False Expectations and Confusion with 802.11ax

There is a lot of confusion and false expectations regarding 802.11ax flavored Wi-Fi these days.  Articles that compare 802.11ax to switches sets a dangerous president and leads to confusion in the marketplace as to what 802.11ax is and what it can do.

Take this article as an example:

https://www.arubanetworks.com/assets/wp/WP_Multi-User-802.11ax.pdf

"802.11AX TO THE RESCUE"

"With 802.11ax, MU-MIMO has been enhanced to support uplink traffic, from the client to the AP, and will support up to eight clients at a time (802.11ac allowed for eight, but no one implemented more than four). This doubles the number of devices to which an AP can talk, and because traffic is supported in both directions, clients can transmit simultaneously back to the AP, similar to how an eight-port switch would work on a wired network."

Here is where the confusion lies:  A client cannot transmit simultaneously back to the AP while receiving a frame from the AP - meaning, full duplex.  That is not in any 802.11ax draft amendment.  If this was possible, this is still not similar to how an eight port switch works.  Ethernet switching allows clients to send traffic on the wire whenever they want because the connection to the network is not a shared medium.  Wi-Fi is an unbounded, shared medium in unlicensed frequency band.

 

"Wider channels can support even more sub-channels. An 80 MHz wide channel can support up to 37 clients at a time. Like MU-MIMO, OFDMA supports downlink traffic, from the AP to the clients, and uplink traffic, from the clients to the AP. If MU-MIMO is a high speed 8-port switch, then OFDMA is a lower speed 37 port switch."

 

Here is where the confusion lies:  A client cannot transmit simultaneously back to the AP while receiving a frame from the AP - meaning, full duplex.  That is not in any 802.11ax draft amendment.  If this was possible, this is still not similar to how a thirty seven port switch works.  Ethernet switching allows clients to send traffic on the wire whenever they want because the connection to the network is not a shared medium.  Wi-Fi is an unbounded, shared medium in unlicensed frequency band.  Also, an 80 MHz channel is simply the aggregation of four 20 MHz channels into one channel.   If there are twenty 20 MHz channels and five 80 MHz channels, they are using the same spectrum.  Currently, date frames are transmitted consecutively using the entire channel. For example, if a client is connected to a 20 MHz wide channel and sends data, the entire channel is taken up and then the AP and clients take turns, one at a time, sending data on the channel.  This will continue until all of the legacy 20 MHz client devices (802.11a, 802.11n, 802.11ac) are removed from the network.  This includes the WLAN's access points and client devices, and also neighboring WLANs that are within earshot of the 802.11ax network.  It is not uncommon to do a survey of an existing building and see 700+ neighboring access points.  This number does not include the client devices on those 700+ access points.

 

"This has led to designs with more lower-bandwidth channels to reduce interference. 40 MHz-wide channels are the norm in office deployments, with 20 MHz wide channels used in high density offices or in environments where there are fewer channels available because of noisy RF neighborhoods.  BSS coloring allows the network to assign a “color” tag to a channel and reduce the threshold for interference.  Network performance is improved because APs on the same channel can be closer together and still transmit at the same time as long as they are different colors. Because we can have fewer channels, it may also be possible for organizations to use wider channels, such as 80Mhz channels in some or all of their network." 

 

Here is where the confusion lies:  The previous paragraph states "This has led to designs with more lower-bandwidth channels to reduce interference. 40 MHz-wide channels are the norm in office deployments, with 20 MHz wide channels used in high density offices or in environments where there are fewer channels available because of noisy RF neighborhoods."  Enterprise Wi-Fi networks should usually be set to 20 MHz channel plans because of the density of access points due to the data/voice/RTLS/higher density design and the noisy RF neighborhoods of the facilities.  80 Mhz channels are not usually used in Enterprise environments, therefore that does not apply.

"If MU-MIMO and OFDMA make the wireless network behave more like a switched environment, then BSS coloring adds switching capacity to the network."

 

There is a lot of confusion regarding 802.11ax - especially with this subject.  MU-MIMO and OFDMA does not make the wireless network behave like a switched environment, therefore that statement is not accurate.  Switching uses CSMA/CD, (carrier sense multiple access with collision detection) and Wi-Fi uses CSMA/CA, which is carrier sense multiple access with collision avoidance.  The two are massively different than each other - CD can sense a collision, which usually does not happen now that we plug hosts into switches and the collision domain exists between the host and the switch.  Collision Avoidance is still the same.  The medium is still in a shared spectrum with all those cordless phones, video cameras neighboring networks and everything else I have forgotten to mention.  None of those things exist in the copper cabling between a host and a switch with CSMA/CD.  There is no way to "detect" a collision with Collision Avoidance - if the station did not receive an acknowledgement for the unicast packet it transmitted, it will retransmit the packet, assuming the receiver did not receive it.

 

"With MU-MIMO, an AP can behave like a high speed, 600 Mbps per client 8 port switch, great for large file transfers and high-performance clients. OFDMA allows an AP to behave like a lower speed, ~25 Mbps per client 37 port switch, good for normal network use and voice and video streaming.  An AP can switch back and forth between these two modes every transmit cycle as the needs of the clients change. We don’t have a dedicated connection for each device like you would in a wired network, but we are able to adjust the amount of network capacity allocated to each device depending on need, which is something wired networking can’t do. Although we don’t have a full-duplex switch, we effectively have time-division duplexing that emulates full duplex over shared/half-duplex mediums. In the end, we gain a lot of the benefits of switching and wireless and 802.11ax is likely to be the point where we think of Wi-Fi less like an old bridged network and more like a high-speed modern switched network."

 

Even more confusion:  An AP cannot behave like a high speed, 600 Mbps per client 8 port switch.  Switches use CSMA/CD, and 802.11ax will use CSMA/CA.  A switch is full duplex - meaning simultaneous transmit and receive to a client, as long as the port negotiates properly.  802.11ax is not full duplex.  Switches commonly use gigabit speeds on every host port, and 802.11ax does not equal an eight port (or 37 port) 1000 Mbps full duplex (no collision domain) Ethernet switch.  We do not "gain a lot of the benefits of switching" with 802.11ax.  We do not have collision detection, we still share the frequency/channel with all the other wireless devices on that frequency, and we do not have full duplex (simultaneous) transmit and receive to a client.