In the Eddystone beacon spec the word 'slot' is used

mtailor gravatar image

asked 2017-09-13 12:07:33 +0100

updated 2017-09-13 14:22:44 +0100

The term 'slot' is used in the context of the Eddystone beacon specification.

I cannot find a definition of that word.

For example, the sample app from Nordic says "The example provides five advertising slots" and despite working in Bluetooth for 17 years I am confused :-)

Does it mean that if you have 5 slots, then each advert will contain one frame and so 5 adverts are required to advertise all of them, OR Does it mean that you can fit 5 AD elements in a single advert ( which means it will not fit in pre v5 adverts)

I think in my case the confusion comes from a little bit too much knowledge to be able to perceive the ambiguity.

Look forward to hearing from someone.

Note to Nordic guys: it may be worth putting whatever feedback comes back as a response to this in the Nordic Inforcenter page related to "Eddystone Beacon Application"

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AmbystomaLabs gravatar image

answered 2017-09-13 14:04:19 +0100

Time slotting is a standard approach for time division duplexing (TDD) for many wireless transmission systems and Bluetooth supports it. You can find it discussed in the Core spec v4.2 section 2.2.3.

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Thank you for a quick response, unfortunately this does not answer my question as my reading of the eddystone documentation is that the designers had something something specific in mind when they used the word 'slot' in their narrative.

Your response is true about TDD in wireless - just that it is all so implied that why should something like eddystone which is an application even bother to mention :-)

Mahendra Tailor ( 2017-09-13 14:21:27 +0100 )editconvert to answer

Well now I'm curious too. I'll do some reading today to see if there is anything specific to Eddystone.

AmbystomaLabs ( 2017-09-13 15:04:41 +0100 )editconvert to answer

I noticed you were looking around on github too. Yes fascinating question. I would say your knowledge is considerably better than mine on this subject but I will give you my take anyway. I hope what I say doesn't sound stupid... It seems the eddystone folks are reusing the term slot and just not defining it well. Even the spec doesn't define it. As far as I can tell "slot" refers to the encryption time slot used for the EID beacon broadcast. Once the beacon negotiates encryption with its host the timestamp is used to create the following encryption keys. Therefore each "slot" has a unique key. There are 8 slots allowed in an EID frame. Then these data are broadcast over multiple bluetooth TDD slots.

I hope that helps a little and wasn't painfully obvious to you. I must say eddystone is a complicated protocol.

AmbystomaLabs ( 2017-09-13 17:34:28 +0100 )editconvert to answer

I like your thinking. However, I seem to recall that the term 'slot' has also been used in the context of the other frame types which don't have the encryption time cycling. So if my memory is correct (which I would not bet on ) would mean that your interpretation would not apply.

I am hoping the developer of the sample app at Nordic can comment as he is obviously much higher up the learning curve

Mahendra Tailor ( 2017-09-13 17:44:18 +0100 )editconvert to answer

It's a worthwhile question to resolve for people getting into Eddystone Beacons. Try sending a PM to Terje Schjelderup. He commented on an earlier eddystone question. https://devzone.nordicsemi.com/questi...

AmbystomaLabs ( 2017-09-13 17:51:18 +0100 )editconvert to answer

Having tried the eddystone sample app and the android application, I made it so that it had 2 slots. I saw that I saw alternate adverts of the two URLs that I had in the 2 slots.

From that I will assume that slots mean sending frames in different adverts and that may change in extended advertisments of v5 as the payload in that case can be multiple of 250 bytes.

I will have to revisit all this when Nordic expose extended adverts

Mahendra Tailor ( 2017-09-14 10:53:54 +0100 )editconvert to answer
mtailor gravatar image

answered 2017-09-13 17:54:09 +0100

I work for Laird and we sell BLE modules based on the nrf51 and nrf52. Have been working with them since 2012 and have access via their support portal so I am now inclined to ping then the same question there. But I will give it until tomorrow before I do that as they get irritated when I post in both places :-)

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jornfr gravatar image

answered 2017-09-15 15:15:29 +0100

Hello Mahendra Tailor

As you have mentioned in your comments the slots refer to different advertisement payloads for the beacon. The beacon will then switch between the different payloads when it is advertising.

Best regards

Jørn Frøysa

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I can see that when I enable multiple slots, each slot appears in an advert on its own. So when scanning you see adverts cycling through the number of slots.

My confusion was knowing that two EIDs could fit in a single advert ( if you put the other in the scan response ) and so got stuck in the ambiguity.

Then to make matters worse, when extended adverts start getting used in anger, a single advert will definitely be able to send out all slots in a single advert. Perhaps that is a bridge to cross when we come to it

Mahendra Tailor ( 2017-09-15 16:09:02 +0100 )editconvert to answer

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Asked: 2017-09-13 12:07:33 +0100

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Last updated: sep. 15 '17