (DTX) and Discontinuous Reception (DRX).

Standard voice is packetized in 20 ms intervals i.e. each voice packetincludes 20 ms of voice. Cellular radio resources are divided in thetime domain into Transmission Time Intervals (TTI), which in LTE are asshort as 1 ms. Because LTE is broadband technology, it is possible tosend one voice packet within one TTI. Therefore, a VoLTE smartphonecan 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 TTIevery 40 ms.


6/16

nsn.comPage 6

3.1.2 VoLTE service quality

VoLTE supports mobile high denition (HD) voice i.e. the AdaptiveMulti Rate Wideband (AMR-WB) codec, which is already used in tensof WCDMA/HSPA networks as well as in a few GSM networks. Thewideband codec improves voice quality by transmitting a broaderspectrum of human voice frequencies than narrowband codecs.

Perceived voice quality depends on the audio codec used and themouth-to-ear delay, as well as transmission impairments such as jitter,bit errors and packet loss. VoLTE improves performance, particularlyduring busy hours, by establishing a dedicated Guaranteed Bit Rate(GBR) bearer for voice. The GBR bearer oers a low latency and low

jitter connection.

Voice service experience is further aected by the call-setupperformance and call reliability. LTE can improve mobile voice call-setup time signicantly due to the high capacity and low latency LTEradio access available for the signaling connection between the UE andvoice service core.

3.1.3 VoLTE coverage

In poor radio conditions, data can be received with errors. This

eect is particularly visible at the cell edge due to the limited UEtransmission power. In order to improve the reliability of VoLTE packettransmission in the uplink direction, it is possible to use a techniquecalled Transmission Time Interval (TTI) bundling. This relies on sendinga few redundant versions of the same set of bits in consecutiveTTIs. TTI bundling is estimated to provide 2 - 4 dB uplink coverageimprovement 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 LTEcoverage area with GSM/WCDMA is solved with Single Radio Voice CallContinuity (SRVCC) technology, which dierentiates the VoLTE serviceexperience from alternative VoIP services.


7/16

nsn.comPage 7

3.2 VoLTE trac considerations innetwork design

3.2.1 Quality of Service

Trac between the UE and the network is carried over bearers, whichcan have dierent QoS characteristics. When a LTE UE attaches tothe network for the rst 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 adefault bearer for Internet trac. Both are non-Guaranteed Bit Rate

(non-GBR) bearers. IMS signaling bearer has higher priority. When aVoLTE call is setup, a dedicated GBR bearer is established for the voiceconnection. Thus, QoS dierentiation must be taken into accountin VoLTE network design and dimensioning. In contrast, OTT VoIPapplications always run on the default bearer for Internet access andtherefore dierentiated QoS cannot be guaranteed.

3.2.2 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 ecient use of radio resources for bursty trac, but dueto dynamic resource allocations, control channel information mustbe sent along the data transmission both in uplink and downlinkdirections. To overcome this, the LTE base station can use analternative scheduling method called Semi Persistent Scheduling (SPS),which assigns predened radio resources for the VoLTE user. SPS doeshowever, have some drawbacks. It must use xed link adaptation anda xed resource block in the frequency domain, which prevents anyadvanced link adaptation or scheduling. SPS is also not well suitedfor mixed voice and data trac, because xed SPS allocations limit

data scheduling. Another method to reduce the control overhead ispacket aggregation, resulting in a 40 ms transmission interval. As aconclusion, in realistic network deployments with mixed VoLTE anddata usage, instead of using SPS, the optimal solution is to use packetaggregation with dynamic scheduling.

Use of Robust Header Compression (RoHC) with a GBR bearer willreduce the user plane trac by several bytes over the air interface.For VoIP packets, the size of IP headers (IP/UDP/RTP) is larger than thevoice payload itself. RoHC can compress the header size from 40 bytesto two or three bytes between the user device and base station. Thishigh compression ratio not only increases network capacity but also

provides coverage improvements for VoLTE users compared to OTTVoIP of up to 3 dB due to the lower bandwidth required at thecell edge.


