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Inssider 4.01/9/2024 ![]() ![]() The logarithmic nature of the dBm measurement, coupled with the fact that the RSSI range used for measurement contains dBm “gaps” (due to the integer nature of the RSSI value), has led many vendors to map RSSI to dBm using a lookup table. The main use for the signal level measurement is to determine when signals are so weak as to warrant looking for another AP (roaming) - typically at a 20% reading. Worse, wireless NIC manufacturers do not measure signal strength so accurately at the upper levels. Depending on what wireless chip (and hence RSSI-MAX) is used, the user may see this as anything between a 3dB and 12dB change! This is obviously absolutely useless as a measurement tool. A very slight change in the path loss on a channel can push a measurement up or down by 3-4 dBm. Just this one artifact alone is enough to nullify the majority of the "tests" conducted by forum members. In the case of Cisco, the jitter would be small, with Symbol, it would be perhaps three times larger. ![]() You've all seen the signal level jumping by three or four dB between readings I am sure. Right away we can also see that the lower the RSSI_max value used by a wireless card's manufacturer, the larger the "jitter", or jump by several dB in the signal indicated level as the signal increases slightly and crosses the threshold into the next "step". And so all of us are making comparisons between varying situations with tools that are also varying in effectiveness (I hesitate to use the term "accuracy" here - there is no implied accuracy whatsoever in the RSSI system!). Every wireless card manufacturer, and there are many, can choose his own RSSI-Max level, and this may vary depending on the version of their particular chipset. ![]() ![]() Herein lies the reason that making measurements with simple tools like inSSIDer are so difficult. Secondly, it must be recognized that all possible energy levels (mW or dBm values) cannot be represented by the integer set of RSSI values. First, it is necessary to consider what range of energy (the mW or dBm range) that’s actually being measured. There are, therefore, two important considerations in understanding RSSI. Therefore, if RSSI changes by 1, it means that the power level changed by some proportion in the measured power range. Whatever range of actual energy is being measured, it must be divided into the number of integer steps provided by the RSSI range. Therefore, it can be seen that the RF energy level reported by a particular vendor’s NIC will range between 0 and RSSI_Max. The Atheros chipset uses an RSSI_Max value of 60. Symbol uses an RSSI_Max value of only 31. For example, Cisco chooses to measure 101 separate values for RF energy, and their RSSI_Max is 100. However, no vendors have actually chosen to actually measure 256 different signal levels, and so each vendor’s 802.11 NIC will have their own specific maximum RSSI value (“RSSI_Max”). RSSI is an arbitrary integer value, defined in the 802.11 standard and intended for use, internally, by the microcode on the adapter and by the device driver. Notice that nothing has been said here about measurement of RF energy in dBm or mW. This numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Receive Signal Strength Indicator (RSSI). The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. Why RSSI isn't very useful for signal measurements I'll post some more charts here shortly, watch this space. As a quick guide, if you do not get about a very noticeable 2-3dB increase when changing from 84 to 251, then the output is being limited by the driver. The easy way is to answer the question yourself, for your own particular set of circumstances, by varying the power setting on your router while monitoring the strength on your wireless card - use inSSider as an addon if necessary. But anyway, the easy answer is "Yes" but this may vary with the prevailing wind. That's why I said, everyone on the forums (and I'm talking of at least 20 different forums I've looked at in many countries) keep coming up with different answers. It depends on how you measure it, at what speed, mode, modulation class, country regulatory mode, and which bits of the 80211 specifications are implemented in wireless cards, routers, and if they work properly together, or even at all, how the user has set them, what other clients using what mode are sharing the channel or are associated. ![]()
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