Single Radio Voice Call Continuity (SRVCC) Testing Using Spirent CS8 Interactive Tester

(1)

Single Radio Voice Call Continuity (SRVCC) Testing Using Spirent CS8 Interactive Tester

September 2013

(2)

(3)

Using Spirent CS8 Interactive Tester

1. exeCUTIVe SUMMARy

Voice continues to be a viable source of revenue for Network operators. Consumers have been accustomed to assured Quality of Service (QoS) standards while using voice services on their mobile devices. According to Infonetics Research, an international market research and consulting firm, voice will continue to comprise 60 percent of the mobile services market by 2014, signifying $522 billion in global mobile revenues (figure 1).

Figure 1: Mobile Services Market Forecast For 2014 Source: Infonetics Research

As lTe networks are continuing to be deployed alongside legacy networks such as GeRAN/

UTRAN/1xRTT, the ability for multimode 3G/lTe mobile devices to connect to different network technologies will be an important part of providing the best possible mobile voice and data experience to customers. However, adoption of lTe and its all-Internet Protocol (IP) Radio Access Network (RAN) has produced one of the key challenges of lTe deployment: delivery of voice services in an all-IP network.

Three approaches were decided by the wireless industry to overcome these challenges:

• VolTe (Voice over lTe)

• CSfb (Circuit Switched fallback)

• SVlTe (Simultaneous Voice and lTe)

Unlike CSfb, VolTe allows call continuity. With cost, size, and battery efficiency advantages over dual radio solutions such as SVlTe, the industry is standardizing on VolTe for the future.

VolTe is based on the IP Multimedia Subsystem (IMS) network, with voice services being delivered as data flows within the lTe data bearer. As a result, Single Radio Voice Call Continuity (SRVCC) is required in order to execute a seamless handover of a voice call from an lTe network to a 3G network and provide continuity for traditional circuit-switched networks.

This application note provides an overview of the SRVCC technology and outlines SRVCC mobility testing using Spirent solutions. Since SRVCC implementation introduces a number of challenges to development and test teams – its testing is as important as ever.

(5)

2. SRVCC TeCHNoloGy oVeRVIeW

SRVCC is a method for ensuring fast and reliable handover of an lTe user to a legacy network coverage area while it is in an active IMS based voice session. one challenge with SRVCC is to handover while the Ue is connected to only a single radio at a given time. Two variations of SRVCC have been identified – UTRAN to 3GPP2 1xCS and e-UTRAN to 3GPP UTRAN/GeRAN.

The Ue, lTe Network, and Target legacy Networks should all support SRVCC. In addition, a special interface known as Sv is formed between MMe and MSC Server. The Sv interface is an interface between the Mobility Management entity (MMe) or Serving GPRS Support Node (SGSN) and 3GPP MSC server enhanced for SRVCC. The Sv interface is used to support Inter-RAT handover from VoIP/IMS over ePS to CS domain over 3GPP UTRAN/GeRAN access. To support SRVCC the IMS network should also include application server called SCC-AS. The SCC (Service Centralization and Continuity) AS is responsible for handling the signalling required for the process (figure 2).

Figure 2: High Level Concept for SRVCC from E-UTRAN to UTRAN/GERAN

As the Ue moves away from the lTe coverage area, lTe Reference Signal Transmit Power (RSTP) starts diminishing. The Ue then notifies eNodeb about the change in the signal strength and SRVCC handover is initiated. The lTe network determines that the active voice call needs to be moved from the packet to the circuit domain. MMe then receives the handover request from e-UTRAN with the indication that it is for SRVCC handling, and it then triggers the SRVCC procedure with the MSC Server via the Sv reference point.

A new voice call request is sent to the IMS using a special number known as STN-SR. STN-SR is a unique number that is generated for each Ue and is stored in the HSS. This number is sent to the MMe by the HSS when the Ue first contacts with the network. Receiving STN-SR number indicates to the SCC-AS that the corresponding call needs to be routed to a different network, and it starts the redirection process to the legacy endpoint.

