802.11ax Is NOT A Wireless Switch

802.11ax is fast approaching. Though not 100% ratified by the IEEE, the spec is at the point where most manufacturers and vendors are going to support what’s current as the “final” version for now. While the spec for what marketing people like to call Wi-Fi 6 is not likely to change, that doesn’t mean that the ramp up to get people to buy it is showing any signs of starting off slow. One of the biggest problems I see right now is the decision by some major AP manufacturers to call 802.11ax a “wireless switch”.

Complex Duplex

In case you had any doubts, 802.11ax is NOT a switch.1 But the answer to why that is takes some explanation. It all starts with the network. More specifically, with Ethernet.

Ethernet is a broadcast medium. Packets are launched into the network and it is hoped that the packet finds the destination. All nodes on the network listen and, if the packet isn’t destined for them they discard it. This is the nature of the broadcast. If multiple stations try to talk at once, the packets collide and no one hears anything. That’s why Ethernet developed a collision detection  system called CSMA/CD.

Switches solved this problem by segmenting the collision domain to a single port. Now, the only communications between the stations would be in the event that the switch couldn’t find the proper port to forward a packet. In every other case, the switch finds where the packet is meant to be sent and forwards it to that location. It prevents collisions by ensuring that no two stations can transmit at any one time except to the switch in the middle. This also allows communications to be full-duplex, meaning the stations can send and receive at the same time.

Wireless is a different medium. The AP still speaks Ethernet, and there is a bridge between the Ethernet interface and the radios on the other side. But the radio interfaces work differently than Ethernet. Firstly, they are half-duplex only. That means that they have to send traffic or listen to receive traffic but they can’t do both at the same time. Wireless also uses a different version of collision detection called CSMA/CA, where the last A stands for “avoidance”. Because of the half-duplex nature of wireless, clients have a complex process to make sure the frequency is clear before transmitting. They have to check whether or not other wireless clients are talking and if the ambient RF is within the proper thresholds. After all those checks are confirmed, then the client transmits.

Because of the half-duplex wireless connection and the need for stations to have permission to send, some people have said that wireless is a lot like an Ethernet hub, which is pretty accurate. All stations and APs exist in the contention (collision) domain. Aside from the contention algorithm, there’s nothing to stop the stations from talking all at once. And for the entire life of 802.11 so far, it’s worked. 802.11ac started to introduce more features designed to let APs send frames to multiple stations at the same time. That’s what’s called Multi-user Multi-In, Multi-out (MU-MIMO).In theory, it could allow for full-duplex transmissions by allowing a client to send on one antenna and receive on another, but utilizing client radios in this way has impacts on other things.

Switching It Up

Enter 802.11ax. The Wi-Fi 6 feature that has most people excited is Orthogonal Frequency-Division Multiple Access (OFDMA). Simply put, OFDMA allows the clients and APs to not use the entire transmission channel for sending data. It can be sliced up into sub-channels that can be used for low-bandwidth applications to reserve time to talk to the AP. Combined with enhanced MU-MIMO support in 802.11ax, the idea is that clients can talk directly to the AP and allocate a specific sub-channel resource unit all to themselves.

To the marketing people in the room, this sounds just like a switch. Reserved channels, single station access, right? Except it is still not a switch. The AP is still a bridge between two media types for one thing, but more importantly the transmission medium still hasn’t magically become full-duplex. Stations may get around this with some kind of trickery, but they still need to wait for the all-clear to send data. Remember that all stations and APs still hear all the transmissions. It’s still a broadcast medium at the most basic. No amount of software configuration is going to fix that. And for the networking people in the room that might be saying “so what?”, remember when Cisco tried to sell us on the idea that StackWise was capable of 40Gbps of throughput because it could send in both directions on the StackWise ring at once? Remember when you started screaming “THAT’S NOT HOW BANDWIDTH WORKS!!!” That’s what this is, basically. Smoke and mirrors and ignoring the underlying physical layer constraints.

In fact, if you read the above resources, you’re going to find a lot of caveats at the end about support for protocols coming up and not being in the first version of the spec. That’s exactly what happened with 802.11ac. The promise of “gigabit Wi-Fi” took a couple of years and the MU-MIMO enhancements everyone was trumpeting never fully materialized. Just like all technology, the really good stuff was deferred to the next release.

To make sure that both sides are heard, it is rightly pointed out by wireless professionals like Sam Clements (@Samuel_Clements) that 802.11ax is the most “switch-like” so far, with multiple dynamic collision domains. However, in the immortal words of Tyler Durden, “Sticking feathers up your butt doesn’t make you a chicken.” The switch moniker is still a marketing construct and doesn’t hold any water in reality aside from a comparison to a somewhat similar technology. The operation of wireless APs may be hub-like or switch-like, but these devices are not either of those types of devices.

