Measuring TETRA networks

TETRA network operators need to ensure high service quality, particularly where their infrastructure is used by the emergency services.

Measuring TETRA networks

Professional mobile radio conforming to the TETRA standard differs greatly from the public mobile radio networks that we all use every day.

TETRA networks used by the emergency services are of critical importance for public safety. This key difference poses more stringent requirements regarding the availability
and quality of the network service.

Various parameters with associated measurements can assist here, providing reliable information on the quality level. How can the network be monitored effectively, and what points need to be observed to avoid or quickly resolve errors impacting availability?

Measurements on network components
The network operator obtains a lot of data from its own in-house network operations. Based on statistical evaluations, the Network Management Centre (NMC) is able to establish deviations in quality by analysing the changing operating data. This method has two important disadvantages: the network is viewed through the eyes of its manufacturer - signals from radio devices made by third-party manufacturers are often simply ignored; and if a cell fails completely, this remote diagnostics process is useless because no operating data is sent to the NMC at all.

Measurements carried out on site at the base station require special TETRA base station testers. These instruments evaluate the transmitter and receiver of a base station against the TETRA specifications. The disadvantage of these measurements is that the base station must be in a test mode, making it unavailable to the cell. Network operators are increasingly foregoing the measurements even during commissioning, because both the base station and the spare parts are comprehensively tested at the factory. The aerial system is not included in any tests.

The previous considerations assume direct access to the network. The customer does not have that access, but may want to verify the agreed level of quality.

...and in the field
The relevant performance data can be accessed more simply by both network operators and customers using test receivers. The key benefit here is that with measurements directly at the air interface, actual operating data can be acquired from ongoing radio operations and analysed. If a calibrated measurement device rather than a basic radio device is used, conclusions can also be drawn on the radio coverage of the network. This is either measured directly via the reception level (field strength), or a characteristic value is issued based on the decodability of the received signal. The basic principle here is the measurement of the bit error rate. Basic radio devices do not offer these functions as they do not have suitable receivers, and neither the receivers nor the aerials used can be calibrated. If a test receiver is linked up to a PC-based test data recording system, supplied areas can simply be displayed on maps. Top spec models have two calibrated test receivers, allowing simultaneous measurements of the signals from the base station and the mobile radios.

By using protocol analysers, operators gain an even more in-depth insight into processes and the operating status of a radio network. These devices can also comprehensively demodulate and display the communication on the air interface, separated by signalling data, i.e the control messages between the base station and radio device, and payload data such as voice and data.

If air interface encryption is activated on the radio network, the measurement device must know the relevant encryption algorithms and the code. This is because air interface encryption also covers the signalling commands that are important for the analysis. The end-to-end encrypted payload still remains excluded from the analysis for security reasons, and even the network cannot decode it. Unencrypted payload information, including voice data, can still be intercepted and recorded using the appropriate measurement devices, however.

If the algorithm and code are available, or the communication is unencrypted, a comprehensive analysis of the TETRA protocol can be started. The protocol analyser receives all the normal and unexpected events occurring in a radio cell, and stores them in a database. This data can be used to obtain a statistical overview of the service quality within the radio cell being monitored, consisting of highly relevant parameters such as call set-up times, number of interrupted calls, channel load on signalling and traffic channels and much more. These statistical parameters should be monitored for several hours or days as part of a long-term analysis process.

Protocol analysers also permit the analysis of problematic events right down to the individual bit. Malfunctions can be identified reliably and quickly; this includes continuously signalling radio devices that do not receive a response from the base station and that are therefore severely limiting or even overloading the available radio channel. As the TETRA standard allows manufacturers considerably more leeway with the protocol than is the case with GSM, for instance, these events occur a lot more frequently than one would expect.

Even network parameters set incorrectly or using limit values become obvious in a detailed protocol analysis via the radio interface. Access parameters instructing the radio device when to change cell are often set too high, meaning that a large number of cell changes occur.

With faulty adjacent channel information, the radio cannot change cells easily and instead is required to start a time-consuming search process.

Conclusion
Measuring equipment is critical if network operators want full control of their TETRA radio networks. The 'crunch points' here often lie buried in the configuration, meaning that the use of protocol analysers is becoming essential. Network coverage is the most important parameter for service quality, particularly in the expansion phase, and must be tested using appropriate, calibrated test receivers. For measurements on base stations, easily portable, battery-operated base station testers are desirable.

About the author
Thomas Riedl is the product marketing manager at Willtek Communications, responsible for the PMR product range

Written by Wireless magazine
Wireless magazine

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