8/16

nsn.comPage 8

3.2.3 Signaling capacity

There is no frequent background keep-alive trac associated withIMS based services, which is an advantage over OTT VoIP services.OTT VoIP apps (Viber, Skype, etc.) must maintain active sessions withkeep-alive messages in order to stay reachable for incoming calls.These frequent keep-alive transactions eventually result in largesignaling load in the network. With IMS services, the client deviceperforms periodical re-registrations to the IMS, but the frequencyis signicantly lower than that of OTT apps. A VoLTE service requiresadditional signaling for setting up a dedicated GBR bearer to fulllthe QoS requirement, but the expected network impact is low. It is

worth noting that smartphone platforms such as iOS, Android andWindows Phone tend to have platform specic connections (e.g. to getnotications and automatic software updates), which generate datatransactions and signaling load. The impact of VoLTE on signaling loadis therefore assumed to be negligible.

3.2.4 IP packet forwarding capacity

Generally, trends in mobile broadband trac growth indicate thatvoice will play a minor role in total trac volume, with video content

and browsing being the main contributors to mobile data volumes.Although VoLTE and OTT VoIP are not signicant services when thethroughput capacity of network (bit/s capacity) is dimensioned, theycan have a relatively high eect on the dimensioning of IP packetforwarding elements such as S/PGW (packet/s capacity).


9/16

nsn.comPage 9

Table. 1. Examples of OTT VoIP applications

4. Cellular Voice over IP alternativesOTT communication services are challenging traditional mobilevoice and messaging services. Therefore, operators must take OTTcommunication services into account in their strategies. Operatorscan compete with OTT services, but can also collaborate with OTTservice providers. In both cases, operators must understand howthe user experience with OTT services compares with the operatorcommunication services, including VoLTE. Operators should alsounderstand how the growing adoption and use of OTT communicationapps aects the mobile network.

4.1 OTT VoIP servicesOTT refers to services provided independently over the mobileoperators Internet access services. Skype is probably the most wellknown OTT VoIP service. This and many other VoIP services are todaycommonly available from application stores for all major smartphoneplatforms. Many OTT applications that support VoIP also include richcommunication features, such as instant messaging, le sharing,presence and video calls.

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

iOS, Android,Windows Phone,Blackberry,Symbian

iOS, Android,WindowsPhone,Blackberry,Symbian, Bada

iOS, Android,Windows Phone,Blackberry,Bada

iOS iOS, Android,WindowsPhone

iOS, Android

Other devices PCs, TVs,iPod touch,PlayStation Vita,Skype handsets,Xbox One

PCs iPod touch iPod touch,

Mac PCs

PCs, iPod touch

PSTN interworking Premium

servicesavailable

Viber client

places aregular call,if VoIP is notavailable


10/16

nsn.comPage 10

4.2 Best efort SIP VoIP

While commercial VoLTE is still emerging, operators can consideroering VoIP services based on IMS infrastructure and SIP VoIP clientsover best eort mobile broadband access. Such a pre-VoLTE servicecould be oered over LTE as well as over HSPA and Wi-Fi. The Pre-VoLTE phase is an opportunity to develop the SIP VoIP technologyand the VoIP business, particularly in markets with limited LTEcoverage or in markets missing standard VoLTE smartphones.

The availability of downloadable and congurable SIP compliant VoIPclients is limited to major smartphone platforms and application

markets. For example, Bria and Acrobits are SIP VoIP clients, which areavailable for both iOS and Android platforms.

4.3 OTT VoIP performance in LTE networks

This chapter includes some highlights of OTT VoIP performance basedon Smart Labs measurements and analysis.