CPU Bound and Determined

The other issue that I see that prevents this from becoming a switch is the CPU on the AP becoming a point of contention. In a traditional Ethernet switch, the forwarding hardware is a specialized ASIC that is optimized to forward packets super fast. It does this with some trickery, including cut through for packets and trusting the incoming CRC. When packets bounce up to the CPU to be process-switched, it bogs the entire system down terribly. That’s why most networking texts will tell you to avoid process switching at all costs.

Now apply those lessons to wireless. All this protocol enhancement is now causing the CPU to have to do extra duty to work on time-slicing and sub-channel optimization. And remember that those CPUs are operating on 18-28 watts of power right now. Maybe the newer APs will get over 30 watts with new PoE options, but that means those CPUs are still going to be eating a lot of power to process all this extra software work. Even adding dedicated processing power to the AP isn’t going to fix things in the long run. That might be one of the reasons why Cisco has been pushing enhanced PoE in the run-up to their big 802.11ax launch at the end of April.


Tom’s Take

Let me say it again for the cheap seats: 802.11ax is NOT a wireless switch! The physical layer technology that 802.11 is built on won’t be switchable any time soon. 802.11ax has given us a lot of enhancements in the protocol and there is a lot to be excited about, like OFDMA, BSS coloring, and TWT. But, like the decision to over-simplify the marketing name, the idea of calling it a wireless switch just to give people a frame of reference so they buy more of them is just silly. It’s disingenuous and sounds more like a snake oil salesman than honest technology marketing. Rather than trying to trick the users with cute sounding terms, how about we keep the discussion honest and discuss the pros and cons of the technology?

Special thanks to my friends in the wireless space for proofreading this post and correcting my errors in technology:


  1. The title was kind of a spoiler ↩︎

Maybe MU-MIMO Matters

Wireless

As 802.11ac becomes more widely deployed in environments I find myself looking to the next wave and the promise it brings.  802.11ac Wave 1 for me really isn’t that groundbreaking.  It’s an incremental improvement on 802.11n.  Wave 1 really only serves to wake up the manufacturers to the fact that 5 GHz radios are needed on devices now.  The real interesting stuff comes in Wave 2.  Wider channels, more spatial streams, and a host of other improvements are on the way.  But the most important one for me is MU-MIMO.

Me Mi Mo Mum

Multi-user Multiple-Input Multiple-Output (MU-MIMO) is a huge upgrade over the MIMO specification in 802.11n.  MIMO allowed access points to multiplex signals on different channels into one data stream.  It accomplished this via Spatial Division Multiplexing (SDM).  This means that more antennas on an access point are a very good thing.  It increases the throughput above and beyond what could be accomplished with just a single antenna.  But it does have a drawback.

Single-user MIMO can only talk to one client at a time.  All the work necessary to multiplex those data streams require the full attention of a single access point for the period in time that the client is transmitting.  That means that crowded wireless networks can see reduced throughput because of shorter transmit windows.  And what wireless network isn’t crowded today?

MU-MIMO solves this problem by utilizing additional antenna capacity to transmit multiple data streams at once.  If you have spare antennas, you can send another data stream.  The AP then takes the multiple streams and stitches them back together.  This means an effective increase in throughput for certain devices even though the signal strength isn’t as high (thanks to FCC power limits).  Here’s a great video from Wireless Field Day 7 that explains the whole process:

What I found most interesting in this video is two-fold.  First, MU-MIMO is of great benefit to client devices that don’t have the maximum number of spatial streams.  Laptops are going to have three stream and four stream cards, so their MU-MIMO benefit is minimal.  However, the majority of devices on the market using wireless are mobile.  Tablets and phones don’t have multiple spatial streams, usually just one (or in some cases two).  They do this to improve battery life.  MU-MIMO will help them out considerably.

The second takeaway is that devices without a high number of receive chains will make the AP do more work.  That’s because the AP has to process the transmit stream and prevent the extra streams from being transmitted toward the wrong client.  That’s going to incur processing power.  That means you’ll need an AP with a lot of processing power.  Or a control system that can crunch those numbers for you.

When you consider that a large number of APs in a given system are sitting idle for a portion of the time it would be nice to be able to use that spare capacity for MU-MIMO processing.  In addition, having those extra antennas available to help with MU-MIMO sounding would be nice too.  There’s already been some work done on the research side of things.  Maybe we’ll soon see the ability to take the idle processing power of a wireless network and use it to boost the client throughput as needed.


 

Tom’s Take

Wireless never ceases to amaze me.  When I started writing this article, I thought I knew how MU-MIMO worked.  Thankfully, Cisco set me straight at Wireless Field Day 7.  MU-MIMO is going to help clients that can’t run high-performance networking cards.  The kinds of clients that are being sold as fast as possible today.  That means that the wireless system is already being developed to support a new kind of wireless device.

A device that doesn’t have access to limitless power from a wall socket or a battery that lasts forever.  There’s been talk of tablets with increased spatial streams for a while, but the cruel mistress of battery life will always win in the end.  That’s why MU-MIMO matters the most. Because if the wireless device can’t get more powerful, maybe it’s time for the wireless network to do the heavy lifting.