Measuring the service level in the 2.4 GHz ISM band

Abstract

In this report we provide the findings of the 2.4 GHz service level research. Here service level means the following: can all devices in the 2.4 GHz band fulfill their communication needs. In other words this corresponds to the overall Quality of Service (QoS). The project is a short research exploratory project of about 400 hours in collaboration with Agentschap Telecom, the Dutch Radiocommunications Agency. First of all a survey has been made to investigate which measurement methods can be used to assess the service level in the 2.4 GHz. Here the focus is on IEEE 802.11b/g/n (WiFi) systems. The service level can be measured at several levels of the OSI model: spectrum sensing (physical layer) and packet sniffers (datalink layer). Power level measurements are used to assess the utilization of the 2.4 GHz ISM band. On the other hand packet sniffers are an appropriate method to measure congestion and to pinpoint problems. Secondly, in this project the interferer mechanisms of several sources (microwave, wireless A/V transmitter, Bluetooth, second WiFi network) have been measured in a controlled environment. It turns out that interferers not only increase retry rate, but also trigger unwanted WiFi mechanisms; especially the hidden node mechanism (Request To Send (RTS)/Clear To Send (CTS) packets). So this means that the CTS/RTS control packets, but also the retry rate can be used to identify congestion. The spectrum measurement results allow to identify which interferer source causes congestion. Finally, also a measurement setup is presented that allows to measure the service level. In addition, initial measurements are provided of live environments (college room, office room, city centre). The results show inefficient use of the wireless medium in certain scenarios, due to a large frame rate of management and control packets compared to the data frame rate. In a busy WiFi environment (college room) only 20% of all frames are data frames. Of these data frames only 1/10 are actual data frames as most data frames are so-called null frames; used to keep a WiFi connection alive in power save mode. From all frames about 70% are control frames of which most are ACK frames and in less extend CTS/RTS frames. More research is required to identify the reasons for the high number of control frames. It is likely that there is significant interference, probably due to the many WiFi devices. This is also depicted by the retry frame rate (7%). Combining spectrum sensing with packet sniffing seems to be a good method to assess the service level in the 2.4 GHz ISM band. However, the interferer mechanisms that occur between WiFi networks, WiFi devices and other technologies are complex. More research is needed to enhance the developed proof-of-concept demonstrator and to have a better understanding of the interferer mechanisms in WiFi systems