(6)

MMe starts the handover of non-voice PS bearer during SRVCC procedure based on the information received from e-UTRAN. The handover of non-voice PS bearer(s) is done according to Inter-RAT handover procedure as defined in 3GPP Technical Specification 23.401. After the resource preparation is completed, MMe confirms the handover request provided earlier by the eNodeb (figure 3).

Figure 3: SRVCC from E-UTRAN to UTRAN/GERAN Message Flow

The eNodeb then transmits the confirmation to the Ue and provides the required information about the target network. When the mobile is detected in the legacy network, it switches its internal voice processing from VolTe to legacy-circuit voice and the call is re-established.

3GPP Performance Requirements mandate the measured Voice Interruption Time to be within the 3GPP target of less than 0.3 seconds. Voice Interruption Time is the time between last voice packet on lTe and first on CS domain and is measurable through Inter-RAT handover delay. With legacy voice call retention rates typically higher than 98%, SRVCC handover is targeted to be successful more than 99% of the time. These established QoS standards target less than 0.3 seconds Voice Interruption Time – and call drop rates and handover failure rates of less than one percent.

(7)

3. SPIReNT SolUTIoNS oVeRVIeW foR SRVCC TeSTING

3.1. CS8 Mobile Device Tester

CS8 Mobile Device Tester is a single network emulator designed to address all stages of the mobile device design and testing cycles (figure 4). CS8 can be used in radio protocol development, platform validation, system testing and as the network emulator in automated test systems, bringing value to every stage of the mobile device lifecycle. Available CS8 configurations can immediately address needs ranging from realistic lTe network emulation to advanced multi-RAT mobility testing.

Figure 4: Spirent CS8 Mobile Device Tester

3.2. CS8 Interactive Tester

CS8 Interactive Tester is a GUI that controls the network emulator and offers intuitive control over multi-cell multi-RAT network emulation (figure 6). It integrates multiple radio access technologies with a fully developed real-time IPv4 and IPv6 evolved Packet Core (ePC), providing a multi-RAT system with complete end-to-end emulation. The CS8 Interactive Tester real-time state machine emulates an entire cellular environment including lTe, WCDMA, GSM, HSPA, CDMA, eV-Do and evolved High-Rate-Packet Data (eHRPD) services, with multiple cells available per technology.

CS8 Interactive Tester Software window is divided into four panels:

1. Network View 2. Test Configuration 3. Test Results 4. Message Analyzer

Network View and Test Configuration panels are used to configure network topologies. These panels are used to configure both the evolved Packet Core (ePC) and the evolved UMTS

Terrestrial Radio Access Network (eUTRAN). for instance, PDN-GW, HSS, MMe/SGW, and eNodeb are all configured from within these panels.

(8)

The Test Results panel provides diagnostic interpretation of test progress and outputs real- time interactive message exchange between network and DUT. It also logs the NAS/AS state transitions, ePC status, and system control information.

The Message Analyzer panel displays a real-time log of all RRC/NAS messaging between the network and the Device Under Test (DUT). The left side of the panel lists the message sequences, while the right side shows the ASN.1-based contents for each message.

Figure 5: CS8 Interactive Tester User Interface Overview

3.3. System Architecture

Figure 6: CS8 Mobile Device Tester UMTS-LTE Hardware Setup

Spirent CS8 Mobile Device Tester coupled with the CS8 Interactive Tester provides emulation of all network components and is able to handle testing requirements for SRVCC capable devices including mobility scenarios.

(9)

4. SRVCC MobIlITy USING CS8 INTeRACTIVe TeSTeR

4.1. Step 1: Select Appropriate Test Configuration & Network View

open CS8 Interactive Tester (figure 7) and ensure the Application Password and Annual Support Agreement is up-to-date (figure 8).