4.3.1 OTT VoIP user experience

Voice quality is a very important criterion in providing a satisfactoryvoice service. CS mobile voice has not been excellent, due, forexample, to quite a narrow voice spectrum. Wideband codecs improvethe quality and therefore wideband audio is used by some OTT VoIPservices. If we compare the voice quality in excellent radio conditionsin an empty cell by measuring the MOS (mean opinion score), someOTT VoIP applications provide better speech quality and some provideworse quality than CS voice.

Conversation quality depends on the mouth-to-ear delay. Latency ofmobile CS voice depends on network implementation details, includingphysical distances between network elements. Typical CS voice latencyin commercial networks is 200 300 ms, which is good for smoothconversation. OTT VoIP clients must have adequate jitter buersto manage the best eort quality in todays mobile broadbandnetworks. Furthermore, OTT VoIP services must have a connectivitysolution for the varying Network Address Translation (NAT) and rewallenvironments of mobile networks. Therefore, OTT VoIP calls may haveto be routed via gateway nodes on the public Internet, which increasesthe mouth-to-ear delay. Measurements show that OTT VoIP servicespose challenges in oering satisfactory conversation quality, even in avery low latency LTE network.


11/16

nsn.comPage 11

NAT traversal can be managed by OTT services, but the complex NAT

traversal solutions can increase call setup times. Measurements andanalysis show that OTT VoIP call establishment time varies a greatdeal, in particular due to the NAT traversal protocols.

Battery life is also an important customer experience factor foroperators, because network settings aect smartphone currentconsumption and users can detect the dierence between mobileservice providers.

With VoIP services, the smartphone current consumption can bemeasured during a call and during periods of user inactivity. Becausean average user can spend some tens of minutes in voice calls daily,it is more important to compare the current consumption causedby background activity of a VoIP application. Measurements showthat the background activity of dierent OTT VoIP applications variessignicantly, with a noticeable eect on the UE battery life time.

Current consumption during a OTT VoIP call depends mainly on thehardware components of the smartphone, as in practice, HSPA or LTEradio must be continuously in a high power connected state (Cell_DCHor RRC connected respectively). On the other hand, VoLTE is speciedto support a connected state DRX during the call. Therefore, VoLTE isexpected to have lower current consumption during the call than anyOTT VoIP service.

4.3.2 OTT VoIP network impacts

Trac characteristics during a VoIP call depends on the voicepacketization interval and silence suppression capability, as well asother protocols that may be active while voice media is transmitted.Average packet inter-arrival time (or packets per second rate) isimportant for dimensioning high capacity packet forwarding elementssuch as packet core gateway elements. OTT VoIP trac can beanalyzed even though the content can be encrypted. Measurementsshow that OTT VoIP apps can generate similar trac to VoLTE with a

20 ms packetization interval. However, in some cases even a doubledpacket rate is generated.


12/16

nsn.comPage 12

Another trac characteristic is the average bit rate (kbit/s) during a

VoIP call. This is aected by the codec and IP overhead. Measurementsindicate that OTT VoIP applications can generate two to three timesthe bit rate of 12.2 kbit/s AMR codec VoIP trac. RTP/UDP/IPv4overhead is 40 bytes per voice packet, which for example with12.65 kbit/s AMR-WB voice results in around 29 kbit/s IP trac inone direction. The actual average bit rate with AMR can be signicantlyless due to silence suppression. The gain of silence suppressioncan be estimated assuming 60% voice activity. Currently, some OTTVoIP applications support silence suppression, but not all of them yetdo so.

One of the key concerns of operators is the signaling load generated

by smartphones. Part of the load is caused by the mobile operatingsystem services, but a signicant proportion is generated by onlineapplications. OTT communication services are particularly challenging,because always-on reachability requires a persistent connection tothe network. OTT VoIP applications are a common reason for frequentdata transactions. These transactions include, for example, keep-alive messages for the persistent connections, but there is also otherapplication specic activity. The exact behavior of the OTT appsvaries depending on the OS platform and the application version.Measurements show that some OTT VoIP apps generate frequent datatransactions, which increase the signaling load in mobile networks.


