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 Wi-Fi comes increased data rates. When Wi-Fi 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 Wi-Fi 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. Modulation Simply put, modulation is the process of converting network data into an RF signal that can be transmitted wirelessly. Wi-Fi 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 ] Negotiation The Wi-Fi 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: Image Source – Devin Akin’s awesome blog which dives into this subject matter to greater depths- check it out! 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 Wi-Fi 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

Again with the SNR In my previous post (Eliminate Channel Dysfunction – Your Wi-Fi Will Thank You), I spoke briefly about the signal-to-noise ratio (SNR). SNR is what’s left over of the received signal (RSSI), after you subtract the ambient noise (or noise floor). For example, if the RSSI at the client device is -60 dBm, and the noise floor is at -95 dBm, then the SNR is (-60) – (-95) = 35 dB. Many enterprise controllers show the client’s SNR as the reported signal level, while others will show the RSSI. Too Much of a Good Thing Knowing now how SNR works, you might be thinking, “Well, if I need better SNR, then I’ll just crank up the AP’s transmit (Tx) power.” While that does make mathematical sense, it’s not always the right move when it comes to optimizing your Wi-Fi network’s performance. Let’s get into the reasons why. Wi-Fi Devices are Not Created Equal In the earlier days of Wi-Fi, the primary devices connecting to the Wi-Fi were PCs, mostly laptops and some desktops using an adapter. These devices had pretty decent antennas and transmitting capabilities. Thus, it was common for a network engineer to crank the AP’s Tx power to the max, and therefore get decent coverage with fewer APs. However, nowadays, things have changed. Devices like smartphones, VOIP (or VoWiFi) handsets, and IOT devices are commonly using Wi-Fi, and these types of devices have limited capabilities. Their antennas are much smaller, and due to battery efficiency, they are more limited in their transmit power. Take, for example, the iPhone 5. The table below shows it’s max power capability, which changes based on the channel in use. [ a few years ago, Jerome Henry (principal engineer at Cisco) gave a conference presentation about this same topic, and that’s where I got the iPhone 5 power data above ] Now, considering that the max Tx power of many of today’s APs is between 23 – 30 dBm, you see the significant difference between what the AP can do vs. today’s smaller battery-operated Wi-Fi devices (roughly half). Can You Hear Me Now? Because client devices are more limited in their transmitting capabilities than APs, it’s not uncommon for these devices to have an asymmetrical connection, meaning if the AP’s Tx power level is higher than the client’s, then the client will “hear” the AP, but the AP may not “hear” the client. Again, this is due to the mismatch in Tx power levels, and mismatched power results in mismatched range. Having mismatched power (and therefore an asymmetrical connection) should be avoided for several reasons. Primarily, it will result in a lot of retries. Remember this..? There are several reasons why a Wi-Fi transmission (or frame) must be resent, and we’ll discuss another one here in a moment. What triggers these retries is the absence of an acknowledgement (ACK) frame from the receiving device. You see, a lot can go wrong with Wi-Fi, so the powers-that-be built into the protocol this acknowledging behavior where the receiver will acknowledge the successful receipt of a transmitted frame, by sending its own ACK frame. The scary thing is that these retries are resent over-and-over (and usually slower-and-slower) when the expected ACK isn’t received. What do you think that does to the available airtime? Eliminate Channel Dysfunction – Your WiFi Will Thank You So, when the client’s data doesn’t reach the AP, the client resends it over-and-over again. And, the data either never gets there (resulting in packet loss), or gets there late as it tries lower data rates that are easier to demodulate at lower signals (resulting in latency). This behavior is particularly detrimental to voice applications. Let’s look at another conference call example (see the “Understanding SSIDs (aka WLANs) and Using Them Wisely” post for the first one). If there’s one common theme about conference calls, it’s that there’s usually at least one loud-mouth, and one whisperer. The loud-mouth is at a near scream (like an AP at 23 dBm) causing folks to pull away from their speaker or headset. However, the whisperer speaks so softly (like a VoWiFi device at 11 dBm), that the rest of us can’t distinguish what he’s saying from the background noise. What should be done to resolve this? Obviously the loud-mouth needs to lower his speaking volume, and the whisperer needs to raise his, so they both can be heard at appropriate levels. Bringing Balance to the Wi-Fi I think it’s obvious by now that the way to fix the mismatched power between AP and client, is to set the AP’s power level to match that of the least-capable client device that will be connecting to your Wi-Fi. Now, this may not be as simple as it sounds, as any experienced Wi-Fi engineer will tell you, because you have to consider multiple device types, multiple application requirements, and not to mention the financial budget. So, this balancing process usually takes some tuning and prioritizing to get it right. Some vendors may provide guidelines, or starting points. Take a look at Meraki’s RF profile templates for example, and their min/max Tx power levels. source So, be a good Wi-Fi Jedi and bring balance to your Wi-Fi.

