Application Note

This application note details the high-level functional requirements of LTE testing and presents solutions for assuring high quality of service (QoS) across networks. We will refer to the challenges met at each phase of the LTE technology lifecycle from lab phase to live network, from a mobile network operator’s (MNO) point of view. Solutions are presented for effective testing of LTE elements and interfaces as well as on end-to-end performance focusing on layer-3-to-7 protocols.

1. LTE INTEROPERABILITY TESTING

ChallengeBuilding a completely new network element like eNB that is 3GPP compliant is quite complex, but this complexity increases exponentially when the complete LTE network has to run smoothly with hundreds of different devices and under significant load conditions. The MNO needs to be 100% sure that the network being launched will not fail when a new smartphone, service or network element is added. That is why meticulous interoperability testing (IOT) needs to be performed.

Testing RequirementsIOT must be conducted on a complete or nearly complete LTE network. IOT testing may be done with a complete LTE network in the test-laboratory, or if possible with live networks core elements, and have the separate eNBs and user equipment in the laboratory. Testing must follow the functional and regression test cases, but every interface involved must be analyzed. With purpose-built analyzers, complete networks can be analyzed and end-to-end functionality can be visualized. Using LTE end-to-end analyzers during the IOT testing phase will shorten the time to production and reduce operational expenditures (OPEX), as well as eliminate issues before the network goes live.

Mobile IOT must include detailed testing of the devices that operators sell as well as the most commonly used roaming LTE dongles/smartphones. Test scenarios must include the following groups of test cases per 3GPP 36.523-1:

› Idle mode operations including public land mobile network (PLMN) selection, cell selection and cell reselection on which the multimode scenario MNO network is built

› Radio resource control (RRC) operations including connection management, connection reconfiguration, measurement configuration, inter-RAT handover and minimization of drive-test-related features

› Evolved packet system (EPS) mobility management operations including EMM common and specific procedures, EMM connection management and non-access stratum (NAS) security procedures

› EPS session management operations including context activation, modifi cation and deactivation, packet data network (PDN) context connection and release, bearer resource allocation and modifi cation and routing of uplink packets

› Packet switched service operations including short message service (SMS) over SGs interface and emergency calls over IP multimedia service (IMS)

› Data transfers over various radio bearer combinations with and without multiple-input multiple-output (MIMO) capabilities

› Multilayer operations including call set-up using circuit switched fall back (CSFB), RRC connection reconfi guration, connection re-establishment intra-LTE and inter-system (3G or 2G), intra- and inter-system (3G or 2G) mobility for PS handovers, inter-system mobility (3G or 2G) for voice using single radio voice call continuity (SRVCC), emergency alert system

› 3GPP LTE requirements for signaling latency, user-plane latency, throughputs and QoS for different quality classes.

Ensuring a Successful LTE Launch by Troubleshooting Throughout the Technology LifecycleMurat Bilgic, Director of Wireless Technology, CTO Offi ce, Tommi Tallgren, Senior Product Manager, Network Analyzers, Gerold Haase, Sales Specialist, Network Simulators

Figure 1. Simple LTE network with LTE elements and interfaces

Figure 2. EXFO’s real-time test environment

© 2013 EXFO Inc. All rights reserved.

Application Note

One SolutionIOT is mostly functional testing, so it is essential for a tester to see the network behavior in real-time. Test systems are used to illustrate and generate reports based on test results. LTE requirements mean a drastic increase in user-plane interest as well as in data-throughputs, thus requiring a new kind of test solution that can handle both user-plane and control-plane data in real time. EXFO’s solution includes seamless real-time analysis of both datasets; the user-plane is also analyzed in separate powerful PowerHawk Pro platforms. Refer to fi gure 3 for the architecture of the solution.

In bigger labs, the test environment (test network) will be used by multiple engineers. The analysis tool must therefore be able to provide access to multiple groups concurrently. EXFO’s PowerHawk mobile analyzer allows up to 10 users at once.

