Wireless technologies | Testers Testing IP-based voice services Mobile networks from the fourth generation (LTE / LTE-A) on use the protocol for all services. Another infrastructure is required in addition to IP to ensure that traditional services such as voice communications continue to function. A 4G device only becomes a “phone” with the help of the IP multimedia subsystem. The R&S®CMW500 wideband radio communication tester can solve a variety of test-related problems.

All-IP communications opens up a for these effects since – in the case of applications lack the aforementioned whole new world of applications in telephony – they considerably impact integration, they cannot ensure a certain mobile communications, but it also the voice quality. 3GPP-based networks QoS and therefore do not always deliver means that the circuit-switched services such as LTE offer the ability to ­separate a satisfactory user experience. The user that were common in 2G (GSM) and IP-based services on the transport layer is also dependent on the proliferation (UMTS) networks for voice transmission and to provide an ensured quality of of the application because the different are no longer available. Instead, voice service (QoS) as needed. This is han- services are generally not interoperable. signals have to be packed in IP pack- dled by a dedicated bearer that is trig- The absence of mechanisms for handling ets. In the past year, data services finally gered by the service in use. A dedicated emergency calls is also a significant overtook voice telephony as the most bearer is a type of virtual channel with drawback of these applications. Conse- important source of revenue for net- its own network address (port range) quently, for voice support in LTE, appro- work operators, but voice telephony still and specified performance characteris- priate mechanisms and architectures accounts for a considerable share and tics. The prerequisite for this function- must be implemented in the mobile net- cannot be technologically neglected. ality is a high level of integration of ser- works to ensure high quality for new Most importantly, the voice quality can- vices in the wireless device or modem. features like video telephony as well as not fall below the accustomed standard. tried and trusted features such as classic In practice, such legacy services often Services such as Skype and Face- telephony and emergency calls. cause problems in IP-based networks Time already offer functions like because packet loss, jitter and packet video telephony and messaging ser- delays can occur during transmission. vices. Such functions set user expecta- The IP multimedia subsystem Ideally, the receiver should compensate tions, but since these over-the-top (OTT) A key technology for integrating voice services in an LTE network is the IP mul- timedia subsystem (IMS). IMS offers a framework that supports IP-based mul- timedia services. IMS requires new net- Network architecture with integrated IMS work elements in the network architec- ture (Fig. 1).

Applications IMS was developed by 3GPP during SMS over IMS Voice over IMS (VoLTE) Video over IMS RCS … the UMTS standardization. It was since improved and is now also used for LTE voice services (voice over LTE, VoLTE) IMS (Fig. 2). The session initiation protocol (SIP) is the base protocol of IMS. SIP uses an IP network to establish the con- Broadband Fixed connection Mobile access Broadband wireless access cable access (GSM, UMTS, LTE, C2K) (WLAN) nection between subscribers. To ensure a consistently high QoS, network oper- ators have now started to roll out VoLTE Fig. 1: IMS as key technology in mobile networks. services for voice and video telephony

18 IP multimedia subsystem (IMS) Testing with the R&S®CMW500

Signaling Media transport Utilities Media SDP coding Application layer Voice / video data SIP RTP RTSP RCTP DNS DHCP Call setup

Transport TCP UDP LTE layer WLAN

Network IPv4, IPv6 layer

Access Fig. 3: Parallel testing of LTE and WLAN with LTE technology the R&S®CMW500 wideband radio communi- cation tester. SDP session description protocol RTSP realtime streaming protocol DNS domain name server SIP session initiation protocol RTCP realtime control protocol DHCP dynamic host TCP transport control protocol configuration protocol RTP realtime transport protocol UDP user datagram protocol Fig. 2: IMS protocol structure.

based on IMS – with a guaranteed VoLTE, VoWLAN and WLAN between LTE and WLAN as well as sin- seamless transition to circuit-switched traffic offload testing with the gle radio voice call continuity (SRVCC), services, i. e. to 2G and 3G networks. R&S®CMW500 i. e. the handover between LTE and leg- The R&S®CMW500 wideband radio acy networks such as GSM and UMTS. They are also working to ensure uninter- communication tester comes with rupted service and to integrate WLANs everything needed to test these mecha- Since mid-2015, the integrated IMS during the transition from outside to nisms on mobile devices (Fig. 3). On the server has offered the ability to test sev- inside. Voice over WLAN (VoWLAN) is a data application unit, the tester provides eral DUTs (IMS subscribers) in paral- network operator driven technology that all access technologies and all neces- lel. Now true end-to-end media connec- also uses SIP/IMS to route voice over sary servers, such as an ePDG and an tions between two mobile devices can WLAN access points. Instead of using IMS server (Fig. 4). It tests the handover also be tested. the network operator’s closest base sta- tion for the call, the voice packets are tunneled through the Internet to a gate- VoLTE, VoWLAN and WLAN traffic offload testing way provided by the network opera- tor. This evolved packet data gateway 3GPP structure (ePDG) creates a link between the LTE IMS server, VoLTE VoWLAN network and WLAN. IMS and upcoming generations of smartphones that sup- WCDMA SRVCC port all of these mechanisms as stan- Core network PDN-GW dard will enable uninterrupted and high- quality voice service for all available access technologies. LTE

LTE

IPsec ePDG Wi-Fi WLAN IMS IP multimedia subsystem SRVCC single radio voice call continuity ePDG evolved packet data gateway VoWLAN voice over WLAN PDN-GW packet data network gateway VoLTE voice over LTE IPsec Internet protocol security

Fig. 4: LTE / WLAN traffic offload via an ePDG.

