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FOREWORDWhile over-the-top providers like Skype or Viber continue to be dominant forces in the mobile VoIP space, for network operators deploying LTE, Voice over LTE (VoLTE) promises to become a killer app for their networks between 2014 and 2016. Shored up by successes made by early adopters like SK Telecom, analyst groups, such as Infonetics Research, have begun to re-evaluate and more than double past forecasts1 regarding its growth in use.

As one VoLTE deployment success rolls into another success, competitive forces will drive even greater adoption. Major LTE operators will look to VoLTE as a means to introduce a more efficient way to provide high-quality voice over their networks. We live in interesting, fast-moving times and, where a year ago there was some hesitation around VoLTE in the market, we believe the benefits VoLTE brings will promote healthy deployment.

Voice still remains the most critical service mobile network operators (MNOs) run over their networks. Room for error around voice service is therefore slight to non-existent. As such, it is absolutely crucial that when deploying a VoLTE solution operators get it right the first time.

This whitepaper aims to give network directors, operations managers and network support engineers a head-start toward better understanding VoLTE deployment. For those on the cusp of a deployment, it will help prepare you for successful launch through identifying effective troubleshooting techniques. In it, we will discuss high-level technical and practical aspects of VoLTE testing in live LTE networks. The technical aspects will cover the basics of a VoLTE call scenario and relevant LTE network elements and interfaces, while practical aspects will take an in-depth look at a real-world VoLTE call to highlight where to focus when troubleshooting VoLTE calls.

VoLTE OVERVIEWThere are three main differences between VoLTE voice calls and calls over legacy mobile networks.

First, the LTE network is designed for packet data transfer, so there are no dedicated resources for the voice, such as those found in circuit switched networks like GSM, CDMA or UMTS. In LTE, voice must be transferred over IP packets, just like any other user-plane media.

Second, the LTE Network has separate paths for control-plane and user-plane data. User Equipment (UE) management, mobility control and session signaling messages are transferred in different paths than the user-plane data. In legacy networks, the same interfaces and elements are used for both signaling and user-plane handling.

The third difference is—a result of the other two architecture differences—is that the VoLTE call signaling analysis requires multi-interface access. Signaling to set up the voice stream is available in the user-plane interfaces, whereas the session and mobility is seen in the control-plane interfaces.

PRACTICAL REQUIREMENTS FOR LIVE VoLTE TROUBLESHOOTINGAs illustrated in the call scenario, VoLTE calls incorporate all elements of the LTE network. Because of that, troubleshooting VoLTE calls requires network-wide analysis. Data from the following interfaces is required for end-to-end analysis of VoLTE calls.

› S1-MME (control-plane interface between eNB and MME)

› S1-U (user-plane interface between eNB and S-GW)

› S6a (Authentication signaling, and for key’s to Decipher S1-MME messages)

› S11 (interface between MME and S-GW for Packet Data network bearer signaling)

› S5 (S-GW to PDN-GW interface signaling and user-plane data).

It is impossible to troubleshoot VoLTE calls without the ability to perform extensive and correlated analysis of both the control- and user-plane interfaces. Information about the subscriber identity, mobile attaches and bearer creation procedures is found in the control-plane interfaces, while SIP signaling, IMS register and call-setup messages to the IMS network, and the RTP streams are transferred in the user-plane traffic.

To perform all the troubleshooting requirements, analysis by a live network analyzer is needed. This network analyzer would give MNOs an independent view of what is happening between the signaling of different elements, and allows them to pinpoint troublesome elements and the root causes for the non-functioning calls. Live LTE networks require massive capturing and analysis performance from the analyzers. For root cause analysis, the raw data of each message is essential to analyze the details of the VoLTE sessions. Depending of the sector under analysis, for the amount of raw data that must be captured to disk varies from hundreds of megabits (for one eNB) up to 10Gbps (for thousands of eNBs).

The Analysis performance of the tool minimizes the waiting time and speeds up troubleshooting. Capacity to analyze tens of millions of end-to-end calls in hour allows having the results from test-sessions or live capture available in minutes. Analysis should include the session correlation over every single interface. Effective troubleshooting also requires very deep end-to-end session analysis, where a single user’s behaviour over the network is presented in an aggregated way. Easy detection of subscriber problems in their attaches, bearer creation, SIP signaling and MOS (quality) of their voice connection helps shorten the mean time to resolution—ultimately saving time and resources, while improving service and customer satisfaction.

1 “Mobile VoIP Services and Subscribers Annual Market Size and Forecasts”, June 24, 2013, Infonetics Research.

VoLTE Call Troubleshooting in Live LTE Network By Tommi Tallgren, Senior Product Manager, EXFO

© 2013 EXFO Inc. All rights reserved.

