Monday, January 25, 2016

APs are... not HUBs!?!?

I come across a lot of misconceptions about WLAN networks and how they work in my day-to-day wireless business. This in the end leads to sub-par WLAN designs.

One of the biggest issues contributing to that is the lack of understanding of how the 802.11 protocol actually works. The channel occupation part that is.

As Keith Parsons (@KeithParsons) likes to say: "APs are HUBs!". And while the statement does drive the point of how you should think of wireless communication home, it's only half true (or at least not completely true).

Let's first look at what hubs are. Hub is simply multi port repeater. It's an archaic networking component which upon receiving bits on one port simply repeats that exact sequence on it's every other port. That means only one node can send bits at one time, all the other nodes receive those bits and process them if they are meant for them (if their hardware address is in the frames) otherwise they discard them.

This method of going networking is greatly inefficient and results in very slow speeds, so hubs have long been replaced by switches that have incorporated much better logic and far better hardware. Switches actually separate that one collision domain into as many new collision domains how many ports they have, greatly increasing efficiency and throughput.

What about the wireless world? Considering the hub explanation you might expect that ther's also repetition going on somewhere and that APs might be in charge of that, but you would be wrong. When a client device, associated to a particular AP, sends packets to a destination it sends it through that ap by setting the receiving MAC address of that AP in the frame, but the AP does not repeat those bits sent over the air. What does happen though is that those same bits, due to the the medium being unbounded, go everywhere and this is very important to understand.

In fact that's the way it must work! When a station, either an AP or a client, send out frames, every other station on that channel MUST hear those packets so they can set their timers to not "speak" at the same time. If it does, that's when you get those pesky hidden nodes that negatively impact your networks good-put.

So why is Keith saying that? Well it's a good one-liner that explains the situation and when talking to not technical people it's the one I use also, but as an engineer you should know what it really means.

So understand that APs are in fact not HUBs! They don't repeat the bits sent to them by client stations associated to them. It is rather the channel itself that is the HUB, and it must be that way for your network to function properly.

Thank you for reading my posts.