In practice, passive analysis solutions are installed to sniff all data from the open interfaces. The installation is the same whether the elements are in a laboratory or in a live environment. The ideal analysis solution is to use a tap or mirror port from the switch to connect all signaling interfaces over mobility management entity (MME) to one or multiple 1 Gbit/s Ethernet lines. User-plane data links typically use 30% to 60% of the 1 Gbit/s or 10 Gbit/s line throughputs, and thus the S1-U and S5/8 interfaces must be attached to the analyzer probes with direct taps.

For an example of the analysis results from EXFO’s LTE analyzer, refer to the end of section 2.

2. EUTRAN PERFORMANCE AND FUNCTIONALITY TESTS

ChallengeLTE promises superior user-experience when compared to 3G thanks to lower latency and higher throughput. The reality of new, complex technologies is that network element features and software versions need to be tested for each network before public service can be launched.

Before any new functionality can be launched in an operational network, it is important to build a test lab of the operator’s existing network. LTE testing typically starts with vendors providing a small-scale lab to test their LTE capabilities as well as their solution’s unique features.

After selecting the network equipment provider, operators typically build a test network to meet their comprehensive functional-testing needs. Both in the vendor selection as well as in the fi nal functionality acceptance, a proper third-party analysis solution is essential.

Testing RequirementsLTE functionality testing can be conducted in both a full or minimal test network. A complete LTE test environment includes multiple eNB’s, MME, serving gateway (S-GW), packet data network gateway (P-GW) and registers along with LTE dongles or smartphones.

To speed up complete test scenarios, it is often more effective to use a partly simulated environment, where some of the network elements are replaced with simulations. Below is a diagram of the elements that can be simulated in eUTRAN functionality tests.

Simulators can be used to speed up vendor selection and element testing. Test scenarios and loads can be generated more easily than with real handsets. A simulated environment can also be used for negative testing to see how the equipment recovers after a crash, its limitations and how to prepare for the “unforeseeable.”

Refer to the basic LTE functionality in section 1.

Test SolutionBasic LTE functionalities are tested from the user equipment (UE) end, using LTE dongles, smartphones or simulated LTE mobiles. Test requirements should be identifi ed for each network equipment provider’s network and their network releases. Naturally, every element will also be tested separately to understand its behavior in load situations; functional, load and stress testing simulator products should be used.

A mobile analyzer is needed for fast, accurate testing, test reporting and troubleshooting. A third-party tool is needed to speed up the work and give full standard approval. Other professional tools can be used to show errors and provide a root-cause analysis. EXFO’s family of analyzers is one of the very few to be approved by the main LTE element manufacturers (e.g., Huawei, NSN, Ericsson, ALU). MNOs that work closely with vendors need future-proof tools that allow analyzer or raw fi les to be shared with the vendors in troubleshooting cases.

LTE testing needs a powerful user-plane analysis platform with easy-to-use and multifunctional graphical applications. LTE functional testing might use only 10 or 100 mobiles and its signaling is easy to handle in real-time, thus accelerating the testing process. However, the user-plane data rate easily exceeds the gigabytes per second mark. EXFO’s architecture allows real-time analysis of tens of gigabytes per second with graphical analysis of end-to-end signaling.

Figure 3. RAN testing with real EPC

Figure 4. RAN testing with simulated EPC

© 2013 EXFO Inc. All rights reserved.

Application Note

Below are examples of test cases and how they are reported by EXFO’s mobile analyzer software.

Signaling verifi cation and reporting:

Troubleshooting erroneous LTE sessions:

3. LTE FIELD TRIALS AND LIVE NETWORK TROUBLESHOOTING

ChallengeOperators use fi eld tests to ensure superior end-user experience and fi ne-tune their networks. Field tests are essential to understanding network coverage, interoperability, mobility and actual end-to-end network performance. In this phase, it is also critical to prepare troubleshooting systems for fast error detection and fi xing. Field tests and customer trials are needed to confi gure the network and monitoring systems to generate network-level key performance indicators (KPIs) and troubleshoot the network prior to a commercial launch.

