Which Rate is Which
Data rates can certainly be confusing, even how we refer to them varies by engineer and vendor. Some call them bitrates. Others call them transmit rates or connectivity rates. Then, there are basic rates, mandatory rates, minimum rates, allowed rates, and supported rates. Does it really need to be this complicated?
To help clear this up, let me just say that bitrates, transmit rates, and connectivity rates are synonyms for data rates. Minimum rates and mandatory rates are synonyms for basic rates. Allowed rates and supported rates are synonyms of each other.
[ yes, I know some of you out there may disagree – if so, put it in a comment below ]
So essentially, there are just two types of data rates – basic rates and supported rates. The basic rate is the data rate at which the management frames (beacons, probe requests/responses, etc) are sent, and is typically the minimum data rate. The supported rates are all the other rates that the device supports based on its 802.11 flavor (a, b, g, n, etc).
With each new generation of WiFi comes increased data rates. When WiFi first hit the scene back in 1997, the data rates were limited to 1 – 2 Mbps. In 1999, 802.11b brought additional data rates of 5.5 and 11 Mbps on 2.4 GHz, and 802.11a brought us 6, 9, 12, 18, 24, 36, 48, 54 Mbps on 5 GHz. And the data rates have been going up and up since then, to where we are today with 802.11ax (aka WiFi-6), providing a max theoretical data rate of 10530 Mbps (10.53 Gbps). That’s quite the journey.
With So Many Data Rates, How Does a Device Decide Which to Use?
For starters, it depends on the device’s capabilities – meaning is it an 802.11a device or an 802.11ac device, or somewhere in between. As I stated above, with each new generation of WiFi comes additional higher rates.
The data rate that a device selects to use, the connected data rate, is fluid (meaning it changes frequently), and is the negotiated rate at which transmitted data can be successfully demodulated. Now, you may be thinking “Hold up sparky, what do you mean by negotiated rate, and demodulated what-now..?” Let’s dissect these two items.
Simply put, modulation is the process of converting network data into an RF signal that can be transmitted wirelessly. WiFi uses several different modulation techniques, each one being more complex than the other. The more complex the modulation technique, the higher the achievable data rate. Below is a chart that shows the correlation between the modulation technique and the achievable data rate.
[ there are other factors that impact the achievable data rate, including the number of spatial streams and the channel width – both are topics for another blog post ]
The WiFi protocol includes a process called Dynamic Rate Selection (DRS). It’s during this process where the client device and the AP negotiate the data rate to be used when transmitting data. The logic that DRS uses to negotiate the data rate is primarily based on the signal quality (RSSI and/or SNR). The stronger the signal, the more complex modulation techniques can be used, offering a higher data rate. The reverse holds true as well. As the signal decreases, simpler modulation will be required, which lowers the achievable data rate. Other factors may contribute to the negotiated rate as well, like retries, packet loss, and CRC errors.
It’s Okay to be Judgy When It Comes to Data Rates and Daughters
As a wireless network engineer, you want your WLAN to perform at its best, that’s why you’re paid the big bucks. There are several things that you’ll do to maximize performance (see all my other blog posts), and being selective about data rates should be one of them. Disabling the lower data rates can provide the following optimizations for your WLAN.
Decrease Airtime Utilization
Remember, management traffic is sent at the lowest basic data rate. And if you recall my previous post “Understanding SSIDs (aka WLANs) and Using Them Wisely”, you’ll recall that too much management traffic can bring your WLAN to a crawl by consuming the majority of the airtime, particularly if that traffic is transmitting at 1 or 2 Mbps, or even 6 or 9 Mbps.
Reduce Sticky Clients and Promote Optimal Roaming
Typically, client devices decide when to roam, and let’s face it, some of them just don’t like to. They will just hang onto an AP while connected at a low data rate, even if they’d negotiate a higher data rate by roaming to a closer AP. By disabling the lower data rates, you can force those clingy clients to roam to a more optimal AP sooner, because you’re essentially shrinking the coverage area around the AP. For example, with the basic data rate set to 6 Mbps, the AP’s coverage area looks like this:
However, by disabling the 6 Mbps data rate and making 12 Mbps the basic data rate, the AP’s coverage area looks like this:
Eliminate Legacy Clients from Connecting to and Slowing Down the WLAN
Older legacy clients (like 802.11b devices) can’t take advantage of the WiFi advancements that have been made in recent years. Therefore, they are slower and consume more airtime, and force the more capable clients to wait longer before they can transmit. By disabling the 802.11b data rates, you essentially restrict those devices from connecting to the WLAN, which improves its performance (assuming you don’t need those 11b clients to connect).
Goofy Dad Analogy
I like to look at it this way. I have a 16 year-old daughter, and as her dad, I may not like some of the boys that come knocking on our door. There are the boys who just want to hang out with her all the time, keeping her from focusing on more important things like homework and hanging out with her dad. Then, there are the boys who just don’t know how to move on, and try to keep the relationship going even through they’ve already drifted too far apart. And finally, there are the boys who are just bad news, because they are older and not an appropriate fit for a 16 year-old. So how does dad deal with these misfits? Well, I teach her to have high standards so she can eliminate the riff-raff early on. Then later, when it comes time to “negotiate a connection”, their capabilities and requirements are more likely in line with each others, and they create and maintain a highly functional relationship. You see what I did there?
Word to the Wise
Before you go and disable data rates, please make sure your WLAN design can support it. For example, setting the basic data rate to 12 Mbps requires a fairly dense AP design. If your WLAN doesn’t have an appropriate design, eliminating the lower data rates may create coverage holes, and cause more harm than good. Also, make sure your client devices and applications support whatever changes you may make. Do your homework on this. Survey, design, optimize, test, and test again. You’ll be glad you did.