More About Performance In the previous blog post about SSIDs, I talked about how management traffic can consume the airtime and slow your Wi-Fi network performance. This post addresses similar performance impact created by poor channel use. I’ll attempt to explain why the number of channels, and their size, can have a big impact on the Wi-Fi. Ack-scuse Me? Remember this from my Wi-Fi Airtime post..? ..when the signal drops too low, devices may not receive all of the data being sent, or some of the data be become corrupt. In either case, the data will need to be resent to the receiving device (increasing airtime utilization).. Teaching WiFi Airtime – Part 2 There are several reasons why a Wi-Fi transmission (or frame) must be resent, and we’ll discuss another one here in a moment. What triggers these retries is the absence of an acknowledgement (ACK) frame from the receiving device. You see, a lot can go wrong with Wi-Fi, so the powers-that-be built into the protocol this acknowledging behavior where the receiver will acknowledge the successful receipt of a transmitted frame, by sending its own ACK frame. The scary thing is that these retries are resent over-and-over (and usually slower-and-slower) when the expected ACK isn’t received. What do you think that does to the available airtime? A Bit about Wi-Fi Channels Most already know that Wi-Fi operates in the 2.4 and 5 GHz frequencies, and there are a finite number of non-overlapping channels for use in those spaces. 2.4 GHz has just 3 non-overlapping channels (1, 6, and 11), and 5 GHz has 20-ish channels (ruling out the four channels that can be impacted by Doppler weather radar). Actual available channels may vary by vendor. With the introduction of 802.11n, we were given the ability to combine two channels together to double the connected speed, or data rate. This is commonly referred to as channel bonding, or increasing the channel size or width. One channel equates to a width of 20 MHz, and two bonded channels equates to a width of 40 MHz. Later, 802.11ac gave us the ability to bond four (80 MHz), and even eight (160 MHz) channels, quadrupling and octupling the speed. Okay, that sounds pretty great. The faster our data is transmitted, the less airtime is used. No brainer, right? Well… Why Channel Size Matters Math tells us that as we double the size of the channels, we also halve the number of channels available for use. In 5 GHz for instance, you get: # of 20 MHz channels = 20 # of 40 MHz channels = 10 # of 80 MHz channels = 5 # of 160 MHz channels = 2 5 GHz Channels – (image taken from the Meraki dashboard) Now, let’s say you are in change of a facility that has 25 APs deployed, and you are using 80 MHz wide channels (because that’s fast). This means that each channel must be reused five times. When you reuse channels, you have a much greater likelihood for co-channel interference. Co-channel interference can slow your network down as devices must wait longer for their transmit opportunity, and it may also result in corrupted frames. Corrupted frames can’t be interpreted by the receiver, so the receiver won’t send back an ACK, which we now know results in the transmitter re-sending the frame over-and-over until it gets an ACK, or gives up (resulting in packet loss). So, using wide channels decreases available channels, which can increase co-channel interference, and reduce network performance. Strike one! Big Channels, Big Noise Another impact of using wider channels is that there is more noise (ambient background radio energy). Again, as you double the channel size, you also double the amount of background noise, which then increases the noise floor, and has a direct impact on the all important signal-to-noise ratio (SNR). [You see, when an AP and its connected devices communicate, they must distinguish signal from noise, and if the noise floor rises, it’s much harder for them to do that.] Specifically, the noise increases by 3 dB, so the SNR at the client device drops by 3 dB. If the channel width is 80 MHz, then the noise increases to 6 dB, and so on. Wi-Fi Nigel wrote about his experience testing this theory, and you can read about it here. Another way to look at this is from the perceptive of cell size, or the distance from the AP where you will get a particular signal level. When designing a Wi-Fi deployment, you typically design per the requirements of the client devices that will be in use. If a client device requires a minimum SNR of 25, you can measure how far you can be from the AP upon reaching that level. Assume that using 20 MHz wide channels, you can have an SNR of 25 at a distance of 40 feet. However, if you are using 40 MHz wide channels, and considering its increased noise, you may hit an SNR of 25 at 30 feet. Your AP cell size just