hellow my friends
according to the image down…
if the data rate= 54mbps is that mean that the signal strength which arrive the client will be -75 ?
why the number -72 = a less amount of data rate ?
anyone give me an idea..with thanks
if the data rate= 54mbps is that mean that the signal strength which arrive the client will be -75 ?
No, these are the signal that you hear (receive).
When the radio was testing the modulations (6,12,18,etc…) it heard itch modulation (data-rate) with that signal.
Please take a look at a radio datasheet: http://routerboard.com/pdf/97/R52H.pdf
You see this:
IEEE 802.11g:
25dBm / -90dBm @ 6Mbps
20dBm / -70dBm @ 54Mbps
This means that when the radio is transmitting at 6Mbps, it’s tx-power is 25dB and the signal can be so small like -90
For 54Mbps it transmits with less power (20dB) and this means that you receive less signal at the other side. And the signal for Rx needs to be better that -70dB to be able to work with 54Mbps data-rate.
So we will have:
“6Mbps TX 25dB” ↔ loss in the air ↔ “6Mbps RX signal < -90dB”
“54Mbps TX 20dB” ↔ loss in the air ↔ “54Mbps RX signal < -70dB”
For 54Mbps it transmits with less power (20dB) and this means that you receive less signal at the other side. And the signal for Rx needs to be better that -70dB to be able to work with 54Mbps data-rate.
but when the radio trasmits a larger amount of data .it will need more power(more dbm) not less
i mean if we have a car with 1 ton of load and want this car run with 50km/s speed not as we say to want this car run at 50km/s with 2 tons of load actually that car need more power
any one have something to say?
If you load more in a car to go with same speed you need more power in engine (higher gain antenna or more powerfull radio card).
If your radio card will be transmiting with higher modulation (data rate) your amplifier cannot be that accurate so output power decreeses.
on reciever side you need more power to decode signal, but mikrotik shows you what level of signal you actually recieve
On most 802.11 or “off-the-shelf” radios, the transmit power is decreased based on reached a desired EVM amount. When transmiting using simple modulations the amount of error in the recieved signal vs. what is expected can be fairly high, thus you typcially have better recieve sensitivity, and you can transmit at high power even though it is introducing error.
To achieve high data rates (ie. use more complex modulations), the amount of error in the recieved signal vs. what is expected needs to be low, thus the reieve sensitity is not as good, you need high SNR, and you want to minimize transmit errors as well. If you look up what a constellation diagram is, and look and the difference between simple BPSK/QPSK and the 16-64 QAM modulations, and you think of error as the dots moving from the expected position, it makes alot of sense why you need to minimize the error whereever possible.
Sometimes you will see posts from people that talk about lowering the TX power and getting better results, this is part of the reason why. Same goes for amplifiers, and why they don’t work very well in high-data rate applications.
Cheers
really..the mikrotik is a sea
i feel im just a drop in that sea