Figure 7: CS8 Interactive Tester Icon

Figure 8: Application Password and Annual Support Agreement

Click on the Network View dropdown list from the CS8 Interactive Tester User Interface to select from combinations of different technologies. Selecting a network topology brings up the network components as well as configuration options for those components. each icon represents a network entity. Select appropriate Test Configuration based on the testing requirement by choosing the applicable network from the Network View pane. In this example, SRVCC from e-UTRAN to UTRAN is intended to be tested, therefore LTE, one UTRAN network topology needs to be selected (figure 9).

Figure 9: Test Configuration In Network View

The network components for both e-UTRAN and UTRAN networks are emulated and controlled by CS8 Interactive Tester User Interface. The evolved Packet Core, including PDN-GW, HSS, MMe, and SGW, are all emulated by Spirent’s SR3620. both ePC and e-UTRAN are controlled by a single user interface, providing unified control and a complete emulation of the lTe network.

(10)

4.2. Step 2: Configure Appropriate Downlink Configuration & PDN-GW Settings

To attach to the lTe network successfully, correct Downlink Configurations such as frequency band, Downlink eARfCN, and Transmission Mode need to be inputted in eNodeb entity based on the DUT’s capability (figure 10). In addition, appropriate PDN values need to be assigned in PDN-GW for a successful SIP Registration (figure 11).

Figure 10: Downlink Configuration Settings

Figure 11: PDN Configuration Settings

(11)

4.3. Step 3: enable SRVCC In MMe Test Configuration Setting

As SRVCC is Core Network related its setting is under the MMe entity. In the SRVCC

Configuration section, set Support SRVCC field to True and select the appropriate SRVCC Type.

In this example CS + PS was selected (figure 12). CS8 Interactive Tester will manage and handle the entire network configurations required to enable signalling messages.

Figure 12: SRVCC Configuration Settings

4.4. Step 4: Connect To Instruments/load IMS Message flow/enable Call Processing

Click on the Connect button to connect the CS8 Interactive Tester to the instruments (figure 12).

This connects the lTe eNodeb emulator and enables ePC emulation.

Figure 13: Connect To Instruments

At this stage IMS Message flow script (DMf file) needs to be loaded. IMS Message flow script is targeted for testing of IMS Applications. To load the file from CS8 Interactive Tester software navigate to (figure 14):

(12)

Network è IMS Service è Load an IMS Msg Flow File

Figure 14: Load IMS Message Flow File

This will open the LoadIMSMsgFlow window. In this window, ensure P-CSCF #1 is selected and click on Open File (figure 15) to browse to the IMS Message flow script (DMf file) located in:

C:\Program Files\Spirent Communications\CS8\CS8 Interactive Tester\DMF Scripts

Figure 15: DMF File Browse Window

This folder contains pre-defined IMS Message flow scripts. Select one of already configured SRVCC IMS Message flow files. In this example, a VolTe Mobile originated (Mo) call will be initiated. Therefore SRVCC_verified_MO_main script is selected (figure 16):

Figure 16: Load IMS Message Flow File

(13)

The eNodeb is now ready to transmit signals and the ePC is waiting for the DUT to attach. To enable call processing, click the Enable Call Processing button (figure 16).

Figure 17: Enable Call Processing

Check to see if the connection to instruments succeeded, IMS Message flow script is loaded, and call processing is enabled (figure 18).

Figure 18: Instrument Connection Successful

(14)

4.5. Step 5: Initiate Mobile originated VolTe Call

CS8 Interactive Tester controls the network emulator and runs a real-time state machine to handle RRC/NAS procedures. After the DUT detects the broadcasting information and starts RACH to camp on the cell, the Attach Procedure initiates the state transitions on the RRC, eCM and eMM. The Test Results panel logs the state transition and system status during the attach procedure and PDN establishment. At this point, powering up the DUT will start the cell selection process.

ensure Ue successfully attaches to the lTe network and IMS Registration is successful by monitoring following messages in Test Results panel (figure 19):

• ePS Authentication Succeeded

• ePS Attach Succeeded

• Ue Attached to PDN Successfully

• SIP Message is Received è ReGISTeR

• SIP Message is Sent è 200 oK

Figure 19: EPS Attach & IMS Registration Messages

(15)

