RUCKCast #121: The Minimum Problem with Minimum Basic Rates

You sit down with your design tool, paint coverage to -67 dBm, target 100+ Mbps everywhere, validate it, and present a beautiful heatmap. Then you open the controller and leave the minimum basic rate at 6 Mbps — an MCS 0. Your design says MCS 0 should never happen. So why are you protecting it?

In this episode, Jim walks through the real cost of low MBR settings: beacons and management frames burning airtime at the lowest configured rate, multicast traffic crawling while unicast flies at MCS 9+, inflated cell sizes creating co-channel interference you didn't design for, and neighboring APs deferring to frames serving clients that shouldn't exist in your coverage area. Using field data from a recent network audit, Jim shows what beacon overhead actually looks like when 11b is still enabled and MBR is left at defaults — and what changes when you raise it to 24 Mbps.

The episode also tackles the IoT pushback head-on: possible vs. probable, why segmentation beats accommodation, and why the answer to "but what if a device can't connect at 24?" is to handle the exception, not punish a thousand clients for it.

Design for probable, adjust for possible. Trust your own design.

Intro music by Alex Grohl, available here:
https://www.youtube.com/watch?v=ZsRWpx8VJ_E
and
https://pixabay.com/users/alexgrohl-25289918/

Transcript

0:00

Hey everybody welcome back to another episode of the RUCKCast my name is Jim Palmer I'm your host um today I want to talk about something that's a little nerdy And it's also kind of on topic I've seen some conversations just uh yesterday about this Exact topic and so I wanted to spend a little bit of time going over it mainly because I was Already going to talk about it as you'll see here in a minute and if you're paying attention and You're looking on YouTube and watching the video you already know that today I want to talk about Minimum basic rates or MBR um it's it's kind of an interesting topic because we don't really talk About it too much and everybody just kind of goes oh well you know this is the minimum basic rate and So we're just going to kind of leave it you know just let it run whatever the default is we're going To leave it alone and just kind of go with it but I want to talk about it because there's something Else interesting that's going on here at least it's interesting to me maybe it's interesting to you And I want to start with when you first sit down and start talking about a network You sit down and you go okay I want to design a network you know you pick up get your floor plans You get all your stuff and then you set your criteria and one of the first criterias we really Talk about we set to is what is going to be my coverage you know what what is my signal level that I want to design to now I can tell you there's some other trains I thought about this but one of The things that you end up setting in your design tool whatever design tool you use Is you set a minimum um signal coverage you know what is my minimum what is my RSSI value that I'd Like to get to now most people will set like a -65 dBm sometimes it'll be -67 depending But you set that in your design tool right and so you go ahead and you set that up you set up you Know you go hey I want my channels to be 20 MHz wide 40 MHz wide maybe 80 MHz wide if You're in six GHz and you go through and you do all your design and you and you validate it and You go look how pretty my design looks I'm meeting all these criterias for you know how many AP's are On a channel what my minimum signal level is what my data rate is and if you do it right you're looking At you know at least a -67 across most of the the map and you know at anywhere from a 20 to a 40 MHz wide channel you're going to get easily above 100 Mbps And so you know when you look at the mc uh MCS charts on MCSindex.org or .net um and you see Like oh yeah you know I'm going to set it in here and so cool you get it approved you go deploy it Everything hits you know you you get it on the controller you go back and you look okay what are My channel bandwidths what are my you know and then you do a validation survey and you all this stuff And everybody's like yeah cool cool cool cool cool it's all said and done But the one thing you never said is the MBR you just go you know what we're just going to leave That alone we're going to leave it at whatever the vendor default is most vendor defaults and 2.4 GHz is one and in five GHz it's six turns out that in six GHz it's also six Um and so you set that but have you ever gone to look in the MCS chart MCSindex.net to see Well what what's required to get to that I have and I'll tell you with um Two spatial streams and a 20 MHz wide channel in order to get to six Mbps it's an MCS Of zero same thing on 2.4 MCS of one or you know or to get them one Mbps data rate You need an MCS of zero but think about what you design you design for all this fantastic coverage Where you're getting 100 Mbps but then you don't change that one thing and you go You go oh well because I might have a I might