Throughput and RSSI are two of the most mysterious entities of the AP measurement world.
For Throughput measurement is an issue and for RSSI the interpretation of the measured value is a cause of concern. They vary from device to device and from one environment to other. This makes it almost impossible to define a standard set of values. For you define a standard then a room where the measurements are taken, the devices with which the measurements are taken including the wifi adapters and drivers and the exact same conditions have to be replicated.
The hardware can be duplicated but the environmental conditions, how do we create a mirror of these ?
If this was not complex for SISO systems we now have MIMO, where complex ascends to a totally different level.
As per my understanding RSSI values are generally scaled values not raw power at the antenna. The way these values get scaled depends on the manufacturer of the card and varies with different manufacturers. For e.g. some may choose 10 levels to scale the received power, some may choose 100 levels to scale the received power. Based on the scaling the same number can mean a different value for each manufacturer. There's no specific standard for this. One reason is because RSSI is mostly used for internal calibration. So different scales imply different calibration, which is fine.
A general trend suggests with increasing distance, RSSI decreases and so does the throughput. However, this is not linear. In MIMO systems, RSSI determines whether a client (any laptop or phone) can connect to the AP in consideration. If the RSSI is so low that no signal is detected then it doesn't matter what technology we use, we don't get anything. The noise floor is generally considered to be around -96 dbm. Now, depending on your client's sensitivity the minimum RSSI level will be decided.
It would be our first guess that plotting RSSI vs Throughput will give a straight line. It holds if we have a SISO system and in MIMO if we have very few obstacles. But here comes the troublesome part. In MIMO if we go in the shadow zone of an obstacle the RSSI will most likely drop but the throughput might increase or decrease depending on the processing gains obtained by various paths.
So now how do we measure the actual throughput in an office like environment with different cubicles. If we try to plot the RSSI vs Throughput graph now we will see points all over the place. This does not give a very good picture as to how these quantities behave.
Now lets make things even more complicated. Let the environment change as a function of time. Lets measure the RSSI and throughput at some fixed places. What we now expect(and if we are lucky we might get to see) is a band of values. I shall leave the interpretation to a keen observer.
For Throughput measurement is an issue and for RSSI the interpretation of the measured value is a cause of concern. They vary from device to device and from one environment to other. This makes it almost impossible to define a standard set of values. For you define a standard then a room where the measurements are taken, the devices with which the measurements are taken including the wifi adapters and drivers and the exact same conditions have to be replicated.
The hardware can be duplicated but the environmental conditions, how do we create a mirror of these ?
If this was not complex for SISO systems we now have MIMO, where complex ascends to a totally different level.
As per my understanding RSSI values are generally scaled values not raw power at the antenna. The way these values get scaled depends on the manufacturer of the card and varies with different manufacturers. For e.g. some may choose 10 levels to scale the received power, some may choose 100 levels to scale the received power. Based on the scaling the same number can mean a different value for each manufacturer. There's no specific standard for this. One reason is because RSSI is mostly used for internal calibration. So different scales imply different calibration, which is fine.
A general trend suggests with increasing distance, RSSI decreases and so does the throughput. However, this is not linear. In MIMO systems, RSSI determines whether a client (any laptop or phone) can connect to the AP in consideration. If the RSSI is so low that no signal is detected then it doesn't matter what technology we use, we don't get anything. The noise floor is generally considered to be around -96 dbm. Now, depending on your client's sensitivity the minimum RSSI level will be decided.
It would be our first guess that plotting RSSI vs Throughput will give a straight line. It holds if we have a SISO system and in MIMO if we have very few obstacles. But here comes the troublesome part. In MIMO if we go in the shadow zone of an obstacle the RSSI will most likely drop but the throughput might increase or decrease depending on the processing gains obtained by various paths.
So now how do we measure the actual throughput in an office like environment with different cubicles. If we try to plot the RSSI vs Throughput graph now we will see points all over the place. This does not give a very good picture as to how these quantities behave.
Now lets make things even more complicated. Let the environment change as a function of time. Lets measure the RSSI and throughput at some fixed places. What we now expect(and if we are lucky we might get to see) is a band of values. I shall leave the interpretation to a keen observer.