Use the VolTe Application on the DUT to initiate a VolTe Mo call. Monitor the Test Results panel for real-time SIP messages sent and received to confirm the call is active and Mo VolTe Call is established (figure 20):

• SIP Message is Received è 180 Ring

• SIP Message is Sent è PRACK

• SIP Message is Received è 200 oK

Figure 20: VoLTE Call Established

(16)

4.6. Step 6: Initiate SRVCC Procedure

To initiate the SRVCC handover in CS8 Interactive Tester navigate to (figure 21):

Call è Initiate Handover Command

Figure 21: Initiate Handover Command

In the Handover Command window, ensure Handover to UTRAN box is checked and correct Cell ID is selected from the drop down menu (figure 22).

Figure 22: Handover Command Window

(17)

The CS8 Interactive Tester then determines that the active voice call needs to be moved from the packet to the circuit domain. Inter-RAT handover from lTe to UTRAN is initiated. As CS + PS SRVCC type was selected prior to the test in MMe, CS + PS type is observed in the handover message (figure 23).

Figure 23: Inter-RAT Handover from LTE to UTRAN Initiated

(18)

Monitor real-time messages in Test Results panel to determine if the Inter-RAT handover from lTe to UTRAN is successful and both CS and PS calls are established on UTRAN. Content of handover success message indicates the SRVCC type and its status. If the SRVCC procedure fails due to call drop or handover failure, the Test Result panel will highlight it through its real-time logging. by analyzing the logs, it can be concluded that SRVCC procedure was successful and both CS and PS calls were established on UTRAN (figure 24).

Figure 24: SRVCC Procedure Success

(19)

Message Analyzer panel can also be examined for further investigation to measure the SRVCC Key Performance Indicators (KPI). The left side of the panel lists the message sequences, while the right side shows the ASN.1-based contents for each message. In this specific scenario mobilityFromEUTRACommand message is viewed (figure 25). It can be confirmed that the Access Stratum Release 9 mobility from e-UTRAN to UTRAN was completed (figure 26).

Figure 25: Message Analyzer Message Sequence

Figure 26: Message Analyzer ASN.1-Based Content

5. CoNClUSIoN

SRVCC is an extremely complicated technology. for an SRVCC call to function properly, key factors such as MobilityforeUTRAN Ies, SRVCC Type, Carrier frequency, and SIP Negotiation need to be configured appropriately by the network elements. Spirent’s proven leadership in Mobility and Channel emulation in addition to expertise in VolTe has made SRVCC testing comprehensive and intuitive. As a result, testing cycles can be drastically decreased for the users. As highlighted in this Application Note, SRVCC testing using CS8 Interactive Tester is simply faster.

Message Analyzer

(20)

6. ACRoNyMS

1xRTT 1x (Single-Carrier) Radio Transmission Technology 2G 2nd-Generation Wireless Telephone Technology 3G 3rd-Generation Wireless Telephone Technology ASN.1 Abstract Syntax Notation one

CDMA Code Division Multiple Access

CS Circuit-Switched

CSfb Circuit-Switched fallback

DMf Data Message flow

DUT Device Under Test

e-HRPD evolved High-Rate-Packet Data

e-UTRAN evolved UMTS Terrestrial Radio Access Network eARfCN eUTRA Absolute Radio frequency Channel Number eCM ePS Connection Management

eMM ePS Mobility Management ePC evolved Packet Core ePS evolved Packet System eV-Do evolution Data optimized fDD frequency Division Multiplexing GeRAN GSM eDGe Radio Access Network

GSM Global System for Mobile Communications HSPA High Speed Packet Access

HSS Home Subscriber Server IMS IP Multimedia Subsystem

IP Internet Protocol

IRAT Inter-Radio Access Technology lTe long Term evolution

MMe Mobility Management entity

Mo Mobile originated

MSC Mobile Switching Center

NAS Non-Access-Stratum

P-CSCf Proxy Call Session Control function PDN Packet Data Network

PDN-GW Packet Data Network Gateway

PS Packet-Switched

QoS Quality of Service RACH Random Access Channel

RAN Radio-Access Network

RAT Radio-Access Technology RRC Radio Resource Control

RSTP Reference Signal Transmit Power

SCC-AS Service Centralization and Continuity Application SGSN Serving GPRS Support Node