13/16

nsn.comPage 13

5. VoLTE comparison with VoIPalternatives in LTE

The following table summarizes the expected key dierences betweenVoLTE and alternative VoIP services.

Table. 2. Comparison of VoLTE and VoIP alternatives

VoLTE SIP VoIP OTT VoIP

User Experience

Voice quality-audio Wideband codec Wideband codec Wideband codec

Voice quality latency Minimal Minimal. Potentially longerjitter buer.

Large variance due to tracrouting, jitter buers &dierent packetization

Voice quality loaded cell GBR bearer Best eort Best eort

Voice coverage Optimized voice coverage withTTI bundling

Lower voice quality at celledge

Lower voice quality at celledge

Call setup time Minimal Minimal Large variance due to NATtraversal

Battery life talk time 20 and 40 ms DRX No DRX No DRX

Battery life standby time Minimal activity in background Minimal activity inbackground

Frequent keep-alive andpresence activity

Network impact

Signaling call setup SIP call control.

Policy control. Dedicatedbearer establishment

SIP call control.

Policy control

Proprietary call control over IP.Optional policy control

Signaling load backgroundactivity

Minimal activity. Negligibleimpact on smartphonesignaling load

Minimal activity. Negligibleimpact on smartphonesignaling load

Frequent background activityincreases smartphonesignaling load

Trac packet per second 20 ms packetization 20 ms packetization Varies

Trac bit per second Coding rate + IP overhead.

ROHC over radio

Coding rate + IP overhead Coding rate + IP overhead


14/16

nsn.comPage 14

6. ConclusionsOperators need a voice evolution strategy, which must include theright timing for the introduction of VoLTE, as well as a dened positionon OTT VoIP and alternative SIP VoIP. Strategic and product decisionssuch as the selection of a device portfolio can be improved by havingexact information about device and service performance.

VoLTE is a standardized service and correct functional interworkingwill be checked in IOT testing between device and network vendors.However, the scope of IOT testing does not fully cover user experienceand measurements and analysis of network eects. In particular, thecomparison to OTT and other alternatives is missing.

Operators must understand the user experience dierencesbetween VoIP alternatives and how the network should be designed,dimensioned and congured for VoIP services. For example, VoLTEtalk time optimization is not straightforward and it is not enough tosimply check the device capabilities for VoLTE and DRX. Specializedtest methods are needed to verify the current consumption of VoLTEwith dierent network congurations and in dierent radio conditions.Other special testing and analysis methods are also needed to verifythe user experience and network performance with dierent devicesand VoIP services.


15/16

nsn.comPage 15

7. About Smart LabsNokia Solutions and Networks (NSN) opened its rst Smart Lab in2009, and has since established labs across North America, Europeand Asia.

By engaging with all the players in the mobile ecosystem applicationdevelopers, smartphone manufactures and mobile operators thelabs bring the industry together to improve network performance.

Smart Labs support mobile network operators to nd the idealbalance between the best user experience and the lowest impact onnetwork performance.

The Smart Labs help clients implement technology improvementswith our two-phased project process. Each phase includesself-funding performance guarantee, and delivers meaningful,measurable benets.

The Smart Labs pioneered consulting projects to capitalize on themobile eco system. Our library of templates goes beyond conventionalindustry tests and tools to identify increases in network performanceand user experience.


16/16

Nokia Solutions and Networks

P.O. Box 1FI-02022Finland

Visiting address:Karaportti 3, ESPOO, FinlandSwitchboard +358 71 400 4000

Product code C401-00874-WP-201311-1-EN

2013 Nokia Solutions and Networks. All rights reserved.

PublicNSN is a trademark of Nokia Solutions and Networks. Nokia is a registeredtrademark of Nokia Corporation. Other product names mentioned in thisdocument may be trademarks of their respective owners, and they arementioned for identication purposes only.

nsn_from_voice_over_ip_to_voice_over_lte_white_paper.pdf

Documents