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TEST & MEASUREMENT readily available live networks, and is available off-the-shelf for a reasonable cost. Typical causes of M2M communications malfunctions With a cellular network simulator to hand, device developers can use their imagination to devise stress tests for their M2M system and answer some basic questions. Do handovers work between cells, between networks or between different wireless protocols? What happens to the application’s data when a handover fails? Does a device connect or re-connect to the network correctly? Can the device successfully register and authenticate a SIM card? What is the data throughput when connected? And when performing a handover? Does the device handle the simultaneous reception of a large number of SMS messages correctly? How sensitive is the M2M device’s RF reception? In practice, how weak does the signal have to be before it loses its connection to the network? For engineers with no previous experience of communications testing, it will often not be obvious how to design test routines to implement scenarios such as these. Network Fig. 3: The MD8475A network simulator from Anritsu. simulators therefore feature software tools that abstract the user away from the underlying test system code, and that provide an intuitive graphical user interface (GUI) for configuring and running many kinds of test scenario. To illustrate the operation of such a GUI, figure 1 shows Anritsu’s SmartStudio, which is built into its MD8475A Network Simulator. SmartStudio provides pre-defined menu options for configuring multiple cells and cellular standards. It starts by requiring the user to choose which radio access technology (for instance, ‘WCDMA to GPRS handover’) should be used in the simulation. This choice will be based on the cellular standards the M2M device supports. The next step is to set up the desired cellspecific parameters, or simply to select one of the pre-defined standard cell parameters. This can be done quickly and with a minimum of knowledge of cellular technologies. Then the user just has to click the ‘Play’ button, and the network is created and available for testing. The cell configurations allow the user to reproduce the network settings of any available mobile network in the world. The typical settings are for the mobile network code (MNC) and mobile country code (MCC). For instance, the codes MCC 262 and MNC 02 are used by Vodafone D2 GmbH in Germany. These settings can be used on the MD8475A to test the ability of an M2M GSM/GPRS device to register with a network and to be authenticated using identification data stored on the device’s SIM card. The instrument logs the stages in this authentication procedure for viewing by the user. The instrument can also test the device’s ability to register with other networks. For instance, a device that is pre-programmed to work on Vodafone will be configured to register with Vodafone cells where appropriate. But the device might also be capable of registering with cells operated by other network providers. The instrument can test the device’s cell-registration behaviour in the presence of cells of multiple providers, taking into account the PLMN (Public Land Mobile Network) identifying information, restriction status, and downlink signal level of each cell. It also tests whether the device successfully performs location registration to that cell. The instrument can also perform a cell re-selection test, which is required should a cell change its restriction status or downlink signal level. The final step in this family of tests is to test the device’s ability to change cells – an action that might be required when a cell’s downlink signal level changes after the initiation of voice or packet connections. This test checks that the mobile device tries to maintain service by re-selecting the optimal cell after comparing downlink signal levels, and moving to the new optimal cell. Assessing device performance The select/re-select and handover set of tests is just one among many that can be easily configured and run using a network simulator and software such as SmartStudio. For each test set, the developer must make a judgement about the application’s tolerance of failure, taking into account the end user’s requirements and expectations. In the case of the select and handover tests, for instance, a device should pass every time as these are basic functions in cellular telephony. A comprehensive wireless network simulator will be able to perform all the tests required to verify the performance and functionality of any M2M application - see figure 2. Instruments from Anritsu – the MD8470A (for 2G and 3G) or the MD8475A (for 2G, 3G and LTE- figure 3) are able to hide the complexity of the wireless standards, and let the engineer focus on the application itself, delivering a stable test environment and a simple user interface for the set-up of complex scenarios. And integrated tools for monitoring, logging and failure analysis enable the user to quickly identify and analyse problems in the system. 34 Electronic Engineering Times Europe September 2013 www.electronics-eetimes.com


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