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CALL SCENARIOS AND NETWORK ARCHITECTURE VoLTE calls use the IMS network, and the signaling for the voice stream (using RTP protocol) is performed through SIP protocol transferred over the “user-plane” in LTE networks. Diagram 1 (below) walks through the basic call scenario and how the different LTE network elements are utilized during the call. In this example, as well as throughout this paper, we will focus primarily on the LTE network from the eNB to PDN-GW.

For simplification, the policy and charging signaling, as well as signaling inside the IMS network, are not shown in our example.

1. User Equipment (UE) Attach and Authentication. First the UE has to attach to the LTE network in order to request any service. Mobile Attach and Authentication procedures include eNB (LTE Base Station), MME (Mobile Management Entity) and AAA/HSS (Authentication and Subscriber Databases) network elements. This signaling is utilizing S1-AP, NAS, and Diameter protocols. In this call phase only the control-plane interfaces (S1-MME and S6a) are used

2. Network access

2a. Data bearer establishment. For the VoLTE call, the bearer has to be established with correct APN (Access Point Node). Bearer establishment requires signaling over S1-MME, S11 and S5 elements using NAS and GTPv2 protocols.

2b. Data connection. Once the Bearer is established between each of the network elements, the information is sent via ENB to the UE. It is a direct GTPv1 tunnel connection from UE to, for example, IMS (via the correct APN).

3. IMS Access and Registering. UE must register to IMS and request the VoLTE call via SIP protocol over the established bearer. Only user-plane interfaces and elements are utilized.

4. VoLTE Call Establishment. For the VoLTE call, the previously established bearer can be used to send the SIP signaling to the IMS network to request the call. Once the SIP session is completed the voice connection is ready to be setup, the voice stream connection is established in two steps:

4a. Dedicated bearer signaling. Dedicated bearer between the UE to the IMS core is established. Quality of the Bearer over LTE network is assured by signaling voice class bearer (CQI = 1) via control-plane (S1-MME, S11, S5) interfaces.

4b. Voice Stream. Finally, once the connection is established, the top-quality voice stream bearer (RTP-protocol) carries the conversation from UE to the other calling party via IMS Core.

For simplification of the example scenario above the policy and charging signaling as well as signaling inside the IMS network are not shown.

S1-U

S5/8

S6a

S1-MME

SignalingData flowControl-plane InterfacesUser-plane Interfaces

(SIP)(RTP)

S11

LTE

Uu

eUTRAN

EvolvedPacket Core

eNB

MME

S-GW

PDN GW

IMSHSS/AAA1

4b

4a

2b

3

2a

Abbreviations/Legends

APN: APN is the gateway from the LTE network to the needed services. In the case of VoLTE, it must be the IMS network.

Data Bearer: Bearer is a data tunnel from UE to PDN-GW (APN). This bearer is established via control-plane signaling and utilized by UE to access the services outside of LTE.

QCI: Quality Class Identifier. In LTE network, the QoS (Quality of Service) for different services are assured by giving the QCI for the bearers. VoLTE Voice stream gets best service level (conversational voice) identified by a QCI 1 class bearer.

Diagram 1: VoLTE call across the LTE network.

© 2013 EXFO Inc. All rights reserved.

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VoLTE CALL TROUBLESHOOTING EXAMPLESVoLTE call troubleshooting can be started from different details level depending of the use case. For network optimization use cases, the ideal level to start at is investigating statistical data and working further down to the details in order to troubleshoot found issues. To troubleshoot specific subscriber or VoLTE sessions requires correlated end-to-end session view, so engineers are able to quickly find the specific subscriber by utilizing, for example, IMSI, Mobile IP or other method of identification.

Diagram 2 shows correlated LTE session phases over the network. Errors are shown clearly in the session view, indicating erroneous call phase icons in red. In this snapshot, the wider analysis of the sessions is not seen, but an effective live LTE network analyzer should naturally present all session information, identifications and network info correlated in one view.

The failure causes (like the ESM cause seen above) are an important part of the session view itself, but from time to time it is needed to dig deeper to the message level to understand what and where things went wrong.

VoLTE call troubleshooting can be split to focus to three main phases: first, look at whether the attach phase was successful and correct; second, check the SIP Session and voice-related bearer establishment; and finally, analyze the voice quality itself.

If the UE attaches are working fine, next the focus should be put to the correct settings in the default bearer establishment. For VoLTE call, we are looking that the mobile would have connection to the “IMS” access point name (APN) allowing the SIP calls. Once the connection is available, the VoLTE-capable UE should register itself to the IMS core using SIP protocol, and only then the UE is able to establish or receive VoLTE calls.

