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Why is the wireless signal (RSSI) negative? In fact, why is the received wireless signal negative? Is it easier to understand this way? Because the wireless signal is mostly at the mW level, it is polarized and converted to dBm, which does not mean that the signal is negative. 1mW is 0dBm, and less than 1mW is a negative dBm number.

Just figure out the definition of signal strength:

RSSI (Received Signal Strength) Received Signal Strength Indicator

Rss=10logP,

Just substituting the received signal power P is the received signal strength (sensitivity).

[Example 1] If the transmit power P is 1mw, it is 0dBm after converting to dBm.

[Example 2] For a power of 40W, the converted value in dBm should be:

10lg(40W/1mw)=10lg(40000)=10lg4+10lg10+10lg1000=46dBm.

Why are the measured dbm values all negative?

A: The first thing we need to know is that the wireless signal dbm is all negative, and the maximum is 0. Therefore, the measured dbm value must be negative. Because the dbm value is 0 only in one case, which is the result of experimental measurement under ideal conditions, we generally think that dbm is 0 as its maximum value, which means that the receiver has received all the wireless signals transmitted by the transmitter. , that is, as much power as the wireless router transmits, the received wireless network card gets as much power. Of course, this is measured in an ideal state. In practice, even if the wireless network card is placed next to the transmitting antenna of the wireless router, it will not achieve the effect of dbm being 0. Therefore, the measured dbm values are all negative numbers. Don't blindly think that negative numbers are bad signals.

power unit

dBm

dBm is a value that evaluates the absolute value of power, and the calculation formula is: 10lgP (power value/1mw).

[Example 1] If the transmit power P is 1mw, it is 0dBm after converting to dBm.

[Example 2] For a power of 40W, the converted value in dBm should be:

10lg(40W/1mw)=10lg(40000)=10lg4+10lg10+10lg1000=46dBm.

dB

dB is a value that characterizes the relative value. When considering how many dB the power of A is larger or smaller than the power of B, the following formula is used: 10lg (power of A/power of B)

[Example 6] A's power is twice as large as B's power, then 10lg (A's power / B's power) = 10lg2 = 3dB. That is, A's power is 3 dB greater than B's power.

[Example 7] The 100-meter transmission loss of a 7/8-inch GSM900 feeder is about 3.9dB.

[Example 8] If A's power is 46dBm and B's power is 40dBm, then it can be said that A is 6 dB larger than B.

[Example 9] If antenna A is 12dBd and antenna B is 14dBd, it can be said that A is 2 dB smaller than B.

How to see the received power from the dbm value

dbm is a unit that represents the absolute value of power, and its calculation formula is 10lg power value/1mw. For example, if the received power is 1mw, the converted value according to the dbm unit should be 10lg 1mw/1mw=0dbm. Of course, in the actual transmission process, it is difficult for the receiver to reach the receiving power of 1mw. Therefore, we can reversely deduce the power value received by the receiver from the dbm value through this formula.

Misunderstanding 1: The smaller the dbm value, the better

Since the dbm value mentioned above is all negative, many people think that the smaller the dbm value, the better. In fact, this perception is wrong. As mentioned earlier, the maximum value of dbm is 0, and it is an ideal state. The closer it is to the dbm value in the ideal state, the more it means that the power transmitted by the wireless router is received by the wireless network card. Therefore, the dbm value should be as large as possible. -50dbm means that the received wireless signal is better than -70dbm.

Myth 2: The bigger the dbm value, the better

There are many friends who think that since the dbm value is 0, it means that the effect of receiving and sending signals is the best, so we should make the dbm value of the enterprise wireless network as large as possible. In fact, this view is also wrong. Although the larger the dbm value, the better the effect of sending and receiving signals, but at the same time, we also need to install enough wireless signal relay devices for the internal wireless network of the enterprise, which is not a small cost. Experiments show that when the wireless signal scanning component of the XP system is displayed as "very good", it can meet the network transmission requirements, and there is no problem with speed and stability, and the dbm value corresponding to this "very good" status is 0 to -50dbm. Therefore, we only need to ensure that the dbm value of each part of the internal wireless network of the enterprise is not greater than -50dbm, and the wireless network established in this way is a high-speed and stable network. Our investment in wireless network will be the most cost-effective.

hint

Of course, sometimes due to financial considerations, we cannot guarantee that the dbm value of each part of the internal network of the enterprise is between 0 and -50dbm, so it is also guaranteed to be between 0 and -70dbm during measurement. Because when the wireless signal received by the XP system is less than -70dbm, the transmission is unstable and the speed is slow, so our wireless network cannot be used normally.