have a device that can only do six Mbps Or one Mbps but what does that cost you and today I want to spend a few minutes and go Through and figure out what does that actually cost you so let's talk about what that is so beacon and Management frames transmit at the lowest configured basic rate and the highest transmit power now there's Some other frames that go out at the lowest rate period you know so you can't get off of those so When you do when you do a pcap and you look you know even if you have your minimum basic rate set to Something higher you're still going to see some frames that go out at at the lowest so it's going to be One or six depending on the band you're looking at and we can't adjust those that's in the standard And they will always go out at that but beacon frames and management other management frames like probe Requests will go out at the minimum basic rate that you configure and so if you think about it if you Set something to if you leave it and we're going to take a we're just going to focus on five GHz And that six Mbps here for a second and if you if you leave it at that It's going to go out you know every 102.4 milliseconds you're going to send out a beacon Frame and that beacon frame is going out at six Mbps now the beacon frame contains A whole bunch of data right it has all the data that you need to run the network to advertise what The network is what devices need to support all the other little fun bits and bobs you know the type Of security everything else that you configure all those little nerd knobs get advertised in the beacon Frame so depending on how many nerd knobs you turn on if you watched my video on the RUCKUS tx vap and the Multi-BSS ID you know the more of those little nerd knobs you turn on the more data you have to send Out in that beacon frame well if you're sending it at the lowest possible data rate on the entire you know Band it's going to take longer to send it out because it's sending it at a lower MCS rate Right so you take extra time to do that but if you were to change that MBR from say a six to a 12 Or to a 24 it's you're going to send anything out at either twice the speed or four times the speed Which means that instead of me taking you know six minutes to get to this point if I was talking Four times as fast you know then I would have gotten to this point at you know less than two minutes And that's the same thing right so people who like to turn turn on podcasts or turn on YouTube videos And watch them at like 1.25 speed because they can get through them faster same concept except we're Sending out man you know these management frames at the lowest speed and so the problem you have is Is if you end up with a bare minimum of three SSIDs right and again if you go back and you watch You know my uh Multi-BSS ID video we talk about that right it takes extra time time on a channel Transmit opportunities stuff like that now in 2.4 and 5 GHz we're not using Multi-BSS IDs and Non-transmitted BSS ID information so we need to be very quick at getting on and off and when we set Something to six or even 12 what we're saying is is hey we still want to talk slower even though we Designed our entire network to go out you know and and achieve 100 Mbps and then we go Yeah but cool those six Mbps clients we want those to come on in too because You know we might need to support those and it just seems backwards to me right we design these Really awesome networks that do all this really cool stuff and meets all these other criteria and All these other guidelines and then we go ahead and we just jam it up by putting in a rate limiter On the on the channel that says hey every 10 times a second I'm gonna come in and when everybody else is Talking at 100 Mbps 200 Mbps you know MCS 8 MCS 9 it was really high MCS rates because we designed a really good network and then every 102.4 milliseconds you have A beacon that comes in and goes okay now that you guys are you know talking really good I'm gonna slow Down and I'm gonna tell you about this network that you're connected to same thing with probe requests So um the other thing broadcast and multicast data um that goes at the lowest basic rate the MBR Probe responses MBR ACK and CTS they'll go to that's those are different they'll go at a different rate Um but what you'll see what you'll also see is the probe requests from unassociated clients Those will go in at the lowest possible rate because it doesn't know it doesn't know any better And so when you go in and you and you have a client brand new client device and it says hey I want To connect so I'm going to do a probe request you know I'm not I'm not connecting anything I'm doing A broadcast probe request hey what's out there and your AP sends a probe response as six megabits per Second then the client goes okay cool we're talking at six Mbps even though you designed The whole thing to do 100 it's just kind of you know sort of a contradiction in my mind because we Shouldn't be doing that stuff right and so when you set up all your design tools and you have it to do All these things you know and then