shrank by 25%. Ouch! More noise, and therefore less SNR – that’s strike two! Speed vs. Capacity It’s true that bonding channels gives us faster connected speeds. But, that doesn’t mean that our Wi-Fi network will then be better capable of servicing more client devices effectively. I know, you’re probably thinking, “What’s wrong with you Todd? The faster the speed, the quicker a client device will finish transmitting, and the sooner the next client device can start transmitting. So yes, faster speeds will allow you to service more clients.” And yes, you are right, but think of it this way. Let’s say you have one AP using a fast 80 MHz channel, covering a classroom of 50 client devices. All of the devices need to transmit data to the AP, and they are transmitting very fast, one at a time. Now, let’s say that instead of one AP at 80 MHz, you have four APs on 20

What is (and isn’t) an SSID? Before getting into what an SSID is, we must understand what it isn’t, which is the wireless equivalent of a VLAN. I repeat, an SSID is not the wireless equivalent of a VLAN – seriously, it’s not. While network engineers may use them for similar reasons, they are distinctly different, particularly when it comes to network performance. Now, when you pull out your phone at Starbucks to connect to the Wi-Fi, you’ll see a list of available Wi-Fi networks within “earshot” of your phone. Each network name in that list is an SSID, or service set identifier. Your wireless router at home typically allows for one SSID. Some of the more advanced home wireless routers may allow for multiple SSIDs. In the enterprise world (using the more expensive enterprise-grade APs and controllers), using multiple SSIDs is very common. SSIDs and Network Performance When you’re connected to the Wi-Fi and checking your Twitter feed at Starbucks, what you most care about is for the data that carries all those tweets to make it from the AP to your phone – nothing else matters, to you. But, Wi-Fi has a lot more going on behind the scenes before the tweet carrying data makes it to your phone. There’s a LOT of information exchanging in the air in the form of management frames, control frames, and data frames. When it comes to SSIDs, we’re going to be paying attention to the management frames. In most cases, each AP will have at least one SSID enabled and broadcasting. So Wi-Fi clients know what SSIDs are available, each SSID will advertise its existence in the form of a beacon. It does this in accordance to the configured beacon interval, which is commonly set at every 100 milliseconds (ms). So, 10 times per second, each AP will send a beacon advertising the SSID. Now, if you have five SSIDs enabled, each of those will also need to send a beacon to advertise their SSID. So now you have 50 beacons sent out every second. What’s that? Oh, you have 10 APs deployed in your facility? That’s right, then you now have 500 beacons sent out every second. But wait, there’s more… Now, let’s look at the Wi-Fi client devices (like your phone). When you stroll into Starbucks, your phone sends its own management frame called a probe request, asking what SSIDs are available. Each AP will then send a probe response for each SSID that it has enabled. Math time again. If there are 50 devices sending probe requests, and 10 APs each having 5 BSSIDs enabled, then (oh, my head hurts) that’s 2,500 probe responses. And yes, that’s on top of the 500 beacons that the APs are sending out every second. [ Yes, there’s even more than that, but that’s diving deeper that I want to right now, and I think this is enough to drive home the point. ] Want to know a little secret (it’s not really a secret)? Remember in the previous “Teaching Wi-Fi Airtime” posts, we talked a bit about transmit speeds, and their impact on airtime utilization? Well, management traffic is sent at the lowest allowed basic data rate (don’t ask why right now, that’s a blog post for later). Anyway, do you see how management traffic can really overwhelm the Wi-Fi? Let me try to illustrate. Let’s imagine that I’m attending a conference call with four other people. I dial in and then introduce myself. “Hello, Todd here. Who else is on the call?” Then, everyone else, in turn, introduces themselves. Now, this is an important meeting, and we need to ensure that everyone is present throughout. To do this, we each check-in and repeat our introductions every 10 minutes. A few moments later, someone else joins late, introduces himself, and asks, “Who else is on the call?” Introductions ensue again. You can see that the introductions take up a fair amount of time, especially when folks ask who else is on the call. Now, imagine that we all have multiple personalities (five in fact), and each one is conscious and aware of the other, and