Let’s Talk About The Green Diamond

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Hello, I’m Keith Parsons

and today we’re going to talk about the Green Diamond

Welcome back to another training video

from Wireless LAN Professionals, I’m Keith Parsons

and today we’re going to talk about the green diamond

It’s how clients join an access point

And we’ve we’ve used this term in ECSC training

and training I used to do for Air Magnet

or other or CWNP training for nearly two decades

Now, the reason we use the word green diamond

is because it’s replacing with something that sticky

It’s easy to remember

We all know the color green

and we all know the shape of a diamond

But where it came from

was a graphic from a slide deck I did

oh, almost 20 years ago now

And here’s the graphic

and you can see how old the graphic was

Yeah, it still had a PCMCIA slot for the client device

It was a diamond because the decision of which AP to join

is solely, totally, completely only in the client device itself

So we put the diamond to represent a decision

on the laptop itself

Green just happened to be, the graphic designer used

red and blue already

and so the next up choice was green

So for the last 20 years

I’ve been calling the green diamond

and other people have been calling that

We could call it the

Unique proprietary roaming algorithm

used by your client

now it’s a big, long thing to say,

but that’s actually what’s happening

when a client is going to associate to an AP

it’s going to go through a series of questions

What’s actually going to go through a series of frames

And then the questions that

those frames give answers to

will then be brought into an algorithm

Now, some client devices have very simple algorithms

which AP is strongest

and they just pick one variable RSSI and join

Other clients, have some very

very sophisticated algorithms that look at it

at a whole series of different questions and answers

So the first step is a

client’s going to send out a probe request

Now, it doesn’t even have to do a probe request because

beacons wrapping every tenth of a second anyway

so a client could just listen to the probe

I mean, listen to the beacons and say, oh

of the beacon’s I heard I choose AP number one

But will most clients do today and have for

actually nearly two decades

is the clients have a more sophisticated method

they want to be more involved and

so a client’s going to send out a probe request

Now, you think, wait, APs are sending beacons

once every tenth of a second

How can I make that any faster

Well, probes can go out even faster as soon as you ask

they can send a probe out

Now a client is going to go

and probe across multiple channels

looking for the best AP it can possibly join

And it’s going to use an algorithm

to determine of all those APs I heard either on a beacon

or a probe response that came back from my request

I’m going to categorize the answers and come up with

the best AP I should join

Now, there’s a whole series of questions

that could go through

It could look just at the SSID

I want to join whichever AP has SSID

in my case, WLAN Pros right now.

It could determine of all the APs with SSID WLAN Pros

who is the loudest

Which has the greatest SNR

now realize SNR isn’t per AP

SNR is per channel

So I might have a channel 1 with an AP with SSI

With a RSSI of 67

And on Channel 6 there’s also an AP with a RSSI of 67

So they’re equal RSSIs

But Channel 6 is a higher noise floor

and thus the SNR be lower

The client would then choose

to go to the one on Channel 1

We could choose on authentication method

Do we want open PSK or .1X

We could choose on authentication

Do I want TKIP or ASCCMP

We could choose on based on it

was there a channel switch announcement that I heard

And should that encourage me

We could choose based on 802.11k,v,n,r

And there’s a whole series of

different ways those could influence me

I could send out a request

and when I get back as a neighbor report

And based on that neighbor report

I can either choose one of those

or I could just use the neighbor report to minimize

the number of channels I have to scan

Am I scanning in 5 gigahertz

and in 5 gigahertz can I scale on DFS channels

Well, no, I can’t go and send a probe I could go listen

and then I have to wait a tenth of a second for a beacon

So now I have to slow down to Beacon Rate

in order to listen to those beacons

Now, if I hear someone on the channel

I can then associate to that channel as well

I could look at what the error rates are

I could look at what data rates are being offered

I could have internally

in my own protocol that I’m looking at

a white list or a grey list or a black list

I could know where I am because if I’m a phone or an iPad

and I have GPS, I could look at the GPS location

the time even and say

I’m at home on a couch at eight o’clock at night

Who is the last AP I was associated to

I could have

Heuristics and little history

that if this AP treated me bad last time

I’m not going to go back to that AP again

There’s a lot of things that we could use in this algorithm

It’s not simply just saying who is loudest

Again, depending on the client device

they have a wide range

of algorithms from very simple

who has the highest SNR choose or which SSID

And I could even have SSIDs in a ranking that I’d like,

SSID number one

But if I don’t get that, I’ll pick that SSID number two

over anything else

So again, the green diamond is the decision

of which AP I should join

Once I join, then I have to do the whole

authentication, association, upper layers as well

If we look at that, we can see once we finish

the 802.11 association, probe request

probe response, association authentication request

authentication response, association request

association response

That’s only the 802.11 portion

Then we’re going to do authentication

Open PSK .1x

Based on the results of that will then encrypt

We could have no encryption,

TKIP encryption, AES/CCMP

We then get the ability to go pass the AP support control

and the AP will send us on and take our wireless frames

and send them on to the wire and on the wire

We probably first thing

we going to send out as a DHCP request

We’ll get back answers

And then when all of that’s done

we can now press pass local traffic

We could also at that point then

Then after all of those features are done

then a captive portal shows up

Now a lot of our customers think

the captive portal is the first thing

The captive portal showing up on the screen actually is

is reporting back all the other pieces before it worked

802.11 Worked, the authentication worked

the encryption work, the DHCP and upper layers worked

and then we get to the captive portal

If you do have a captive portal and you pass it

then you get full network access.

So the green diamond is the trigger

It is the part that allows us to find which AP is best

best being defined by a protocol

by an algorithm that the vendors will use

As end users we have very little control over that

We may be able to and should be able to change the SSID

A long time ago used to be able to ask for

I want more or less

I like I’d like Channel 60, I’m sorry

We would like Channel 1 or Channel 6 or Channel 11

And yet the algorithm said, well, you wanted channel 1

but this AP was better

So it just moved over to where the channel was

A lot of features in this green diamond allow us

to have the best performing Wi-Fi we can

You may not know the roaming algorithm

The vendor may not be supporting

and telling you the details for that

But if they do and you can find them

Then you want to use the answers for that

to design your Wi-Fi to making

the roaming algorithm as happy as possible

If the algorithm says, I’m not even going to look

or scan until I hit neg 72,

then you better have a way in your network

to drop below neg 72 to trigger

the algorithm to do its results

Green diamond is nothing more

than the algorithms are used

in client devices to choose which AP

Choose wisely

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