![]() The majority of path loss is due to physical objects in a room or building and the distance between the AP and a wireless client. Typically, the antennas are built right onto the wireless adapters or within a portable PC, or directly connected to the RF electronics in an AP. In an indoor wireless LAN environment, the antenna cabling distances are so short they are negligible. The total loss end-to-end is known as the path loss. The losses are actually cumulative, working together to degrade the signal. Notice that this list covers conditions as they are encountered along the signal path from transmitter to receiver. ■ Cable loss from the receiver's antenna to the receiver ■ Free space loss as the signal travels through the air ■ Cable loss from the transmitter to the antenna Signal loss can come from any of the following: Whenever an RF signal leaves the transmitter, even before it reaches the antenna, it is subject to outside influences that will reduce its strength. Receiver power levels are referred to as receiver sensitivity. This is because receivers must be very sensitive to low signal levels (much lower than the 1 mW reference power) so that poor signals can be received with clarity. Negative dBm values are often given for receivers, rather than transmitters. ■ Whenever the power doubles, the dB change is approximately +3. ![]() ■ Whenever a power value is halved, the dB change is approximately -3. This change is significant because it demonstrates an important rule of thumb: The dB value changed from 0 to approximately -3 dB, respectively. TIP Notice that the sample power of 0.5 mW is half of the previous example, 1.0 mW. For example, a signal power of 0.5 mW would result in 10log10(0.5 mW/1 mW) or -3 dBm. This doesn't mean that a negative amount of power is produced- rather, it means a power level that is less than the reference. Most often, transmitter power levels will have positive dBm values because they are stronger than the reference.ĭB values can also be negative. Positive dB values result from power values that are greater than the reference power. So far in this discussion, dB values have all been positive numbers. Therefore, 0 dBm always represents an output power that is equal to the reference power. If the output power was decreased to 1 mW, the result would be 10log10(1 mW/1 mW) or 0 dBm. In dB, the output power would be represented as 10log10(100 mW/1 mW) or 20 dBm. As an example, suppose that a wireless AP transmits at 100 mW. You will commonly see dBm used with wireless LANs because the power used is around 100 mW or less. ![]() ■ dBw-The signal power is compared to 1 W ■ dBm-The signal power is compared to 1 mW (the "m" in dBm reflects the "m" in milliWatt) In those cases, the decibel abbreviation also changes to reflect the reference power: Most often, a reference power of 1.0 mW or 1.0 W is used. Here, Psig is the actual signal power and Pref is the reference power. ■ dB is logarithmic, representing a wide range of values on a linear scale To compute a power ratio in dB, use the following formula: ■ dB measures a ratio of actual power to a known reference power ![]() The decibel (dB) is a more flexible way to deal with power quantities for two reasons: Power values can vary over such a wide range that making comparisons or computations is difficult. To put signal power into perspective, Table 17-3 shows typical power output from a variety of sources. Other factors that affect the signal strength can be taken into account, too.Īn RF signal can be measured as a function of its power or energy in units of Watts (W) or milliWatts (mW)-one milliWatt is one-thousandth of one Watt. Because so many variables exist in a wireless environment, being able to quantify an RF signal as it is transmitted and received is handy.
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