each is also an important contributor to the call. Now, every person introduces each of their five personalities when they join the call, check-in periodically, and when others ask who is on the call. That’s a lot of introductions, which are now taking the bulk of the time, leaving very little to discuss the actual purpose of the call. Having too many SSIDs can bring the Wi-Fi network to a crawl, because the airtime is consumed with so much slow management traffic (commonly referred to as overhead), that there is very little left for the tweet carrying data frames that you so desperately want. Circling Back I started out explaining that SSIDs are not the wireless equivalent to VLANs. My primary reason for saying this is that VLANs can actually improve network performance by segmenting the network into separate broadcast domains, thus reducing unnecessary traffic broadcasting throughout the network. The more VLANs you implement, the greater potential exists for more efficient network performance. That’s not the case when adding SSIDs. While there are many valid reasons to use multiple SSIDs, a smart Wi-Fi engineer will balance those benefits with the potential negative impact on Wi-Fi performance. In Wi-Fi, the fewer the SSIDs, the better. If you’re using more than 3 or 4, you’re likely negatively impacting your Wi-Fi network. Check out Andrew vonNagy’s SSID Overhead Calculator to see the impact SSIDs will have on your network. BTW – SSIDs can be fun. Instead of using names like “corporate” or “office” or “Smith’s”, try something fun. Did you catch my home SSID in the image above?

*** please read part 1 before continuing on with part 2 below *** “Fill the Front Seats First” Physics tells us that the closer a device is to the AP, the stronger the received signal (RSSI) should be. And, in the world of Wi-Fi, the stronger the signal is compared to the nearby noise (SNR), the higher the connected data rate will be. The higher the data rate, the quicker the data can be sent to the receiver, and the less time other devices must wait to transmit. The speed in which a device can transmit has a direct relation to the wait times of contending devices, thus impacting the overall available airtime (aka utilization). Furthermore, when the signal drops too low, devices may not receive all of the data being sent, or some of the data be become corrupt. In either case, the data will need to be resent to the receiving device (increasing airtime utilization) – and often, in an effort to ensure delivery, this data gets resent at a lower data rate (further increasing airtime utilization). In our classroom example, when students sit near the front of the class, they are closer to the teacher, and better able to hear and understand what is being taught. Students further away may miss certain details and are more likely to ask the teacher to repeat what was said. This slows the learning process for the entire class. “You’re Too Smart. I’m Moving You to the Advanced Placement Class” There are several generations of Wi-Fi (we’re entering the 6th generation now with 802.11ax, or WiFi-6). With each generation comes improved capabilities that ultimately increase the speed at which devices can transmit. That said, the Wi-Fi powers-that-be decided that each generation of Wi-Fi would be backwards-compatible with all previous generations (assuming they operate on the same frequency). So, my trusty old WiFi-2 device (which tops out at 54 Mbps) can connect to my WiFi-5 AP (which tops out at 1000+ Mbps). Well, isn’t that nice? No, not necessarily. Remember, transmit speed impacts airtime. When an older gen (and therefore slower) device is transmitting, all the new faster devices must still wait for the older device to finish. However, if you have a WLAN that only allows the newer and faster devices to connect, then that WLAN will be able to communicate and process data much more quickly and efficiently. This can be especially beneficial when dealing with video and large file transfers.  Returning to our classroom, the smarter (faster) students can help the class as a whole learn more quickly because they require less time process what’s being taught. However, the other students are slowing the learning process for the more capable and smarter students. This is why many schools offer advanced placement classes. Group the more capable students together in their own class, and these students will learn more quickly, and therefore acquire more knowledge in the same amount of time as the non-advanced class. Returning to our classroom, the smarter (faster) students can help the class as a whole learn more quickly because they require less time process what’s being taught. However, the other students are slowing the learning process for the more capable