SGW Serving Gateway

SIP Session Initiation Protocol SRVCC Single Radio Voice Call Continuity STN-SR Session Transfer Number for SRVCC

Ue User equipment

UTRAN UMTS Terrestrial Radio Access Network VoIP Voice over IP

VolTe Voice over lTe

WCDMA Wideband Code Division Multiple Access

(21)

7. RefeReNCeS

1. 3GPP TS.23.216 V11.9.0 (2013-06): Single Radio Voice Call Continuity (SRVCC)

2. 3GPP TS 129 280 V8.3.0 (2010-01): Sv interface (MMe to MSC, and SGSN to MSC) for SRVCC 3. 3GPP TS 22.278 V11.6.0 (2012-09): Service Requirements for the evolved Packet System 4. 3GPP TS 23.401 V10.7.0 (2012-03): GPRS enhancements for e-UTRAN access (Release 10) 5. GSMA, 2010, IR.92 IMS Profile for Voice and SMS V3.0

6. GSMA, 2011, IR.94 IMS Profile for Conversational Video Service V1.0

7. GSMA, 2011, IR.64 IMS Service Centralization and Con¬tinuity Guidelines V2.0

8. Vittal, Shwetha. “Single Radio Voice Call Continuity (SRVCC) with lTe”. Radisys Corporation.

September 2011

9. QUAlCoMM. (october 2012) VolTe with SRVCC: The second phase of voice evolution for mobile lTe devices [White Paper]. Retrieved from www.qualcomm.com/media/documents/files/

srvcc-white-paper.pdf

10. QUAlCoMM. “Qualcomm Chipset Powers first Successful VoIP-over-lTe Call with Single Radio Voice Call Continuity”. Retrieved from http://www.qualcomm.com/media/

releases/2012/02/02/qualcomm-chipset-powers-first-successful-voip-over-lte-call-single-radio 11. CISCo. Voice over long Term evolution Migration Strategies [White Paper]. Retrieved from http://www.cisco.com/en/US/solutions/collateral/ns341/ns973/white_paper_c11-711982.html

(22)

8. TAble of fIGUReS

figure 1: Mobile Services Market forecast for 2014 ... 4

figure 2: High level Concept for SRVCC from e-UTRAN to UTRAN/GeRAN ... 5

figure 3: SRVCC from e-UTRAN to UTRAN/GeRAN Message flow ... 6

figure 4: Spirent CS8 Mobile Device Tester ... 7

figure 5: CS8 Interactive Tester User Interface overview ... 8

figure 6: CS8 Mobile Device Tester UMTS-lTe Hardware Setup ... 8

figure 7: CS8 Interactive Tester Icon ... 9

figure 8: Application Password and Annual Support Agreement ... 9

figure 9: Test Configuration In Network View ... 9

figure 10: Downlink Configuration Settings ...10

figure 11: PDN Configuration Settings ...10

figure 12: SRVCC Configuration Settings ...11

figure 13: Connect To Instruments ...11

figure 14: load IMS Message flow file ...12

figure 15: DMf file browse Window...12

figure 16: load IMS Message flow file ...12

figure 17: enable Call Processing...13

figure 18: Instrument Connection Successful ...13

figure 19: ePS Attach & IMS Registration Messages ...14

figure 20: VolTe Call established ...15

figure 21: Initiate Handover Command ...16

figure 22: Handover Command Window ...16

figure 23: Inter-RAT Handover from lTe to UTRAN Initiated ...17

figure 24: SRVCC Procedure Success ...18

figure 25: Message Analyzer Message Sequence ... 19

figure 26: Message Analyzer ASN.1-based Content ... 19

(23)

(24)

Single Radio Voice Call Continuity (SRVCC) Testing Using Spirent CS8 Interactive Tester