In general, there are three ways to determine the QOS of a GSM network:
• Drive tests. Teams, paid by the network operator, drive on predefined routes in the network and periodically initiate calls. The results (e.g., unsuccessful handover, low-quality audio, dropped calls, signal strength) are transferred from the MSs to a dedicated PC, where the respective data are available for postprocessing. This kind of measure- ment represents most closely the network quality as real subscribers experience it. The disadvantage, however, is that only a limited area and a small time window can be tested and the testing is extremely expensive. Real-time measurement tools for call quality like QVOICE
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belong to the same category but allow for a more objective judgment on the call quality.
• Protocol analyzers. Preferably at a central place, protocol analyzers are connected to BTSs, BSCs, and MSCs over a period of time. Fre- quently, these devices come with remote access capabilities, which eases most configuration changes. Captured trace files can be uploaded to a central office for statistic evaluation. When problems are detected, the trace file needs to be analyzed manually and more thoroughly.
Measurements with protocol analyzers have the advantage that all cap- tured events are available for later, detailed analysis. The disadvantage is that it is not commercially feasible to have them in such great num- bers that an entire GSM network could be observed permanently. (Of course, vendors of protocol analyzers might have a different opinion.)
• OMC. Several counters can be activated in the BSS and in the NSS from the OMC to provide the central office with the most important data about network quality. Various counters for all kinds of events permanently provide the network operator with information about the state and quality of the network. Examples are counters for the number of incoming or outgoing handovers; call drops before, during, and after assignment; and dropped calls due to missing network or radio resources. The major advantage of measurements via the OMC is that it provides results about the quality of the entire network rather than single BTSs or BSCs. On the other hand, this method is the most abstract one and the one that least relates to what the subscribers are encountering. Furthermore, it hardly allows for a detailed postmortem analysis.
Only the combination of the measurement results of OMC, protocol test equipment, and the–most expensive–drive tests, allows to make a qualified and objective statement about the real Quality of Service.
The focus of the following explanation is put on measurements with pro- tocol analyzers, whereas this information can be translated, fairly easily to OMC measurements.
13.1.1 OMC Versus Protocol Analyzers
QOS involves the permanent observation, supervision, and adjustment of the various network parameters in a telecommunication system. GSM makes no exception. One of the most important functions of the OMC is to provide
feedback about network quality in real-time (more or less). For that purpose, a network operator can choose from a large selection of measurable events that may occur in the BSS or NSS, to be gathered in a predefined time period and then displayed at the OMC. It is a typical task of the network operator to deter- mine the network quality from the available data and to decide on eventual cor- rective measures.
The time period that a single TS is occupied, the rate of ASS_FAI, sepa- rated for MOC and MTC, and the number of handovers, based on DL_QUALITY, are more examples of such counters, to name only a few.
The OMC has some advantages in detecting existing and potential net- work problems, compared to protocol analyzers:
• The OMC is part of the standard delivery of a GSM system and as such requires no extra procurement.
• The OMC plays a central part in its function as an O&M platform. If network quality is monitored by the OMC, countermeasures can be taken immediately when problems arise.
• Although protocol analyzers allow the capture of data on the A-interface, other important data have to be gathered on the BTS level, that is, on the Abis-interface. A complete analysis of data related to the entire coverage area by protocol analyzers, including data from the Abis-interface (e.g., idle channel measurements on the Air- interface) is not possible, due to logistical and financial limitations.
That is where the OMC appears as the optimal tool.
• The OMC measures events and provides the results of respective coun- ters to the network operator. Those values later can be processed and evaluated. For that reason, the OMC is the optimal aid for statistics tasks.
The protocol analyzers, on the other hand, can also claim a number of advan- tages as follows:
• A protocol analyzer is not part of the standard delivery of the infra- structure but anindependenttool. To analyze the OMC counters, the field engineer needs system-specific know-how for the measurement.
For the protocol analyzer system-independent know-how is necessary.
• The OMC is of little help for problem analysis during the process of bringing a network or single network elements into service (without subscriber traffic). That is different for protocol analyzers.
• Protocol analyzers allow the network operator and the system supplier to conduct measurements on the lowest level; it allows capture of com- plete call traces.
• Protocol analyzers typically come with additional software that pro- vides for excellent statistical analysis. That enables the field engineer to evaluate a situation quickly. Technologically advanced software makes a manual analysis of data unnecessary in most cases.
In summary, both OMC and protocol analyzers are necessary tools for the net- work operator as well as for the system supplier. That is particularly true for the supervision of QOS. OMC and protocol analyzers perfectly supplement each other in this area, to ensure optimal network quality for both the network operator and subscribers. For those reasons, network operators use the OMC to monitor a GSM network on a broad scale, while they still have specialists with protocol analyzers for the bottom-down analysis of specific network problems.
If the OMC detects a problem but is not able to identify the cause of the prob- lem, the specialist goes on site, usually with a protocol analyzer, to narrow the problem down.
In the laboratory of a system provider or for product development and integration, the protocol analyzer plays a much more important role than the OMC.
13.1.2 Protocol Analyzer
A protocol analyzer is a measurement tool with a high impedance that can be connected between two system devices to intercept the digital data traffic between the devices (Figure 13.1). The focus here is on the analysis of signal- ing, that is, the control information between the devices. For that purpose and to simplify its operation, a state-of-the-art protocol analyzer needs the following:
• The complete hardware and software necessary to detect and adapt to the settings of the various interface configurations and time slots;
• Software that allows decoding of the binary PCM-code in plain text;
• A screen to display the measurement results;
• Facilities for remote access;
• Storage capacity of an appropriate size to store the measurement results for postprocessing;
• Software tools to filter or search for specific data and parameters;
• A software interface to regular PC editor to be able to export data, for example, for reports;
• Flexible and easy-to-use statistic functionality.
The majority of the protocol analyzers available today meet those requirements reasonably well. Operation is fairly simple, because most of the devices are PC- based and offer additional hardware in form of expansion boards.
The protocol analyzer of the late 1990s consists of a mechanically robust PC expansion board loaded with software and offers the above listed function- ality and additional features. Built into a laptop computer or desktop PC, pro- tocol analyzers are portable or can be used in versatile ways in the laboratory.
It is worthwhile to point out what enormous progress has been made in recent years by the manufacturers of such equipment. While there was hardly any choice in the beginning of GSM (1991), system manufacturers and net- work operators quickly reacted to the increased need. Today, a multiplicity of protocol analyzers are available from many European and U.S. vendors. With GSM, a new era has started in this area. High-frequency (radio) measurements clearly have lost importance, compared to protocol measurements. GSM, as a digital standard, carries the analog problems on the Air-interface (e.g., interfer- ence), digitally coded into the BSS. Most radio-specific problems are well suited for analyzing and narrowing down the problem with a protocol analyzer.
FAS / NFAS TS 1 TS 2 TS 3 TS N FAS / NFAS TS 1
FAS / NFAS
FAS / NFAS TS 3 TS 2 TS 1
TS 1 TS N
Forward direction
Backward direction
Protocol analyzer Signaling
Signaling Figure 13.1 Setup of a protocol analyzer.