and smarter students. This is why many schools offer advanced placement classes. Group the more capable students together in their own class, and these students will learn more quickly, and therefore acquire more knowledge in the same amount of time as the non-advanced class. “And Thus It Is With Wi-Fi” I hope my classroom example made sense and helped illustrate how Wi-Fi airtime works. It’s not the easiest of concepts to grasp, but it is one of the most important. And just like a teacher has several techniques he/she can implement in the classroom to improve learning efficiency, so does a Wi-Fi engineer with their WLAN. Some of these include: Limit data rates Use more channels Use 20 MHz wide channels Limit the number of SSIDs Do not throttle the bandwidth Implement Airtime Fairness (assuming the infrastructure supports it and device applications aren’t negatively impacted) Which of these, if any, you choose to use is up to you. Always do your research before making these kinds of config changes, or you may end up booting client devices from your WLAN unintentionally – which would be bad, especially if one of them is your boss’.

What is Airtime? Airtime is a measure of the time it takes for a Wi-Fi device (i.e. AP, wireless router, laptop, smartphone, etc.) to transmit data. And the total amount of airtime available is fixed, meaning it can and will run out (or reach 100% utilization). Physics tells us that the slower the device, the longer it takes to transmit data. Thus, slow devices use up more airtime, leaving less airtime for all the other devices to share. So, the question we should be asking here is, what makes a Wi-Fi device slow? Turns out, there are quite a few things. Read on. “One at a Time Please” Wi-Fi is a shared (and half-duplex) medium, meaning that all devices operating on the same channel must compete (or contend) for the same limited airtime, and only one device can “talk” at any given moment. This reminds me of a school classroom. The teacher is the AP, and the students are the connected devices. No one else speaks when the teacher speaks. And students comment or respond one at a time, by first raising their hand and then waiting to be called upon. This way, the lesson is communicated to the entire class, and questions can be asked and answered in an efficient manner. There are many parallels between Wi-Fi and this classroom example, and we’ll get into these below. Even the enforcement of students raising their hands and waiting to be called upon, exist in Wi-Fi as well (this is a bit more advanced [CSMA-CA] and not specifically covered in this blog post – perhaps later). “Wait Your Turn” Knowing that Wi-Fi is a shared medium, where only one device “talks” at a time, it’s understandable that devices must contend with each other for their opportunity to transmit. Obviously, the more devices you have connecting to the Wi-Fi, the more contention there will be. What many may not realize though, is that if there is another AP operating on the same channel, whose signal is strong enough to be “heard” by our AP and its connected devices, then we must also wait for that AP and its devices to talk when it’s their turn – increasing the contention much, much more. The more contention that exists, the longer a device must wait to transmit. Returning to the classroom example, remember, the classroom has a strict “raise your hand” policy. And the more students attending the class, the longer they must wait to be called upon. In my freshman year of high school, my science class shared a wall and a door with another classroom; and often, that door was left open. Now occasionally, that neighboring classroom got pretty noisy, as did ours, which forced both classes to do one of few things: Wait until the neighboring noise died down enough to not interfere (or contend) with the other classroom (thus increasing the contention and overall wait time) Close the door, creating enough physical interference to eliminate the need for our class to contend with the other (ensuring that our AP and devices can’t “hear” the neighboring AP and devices) Move to another classroom (change the channel upon which you are operating) One might think that another option would be to speak louder (increase the AP Tx power level). However, if only one person is allowed to speak at a time, then it doesn’t gain anything to speak more loudly. But, it can have an adverse effect. Speaking more loudly may cause other classrooms to hear you, causing them to have to wait for you to finish, ultimately increasing the contention. That said, what if we speak in a quieter tone (lower the AP Tx power level)? This may work; however, students in the back of the room may no longer be able to hear the teacher. *** continue reading in part 2 ***