Understanding WiFi signal strength vs WiFi speed
By Michael Lok September 8, 2015
- Stronger signal strength is correlated with higher data transfer speeds
- But past a point, it does not produce additional throughput gains
THE relationship between WiFi signal strength and the speed at which data can be transferred over that signal is something that needs to be understood when it comes to WiFi performance.
One question we constantly get is this:
When I connect my computer to a wireless network, does a stronger signal always imply faster webpage loading and downloads?
The answer, like all answers to WiFi questions, can be difficult to get a grip on. So here's a good, fairly simple explanation that should help clarify things.
All other factors (of which there are many) being equal, stronger signal strength is correlated with higher data transfer speeds, with a couple exceptions and assuming an optimal physical layer data rate selection algorithm.
The diagram below shows a typical relationship for any modern wireless system with adaptive modulation (click to enlarge):
The data transfer speed increases up to a point as signal strength increases since higher signal strengths enable the use of higher PHY (PHYsical layer data) rates, also known as MCS (Modulation and Coding Scheme) in modern WiFi.
A simple way of putting it is to think of different MCS as being somewhat like different gears on a bike or car.
Once there is sufficient signal strength to operate reliably in the maximum supported MCS rate, additional signal strength does not produce additional throughput gains.
In fact at some point, usually a few centimetres away from the WiFi access point (AP), you can eventually run into a signal strength so high that the receiver's front-end is unable to process it, at which point throughput will drop precipitously.
All of the details, especially the scale, of this graph are highly dependent on the capabilities of the transmitting radio, the receiving radio and the environment.
Variability in the environment and in the radios themselves makes real-world wireless throughput a random variable that can only be assessed accurately via statistical methods.
The physical layer data rate selection algorithm is critical to achieving the monotonically increasing relationship shown here up to saturation.
There have been many examples of poor rate control algorithms loose in the wild, in both popular APs and common client devices, that do not actually achieve this monotonic performance, especially when subject to unexpected environmental inputs or certain radio degradations.
What to do? Get smart
Finding the right balance between optimum performance and reliability with adaptive data rate algorithms is what separates the great WiFi systems from those that are good enough.
Rate adaptation is the function that determines how and when to dynamically change to a new data rate. When it’s tuned properly, a good adaptation algorithm finds the right data rate that delivers peak AP output in current RF conditions – unstable as they are.
Though often ignored, rate adaptation is a critical component to any high performance system.
WiFi engineers have been led to believe, and – and for better or worse – site survey software validates the belief, that data rates can be reliably predicted based on a metric like RSSI (Received Signal Strength Indicator) or SNR (signal-to-noise ratio). And some product manufacturers use simple metrics like these to determine the right rate.
Without the right algorithm, the optimal rate for any client at any given moment in time is a crapshoot. And when you're guessing, the safest guess is to err on the side of reliability, which sacrifices throughput and capacity and causes other unwanted problems.
Stable client connections are important in an unstable RF (radio frequency) environment.
To ensure that you have strong and reliable WiFi connection, it’s vital that you choose a vendor that leverages statistically optimised rate selection algorithms, which jointly adapt both the data rate and antenna pattern together to maximise reliability and throughput.
Michael Lok is South-East Asia managing director at Ruckus Wireless
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