Know Your PHY

What’s the PHY?

The physical layer. You need to know how a NIC works. Now NIC are entirely different in wireless than wired. Learn more about the PHY in today’s video.

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Do not put your access points above the ceiling tiles

In today’s rules for efficient Wireless Lan design

We’re going to talk about why you should not put

your access points above the ceiling tiles

Now, I’ve had lots of people over my decades in Wi-Fi

ask, demand, cajole, talk, do

whatever they can to get me to put access points above

the ceiling tiles

And I have many times when they are like,

you’re going to do this or I won’t pay you, OK

OK, I’ll do what you ask

But I want you to sign this little document that says

you are asking me to have poorly

performing Wi-Fi.

And they’ve even signed it

Sometimes you come across a static police

Well, that’s too ugly.

We can’t use it there and they make you change it

but realize it’s going to cause

Some difficulties

Since this is just a video

And what I really should be doing is take you on site

to show you the results, we’re just

going to use this one and talk through

and do some thought exercises.

If you take an access point

And you place it below the ceiling tile,

the signal propagates,

however, the antenna pattern goes

Through freespace, and then eventually it connects

with the client device

Hopefully it doesn’t go through too many walls

and if it does,

we know we’re going to have a loss

If it’s three dB, well over three dB lost

and the signal eventually make it to the customer

Now, the client device, maybe a single stream

two stream, could be a laptop with three streams

And it’s listening to that information

Have you ever done the following you’ve looked

and you have an access point and it’s not moving,

You have a client and it’s not moving

Say your laptop, your desk

and you watch the signal strength

and the signal strength goes

up and down and up and down.

You’re like, I’m not moving,

no one else in the building is moving,

the access point isn’t moving

The power level going into the access point, isn’t changing

Why is my client device receiving this changeable


Hmmm? Well, if you take and you collect that data

in very precise, small little chunks,

sub tenth of a second level a little bucket,

you’re going to put it in, what you realize is

you will see if the room has multipath

Sometimes the energy from the access point

leaves the access point

and goes straight directly to your client device,

that will be one of the stronger signals

if in that same environment,

one of the other little bits of RF

goes the other direction,

bounces off a metal beam, comes back to you,

it traveled further

It had to go the opposite direction,

reflect lost a little energy,

came back even further

And when it gets back to you, you now receive less signal

If you plot all those little teeny signals coming in,

you might see you have a difference,

a low set and a high set

That’s proof that you have multipath in that environment

The higher ones are the direct ones,

the lower ones are the bounce

Sometimes you might see three or four bands of those

depending on where they’re coming from

When you average all those bands out,

it looks like it’s bouncing up and down.

Most of our engines that we use in the software

that’s showing us signal strength,

take an average and plot a new one once a second

So every second you’re getting a new one

Sometimes it has all of one

Sometimes it has the other

Sometimes it’s a mix

And thus it’s bouncing up and down,

telling you I have some multi path here

Now what happens now that gives you an RSSI

an average RSSI

Now what happens if we take the exact same access point

and put it above the ceiling tile

Now two things could happen wrong

One, when you move it above the ceiling tile,

the orientation of the access point stays

the way it’s supposed to dome down

and you place it on top of the ceiling tile dome down

and then your antenna pattern looks the same

The only RSSI loss you would pick up

is that little thickness of acoustic tile

Less than one DB, thus, when you take your access point,

you put above the ceiling tile

and take some measurements, you go around, you go

Hey, the RSSI is pretty much the same

What’s the penalty

Why did I, I’m just go stick it up there


Here’s the downside

What else is up above the ceiling tile

that may interfere with your RF

The structure that the ceiling tiles are hanging on.

the wires, the girders, the air ducts,

big metal plates that are bent

so they have some strength

And now your signal is leaving

the access point and bouncing off of more things,

causing more multipath

Which then bounces around

and eventually makes it back to the client

Now the same client in the same position,

perhaps getting almost identical RSSI

Now has more multipath because it has more multipath,

the signal it’s receiving highs and lows

and highs and lows.

It’s also not able to decode the spatial streams

So maybe when it was hanging below,

you were getting a nice clean two space or streams.

went up above because the extra multipath, we couldn’t

decode that extra spatial stream

when I dropped down to one spatial stream

Sam AP, same client, same distance

And you’re getting half or less the throughput

because you broke the rule

You put the access point above the ceiling tile

Know how Wi-Fi works, know how it can affect you,

learn the rules and realize you can break

any of the rules and Wi-Fi will still work

But if you want efficient Wi-Fi,

follow these Wireless LAN design rules

If you want to learn more about Wi-Fi

come to,

we have lots of resources for you there

Thanks for being part of the community

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