Live network troubleshooting requires a level of detailed analysis similar to fi eld tests. LTE network troubleshooting and optimization will be based on the passive analysis of subscriber signaling and user-plane data. For live network troubleshooting, it is essential to get to the root-cause of the problem fast. Systems have to be able to handle live-network data amounts, whether in real-time or via post-processing.

Testing RequirementsField tests are similar to those of an operational network but with lower performance requirements. The main fi eld-testing element is a real-time system that can generate both network-level KPIs as well as detailed analyses. An analysis system must cover every single interface and provide views of network behavior or troubleshooting options for current as well as past activities. For example, every Monday, operators need to see the network usage and behavior from over the weekend.

Troubleshooting relies on the fast drill-down and analysis of LTE network issues. Issues can be purely at the user-plane or transmission end, like packet delays or retransmissions, or signaling problems for the MME or individual users. More often than not, LTE troubleshooting is a mix of signaling and user-plane data analysis, because customer quality of experience (QoE) can only be analyzed with signaling details (e.g., attach-time or quality classes given), combined with the experience of the network usage (e.g., voice-over-LTE (VoLTE), call quality−mean opinion score (MOS) or the sustained throughput of the data-download).

Figure 5. Full E2E session correlation

Figure 7. Detailed signaling can be visualized with individual node names and exported to test reports, e.g., in .pdf format

Figure 6. Full correlation over E2E LTE network

Figure 8. Phase 1 for troubleshooting problematic calls

Figure 9. LTE session and user-plane correlation and throughput visualization

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.

APNOTE271.1AN © 2013 EXFO Inc. All rights reserved. 2008

Printed in Canada 13/01

Application Note

One Solution EXFO’s passive LTE network analyzer includes a high capacity, real-time analysis platform that uses probes. This portable solution is a cost-effective way to perform a needs-based analysis, by capturing data over 1 or 10 Gbit/s links and post-processing this data for statistical and detailed analysis.

A real-time test environment can be prepared by simply installing a few probes in the complete network or segment. As illustrated below, a complete LTE MME site can be covered by one probe capturing all the signaling messages, correlating them and storing the sessions in a database. Another probe can be used to analyze the user-plane and system components in order to fully correlate the user-plane and control-plane sessions. Finally, a KPI server will correlate all interface KPIs and a client server will allow multiple engineers to access the environment remotely.

For live network troubleshooting, portable tools are the ideal solution. Performance and technical teams need independent tools to be able to pinpoint network issues or fi nd the root cause of user problems. The advantage of a portable tool that can handle live network data amounts is that it can be re-used at any location. EXFO offers a wide range of portable solutions for 1 and 10 Gbit/s LTE links. These tools are used to capture the diffi cult part of the network and then post-process the data to fi nd any issues at the user-plane or control-plane end.

Analyzing, optimizing or network-level troubleshooting starts with statistical analysis. High-level network behavior can be visualized from pre-defi ned and customized views illustrating statistics over the network, over selected timeframes. All KPIs, failures or attempts can be seen per MME or TAC or eNB level, revealing the most likely problematic elements. In terms of statistics, users need to be able to drill-down the LTE sessions (e.g., see the details of failures in PDN contexts in certain areas). The session analysis can be used to focus on sessions with specifi c UEs. There are hundreds of values in the session that can be used to fi lter and group LTE behavior over the networks. A few examples of analysis capabilities are illustrated here.

Figure 10. Analysis and troubleshooting platform installation

Figure 12. Examples of real-time live network troubleshooting tools over LTE networks

Figure 11. Portable TravelHawk Pro for 2 x 10 Gbit/s and 4 x 1 Gbit/s links with 14 TB of storage