you you forget this one little step in your configuration tool We get channels we get you know transmit uh transmit power values we get all this configuration Information from our design tools but have you noticed have you ever put in a minimum basic rate In a design tool you haven't because it's not there and so we just because it's like oh it's not anything We do we're just going to leave the defaults and that becomes a problem now we're going to get to That here in a second um why that's a problem but before I do I want to talk about one of the biggest Arguments against doing something like that or something like this and that is because people will talk About the cell size and they'll say hey we want to increase our you know minimum basic rates our mbrs Because we want to shrink the cell size well but here's the thing like I said when you do the design You've already designed it to be able to hit 100 Mbps in your cell so why are you Setting it six because what happens is you now have an AP that's broadcasting at six now remember it's Going out at the highest transmit power and so it's spreading it out and so the problem you have is You have when you do your designs and you design less than max power what happens is you look at your Co-channel interference and you go you go hey I'm good I don't have that co-channel interference co-channel Contention um that's kind of an interesting conversation I'm seeing I want to see where it goes in the next Year or so is between co-channel contention and co-channel interference but what we're talking about Is having two AP's on the same channel right because if you design for a very low transmit power then You're not going to see the co-channel contention and co-channel interference in your design tool Because it's not looking at that minimum basic rate but if you got to remember if it's going out at the Minimum basic rate of six and the highest transmit power it's actually going to increase the cell size So when I say hey I think you got to turn those things up and um I'll get I'll cover that sort of The end that's my little teaser for you when you get to that point you know and you say well you know I Might I might shrink my cell size well yeah you but your cell size has already shrunk from your design When you said hey I'm going to transmit it at less power and I'm going to get to 100 Mbps So why why stick that out now the when I'm talking about you know AP's hearing each other there's Actually two different things within the 802.11 standard that we have to pay attention to And one is um signal detect and the other one is energy detect and they have different um parameters And they're looking for different things right now and I wrote this down so I wouldn't get it wrong So let me read it so the signal detect is something for close neighbors but that is is uh that value is Usually about a -82 and what happens is if an AP can decode the preamble and read the duration And NAV field and it will defer for the stated frame duration now a frame at six Mbps has A longer duration remember we were talking about going slower and faster but it it has a frame at six Mbps has a longer duration than the same frame at 24 Mbps you know the Highest we can go on RUCKUS so it's the same data but you end up with a longer deferral so you're Slowing down an AP that you never thought you had interference or contention problems with because You designed for a smaller cell but by setting it at a six leaving it that default it gets bigger and so You can actually impede more now the other one is an energy detect an energy detect is a -62 now They can't decode the frame at all they just see the RF energy and they defer as long as it's above and That but the longer frame same thing right it's a longer time on the channel so you get more deferrals And so it's not just on your one cell on that one you know BSS ID it's across all of it so By raising that thing up you're reducing the amount of time which means that even if you have You know some co-channel contention or co-channel interference you end up having those you know You're you're decreasing that by raising your minimum basic rates as opposed to leaving them you know Further down now what happens when we leave them at the at the at the default and I want to go and I Want to show you something that I just collected a little while ago and this is some some pcaps I Collected in the field and I took the pcaps and I dumped it into Wi-Fi Explorer Pro and I said hey Show me what you'll see what you'll see here on On one meg on channel one we have one overlapping channel 2.4 GHz the beacon overhead is 132 Percent come down to channel six it's 164 percent come down here to channel 11 160 we have 58 networks 60 networks at 2.4 GHz and if you come over here and you look at these Minimum rates look at what these minimum rates are one may make it one Mbps and six Mbps and look at these times you're looking at 2.6 milliseconds you know down to a half a Millisecond now this is these are not um there's none of these are Multi-BSS IDs these are all of those Really high um you