Transmitting data over Wi-Fi is in many ways like speaking. You
may tell something to a single person, or announce it to everyone within earshot.
Rules for how loud you're allowed to be depend on where you are. And it's hard,
if not impossible, to speak and listen at the same time. So polite conversation
partners and standards-compliant Wi-Fi devices commonly take turns speaking and listening.
In other words, only one transmission can happen at a time. For devices on a given radio channel,
the time available for transmissions or ‘airtime’ is a shared resource and
the 802.11 standard defines rules for how devices should divide airtime between them.
But let’s say one of those devices is an access point with 2 connected clients and a stack of data
packets in its buffer that need to be forwarded to each client. The AP has to divide its share
of the total airtime between each client and how it does that is up to the AP.
If you were an access point, how would you do it?
One approach is to alternate between clients,
transmitting 1 packet to client A, 1 packet to client B and so on. This straightforward
logic is fair in terms of throughput and can work well, but it also can go wrong in a way
and to see how, we need to go from thinking in terms of packets to thinking in terms of airtime.
The time that it takes to transmit a packet of data can be calculated by dividing its size by
the data rate used. In high-throughput applications the packet size will likely
be close to the maximum, but the data rates used to transmit to each client can differ by orders
of magnitude. Either because one of the clients supports a much slower maximum data rate or just
is much further away from the AP and cannot be reached by high data rate transmissions as those
require excellent signal strength to decode. In such cases, an AP sending equal amounts of
traffic to each client, will be using vastly more of its airtime transmitting to the slower
client. In a sense, the slower client will be limiting the throughput of the faster one.
This can be demonstrated with an AP running the older 'wireless' driver package.
One of our clients can achieve 42 Mb/s when it’s given all of AP's airtime. The
other - 640 Mb/s. When transmitting traffic to both of them, they each get around 40
Mb/s. A huge drop for the faster device. This can be can be prevented by dividing
airtime evenly between clients. We can do that by upgrading the driver package used on our hAP ac^2,
or switching it out for a hAP ax^2 as our Wi-Fi 6 products come with
improved airtime allocation out of the box. With the hAP ax^2, our clients can achieve
60 and 1100 Mb/s of throughput respectively and when transmitting to both of them simultaneously,
they each get roughly half of that. This is worse for the slower client,
which is no longer getting nearly all of AP’s airtime, but much better for the faster
client and the total throughput of the AP. In closing, I'd like to acknowledge that,
starting with the introduction of multi-user MIMO in Wi-Fi 5, it is possible for an AP to transmit
to multiple connected clients simultaneously. But instead of dividing airtime between clients,
it works by dividing some other resource - such as the number of available spatial streams or
channel bandwidth. The main benefit of such operation is reduced latency and protocol
overhead. It cannot double our throughput here. I hope you've found this more technical video
on Wi-Fi informative and interesting. If there are any other subjects you'd like
us to cover, let us know. Until next time!