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Testing with multiple virtual remote stations

Virtual subscriber 2 Virtual subscriber 3 Virtual subscriber 4 Tel.: #2110 Tel.: #2111 Tel.: #2112 Behavior: answer Behavior: no answer Behavior: declined Codec: narrowband Codec: n.a. Codec: n.a. Endpoint: audio board Endpoint: n.a. Endpoint: n.a.

Virtual subscriber 7 Virtual subscriber 6 Tel.: #2115 Tel.: #2114 Behavior: answer Behavior: answer Codec: wideband Codec: wideband Endpoint: audio board Endpoint: loopback

Virtual subscriber 1 Virtual subscriber 5 Tel.: #* Tel.: #2113 Behavior: answer Behavior: busy Codec: narrowband Codec: n.a. Endpoint: loopback Endpoint: n.a.

Real subscriber 1 Real subscriber 2 RF 1 RF 2 # 1220 # 1221 (default user) # 5566778899 # 5544332211

Fig. 5: The virtual subscriber concept in the IMS server of the R&S®CMW500 allows testing under a variety of scenarios.

The new virtual subscriber concept codecs into the existing architec- Precise VoLTE and allows users to create and test multi- ture. The new audio codec enhanced VoWLAN analysis ple virtual remote stations that have dif- voice service (EVS) in particular is a The flexibility of IP-based communi- ferent characteristics and functionalities. milestone on the road to high-qual- cations also comes with unwanted A variety of scenarios such as codecs, ity audio connections. At its high- side effects such as jitter and packet error rates, etc. can be verified quickly est quality level, EVS can transmit the loss. The data is buffered in the and effectively. Real phones with differ- entire human audible frequency range receiver in order to minimize or elim- ent firmware can be tested against each (Fig. 6). The R&S®CMW500 with the inate these effects. To test the effec- other (Fig. 5). R&S®CMW-KS104 option supports EVS- tiveness of implemented measures, the based voice over IMS calls for vari- R&S®CMW500 tester’s data application The flexibility of IP and IMS enables net- ous wireless technologies, e. g. LTE and unit offers IP impairments to deliber- work operators to easily and afford- WLAN. ately cause such errors and IP analysis ably integrate new audio and video to analyze them. The R&S®CMW-KM051 deep packet inspection option allows users to verify the configured jitter pro- Typical codec bandwidths files and measure the data loss. Com- bining and simultaneously analyzing Full band the three areas of RF, protocols and Super wideband data in one device enables users to pre- Wideband cisely analyze the VoLTE and VoWLAN Level Narrowband processes for errors. IP analysis can 0 dB –3 dB Fig. 6: Typical trans- be used to correlate data packets from mission bandwidth of OTT applications to the correspond- voice codes. The new ing applications and draw conclusions 20 50 300 3400 7000 14000 20000 Frequency in Hz EVS codec covers about their communications behavior everything. (see below).

20 Focus on quality of experience voltmeter, supplies the smartphone with measurement capabilities, ­wireless An important criterion when evaluating power. IP-based events can be corre- device manufacturers can analyze smartphones is their single-charge bat- lated with the smartphone’s power con- the VoLTE and VoWLAN features of tery life. Batteries with higher capac- sumption. The R&S®CMW500 tester’s a device in detail. The tester’s inte- ity are too large and heavy to install deep packet inspection engine enables grated data application unit with inte- in smartphones. Developers have to users to quickly see the correlation grated servers for IMS and ePDG increasingly focus on the current drain between an application, its data traf- allows users to test the proper interac- of individual hardware components fic and power consumption. Ineffective tion between hardware and software and especially applications. Application applications with optimization potential for WLAN offload or SRVCC scenar- requirements and the modem’s network are easy to identify. ios in a reproducible laboratory envi- settings need to be compatible. ronment. Event-controlled power con- The setup in Fig. 7 can be extended for sumption measurements on smart- Functions such as ­discontinuous VoLTE by adding the R&S®UPV audio phones show improvement potential in reception (DRX), which is a power analyzer, making it possible to monitor hardware components and applications. saving mode in LTE, reduces the the power consumption during a VoLTE Network operators can identify error power consumption and must also call while simultaneously measuring the sources early on by accurately measur- be tested in connection with VoLTE. audio quality. ing the VoLTE performance under spe- Rohde & Schwarz offers a fully auto- cific network characteristics. Mobile mated solution based on the Summary phone manufacturers and network R&S®CMWrun test sequencer soft- LTE networks and WLAN transmit operators have a universal tool to test ware and R&S®CMW-KT051 software voice just like other data: as a packet- various parameters with regard to func- package. In the test setup in Fig. 7, switched IP stream. Special measures tionality, voice quality and other relevant instead of using the built-in batteries the are needed to ensure the necessary data in the lab before bringing a device R&S®NGMO2 remote controlled power QoS. Thanks to state-of-the-art net- to market. supply, which is also a precise digital work simulation and the R&S®CMW500 Christian Hof; Stefan Diebenbusch

Fig. 7: Event-controlled power consumption measurement on a smartphone. Power-hungry applications are easy to identify.

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