know just really low data rates which ends up causing you more beacon airtime so When we talk about that time like hey how long does it detect what's the duration of that frame On the on the channel that's what I'm talking about is it's driving that up and when you have Multiple networks because you know you're still running one SSID per VLAN now you end up where You have that many channels that many networks on these channels you know and it's just it's not Good you know and here's the thing is when I when I found all this stuff it wasn't a one-off one-time Thing 69 percent 98 percent 51 percent same thing six Mbps one Mbps And look at these really long durations so all of these really long durations that you that we see Right are just really kind of messing it up and even when we come over here and you look channel 11 And what we're looking at here is the the channel and the number of networks it only has 22 networks On it which for you know 2.4 is actually not too bad but it's still 51 percent now nobody was on These networks when I took these readings so there weren't any stations and so what we're looking at Here is just the beacon overhead this is just exactly how much time it takes on the channel And so when you look at a one Mbps data rate and here's the other little kicker one Mbps minimum basic rate on 2.4 GHz that is not OFDM rate that is a DSSS rate so this Is 802.11b so they've still enabled 802.11b on these networks and then they go hey Jim why is my Network not working well well I'll tell you why it doesn't take much it takes a laptop With you know Wi-Fi Explorer Pro and you get stuff that looks like this here's another one 134 155 155 of the time is spent on nothing but beacons when does the data get through Anybody I feel like uh you know Ferris Bueller Bueller... Bueller... when's that day to get through You know take a look at another one 95 74 98 I mean and then you even take a look at This uh channel 64 right here that has 24 networks at 5 GHz 14 of the time is on nothing but Beacons can you I mean I mean you're and what happens is when you get it's not it's not up to 100 when things start to fall apart things start to fall apart sooner than 100 because Wi-Fi still Needs that management overhead and if the management overhead can't function then this the BSS ID starts To fall apart the channel starts to fall apart so we're not trying to compare you know because I'll Tell you even at 95 and 98 those are on fun that's just dysfunctional it won't work and this is with No clients and all we're looking at here is the beacons anyway so the channel utilization is just Maxed out but there's no clients right let's take a look at another one you know this one's a little bit Better take a look at those this one where we ended up with some some adjacent channel interference Of 2.4 if you've never seen that let's take a look at that you got channel 8 and channel 10 There's one network on each one and so the overlap oh look 74 chat for 74 networks are overlapping On that one channel and so you still end up with a one percent on that channel but you're basically Combining the 104 and the 77 and that's just not something that functions and works in real time Even when you come down here and you take a look at at some of these other ones that are cleaner Down here down in the UNII-2 you still 14 13 beacon overhead This is what these low minimum basic rates and this extended beacon airtime oh we got one that's down To 0.4 right that's what kills you right now you know one more this one's this one's probably this Was the best one that I found but it's still I mean look at this you know 0.5 0.5 2.6 2.7 You know oh 50 only 50 of my time is beacons all right that's the best this is why we don't move This stuff right so let's talk let me reference this to something where we've raised the MBR we put it Up on the high one and what you can see here is we're going to start over here on this side and We're going to take a look at the minimum basic rate of 24 Mbps and this is on 2.4 You know the 20 MHz um and you know you can see you know these different these are the max Rates right so you got 20 MHz we got 160 MHz if we come over and look that's the six But look at that zero milliseconds that's a Multi-BSS ID so this is six GHz so I'm running Some 160 MHz channels and six GHz look at these 80 MHz right here come across 24 0.18 compared to 0.4 the only one of these rates that's above a 0.1 is that one that's carrying Two to three different SSIDs because it's a Multi-BSS ID and it's still running Half the amount of time and it's at six Mbps and we're still going half the time Because it's all about the time time is the one thing that I can't get back I can't get back time Once that time's gone the time is gone right so this is really what I'm pushing for when I talk about You know the minimum problem is people look at it and they go hey we can't you know we don't want to Raise that because we're afraid well what are you afraid of right what is it that you are afraid of So let's talk about that for a minute IoT devices um are there IoT devices