The live network VoLTE troubleshooting is mostly done by analyzing higher-level session view (as in Diagram 2), but in case of problems at the message level, that level of details must also be available. Diagram 3 illustrates the message flow from the S1-MME and S1-U interface of the successful VoLTE call. This detailed message flow example is from a mobile terminated voice call established to the UE that was already registered to IMS. For the clarity only the highest level protocols are seen and the transmission and tunnelling protocols are hidden.

Diagram 2 : Correlated LTE Session view.

Diagram 3: Detailed message flow of Mobile Terminated VoLTE signaling.

© 2013 EXFO Inc. All rights reserved.

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VoLTE Call message explanations1. First the default bearer establishment by the NAS protocol Service Request. UE is asking service with the information about itself using

Service Request that is transferred via eNB to MME.

Note: Since the mobile is already attached, the attach procedure messages with authentication to HSS is not seen here. To allow VoLTE calls, bearer connection to IMS APN is established.

2. After the bearer is available, note the SIP signaling to establish connection to IMS, and the signaling messages to answer to the call. With the SIP protocol the mobile is signaling the voice (RTP) connection.

Note: Previously the UE have already attached and registered to IMS domain, so SIP registering is not seen here.

3. After the Sip powered voice connection is established, VoLTE voice specific dedicated bearer is established to voice streams. This is triggered by the Network. See the detailed decoding in right hand side highlighting the message, including the quality class, QCI =1, that is reserved for conversational voice.

4. Finally the RTP (voice conversation) is seen transferred over the dedicated bearer.

As we can see effective VoLTE troubleshooting requires a live LTE network analyzer tool that can handle end-to-end session analysis with correlation to the user-plane. VoLTE specific user-plane analytics must include the SIP signaling details like called and calling party information, used voice or video codecs and error-cause information. For the voice stream, RTP protocol analysis including codecs, as well as Mean Opinion Score (MOS) and other quality indicators are mandatory. Session-level MOS value gives quick analysis of the voice quality and the possibility to listen the voice conversation allows further detection of voice quality over the call. Snapshot from the analysis view below illustrates small set of SIP and RTP protocols information over the S1-U interface.

Independent VoLTE analysis with live LTE network-capable analyzer is mandatory for MNOs launching VoLTE service to give faster responses for the network or subscriber issues.

Diagram 4: Example of VoLTE analysis from S1-U interface.

EXFO Headquarters > Tel.: +1 418 683-0211 | Toll-free: +1 800 663-3936 (USA and Canada) | Fax: +1 418 683-2170 | info@EXFO.com | www.EXFO.com

EXFO serves over 2000 customers in more than 100 countries. To find your local office contact details, please go to www.EXFO.com/contact.

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WHITEPAPER032.2AN © 2013 EXFO Inc. All rights reserved. 2008

Printed in Canada 13/11

About TravelHawk ProThe portable TravelHawk Pro is designed for three main troubleshooting operations: mobile network signaling analysis, IP application data analysis and data capturing. This unique functionality to support all three use cases in one unit makes TravelHawk Pro a very powerful tool for dramatically improving network quality and mean time to repair (MTTR).

About PowerHawk ProThe rack mounted multi-server system PowerHawk Pro is designed for live analysis and deep troubleshooting of LTE and Packet core networks. Solution improves the network quality and minimizes the outages with fastest troubleshooting methods. Due the real time analysis, the results and root causes for network failures are available whenever required. Solution provides correlated real time analysis for mobile signaling and user-plane sessions with full support for VoLTE.

About EXFOEXFO is a leading provider of next-generation test and service assurance solutions for wireless and wireline network operators and equipment manufacturers in the global telecom industry. The company offers innovative solutions for the development, installation, management and maintenance of converged, IP fixed and mobile networks. EXFO has a staff of approximately 1800 people in 25 countries, supporting more than 2000 telecom customers worldwide.

CONCLUSION: GETTING IT RIGHT THE FIRST TIMEEffective live LTE troubleshooting may make the difference between a successful VoLTE launch or compounding service problems resulting in dissatisfied customers and competitive disadvantage. But to do it effectively requires an ability to analyze the network end-to-end LTE and both control- and user-plane behaviour, with specific focus on analyzing SIP and RTP issues transferred on the user-plane.

VoLTE troubleshooting in live networks will bring also other live LTE network analysis challenges, such as deciphering NAS messages, data storage and the availability of tools. More on how to overcome those challenges can be found in the whitepaper “LTE Live Troubleshooting Tools: 5 Essential Characteristics” found on the EXFO website (http://www.exfo.com/travelhawkpro) under resources.

An effective analyzer tool- with end-to-end support for multiple network elements- will save hours of troubleshooting time and improve the network quality. Whether only testing the waters of VoLTE or aiming to ultimately consolidate voice and data services over the LTE network, MNOs must use every tool they can to ensure they get it right the first time.