out there that you know Can't do 24 Mbps maybe I'm not going to say there's not because there's a bajillion IoT devices but are they on your network if they're not why are you designing and preparing for something That's not on your network you're dragging down at your entire network and making it slow and making It inefficient just simply because you're afraid of a device that may come along right I call that the Possible versus probable and so what I would tell you to do is set your minimum basic rates higher Right because if you've designed your network correctly if you've validated your network and You've looked at it and you go we're meeting all of our other criteria and then you go ahead and you Set those minimum basic rates at the vendor defaults and look I work for a vendor I get it and the reason Why they're there is because we got to set it we got something has to be the default there's got to be a We can't not have a minimum basic rate right now so they're there they're there for a reason but what I'm telling you is raise those rates up you know because it's like I said you might have a device That comes along and you know what if that happens you go okay let's deal with your device that's Obviously from you know either so cheap that it doesn't support you know anything recent Or you know why does it all of a sudden become our fault when somebody else buys a really terrible Device we have to deal with it we have to fix it I get that right I totally get that but why are we Letting it drag down our entire network for the thousands of other devices on it because we Might find an IoT device that's not good enough and you know what if that's the case then you put that Damn thing on 2.4 right you change your minimum basic rate don't ever turn on 11b don't let me Hear you if I hear about that I'll come find you I know who you are I'll you know don't do it but You know even 11g 11n I mean 11n came out in 2009 you know it's 16 years you know you think you can Get something newer than 11n in 16 years yeah you probably can right and if you have to set a Minimum a lower minimum rate set it to 12 and then go down to six right but don't kneecap your network Because somebody might have a device that might pop up that might be that one corner case It's possible yes but is it probable probably not so don't do it right No one's gonna say hey when you raise the minimum basic rates from six to 24 all my clients lost Connectivity that's not gonna happen because you want them doing 100 Mbps anyway you've Designed your network to do 100 Mbps so if that happened there's a problem and it's not With the configurations there is a problem somewhere else so go solve that problem and don't kneecap the Other you know go solve that corner case problem and let the rest of your network run and breathe Freely because a device that's connected to your network at six Mbps do you really Want that device on your network because remember you're talking about an MCS of zero and when I say Turn it up to 24 you're looking at an MCS of one I mean is that such a high bar all I'm literally asking You to do is go from an MCS of zero to an MCS of one on your network and cut the time you spend Serving management by a core by 75 that's it that's the minimum problem with the minimum basic rates that People leave them way too low because they're scared about something that might happen and not worried About what is actually happening right we have all these other tools we have all these other things And if and like I said if if you have a device that can't connect worry about that one device if and When it shows up don't take this beautiful network you've designed and decide from the very beginning That you know you're going to by leaving one specific setting At the lowest data rate it's just and and the problem is you won't know it it'll probably still Work fine it'll work well but it can work better because we can always make it better so that's my Story for you today is you know when you design these beautiful networks and you put in these awesome RUCKUS AP's and you're making everything work take a second to go into that WLAN setting and change your Minimum basic rate from the defaults bump it up to 24 and see what happens and yeah you might have to Change it you might have to drive it down a little bit but so what you know my guess is more than likely You're going to find you don't have to turn it down everything's just going to work a little bit Better and a little bit faster everybody's going to go wow you're a genius now you just use the tools That are there so until next time again my name is Jim Palmer and thank you for listening to the RUCKCast hey there thanks for listening if you like the show and haven't done so please like and Subscribe if you want to contact the show directly you can email us at RUCKCast at CommScope.com I do Read those emails to learn more about any RUCKUS products or services that we've talked about on this Or any other prior podcast please check out our about section of the show for the show notes and as Always thank you for listening