How do tags registers for a time slot in TDMA based protocol?

It is explained in PANS library document that every 100ms is divided into slots like Beacon, Almanc and TWR.
But how does a new tag which have not been assigned any TWR slot previously gets a new TWR slot.
Because 100ms do not include any time period where TWR slot assignment takes place for new tags.
What is the mechanism used by tags and Master node for assignment of slot and leaving of slot ?

It looks from the system overview and the description of the messages, like the tag will see the beacon messages from all the anchors which contain slot maps for the TWR slots. It will pick an open slot. The first part of the TWR slot is an opportunity for tags to poll the anchors. This takes the form of a Group Poll message. The tag will pick the four anchors it wants to use and send them a group poll message during the first part of the TWR slot that the tag has chosen. The anchors can then respond to let the tag know if it can range in that slot.

Again, this is only what I can glean from the documentation as well so there may be some errors.

1 Like

Hi All,

Jeffy, you understand it correctly, but let me try to explain it a bit differently.

In general the system contains collision avoidance, collision detection and collision resolution mechanism for all communication, i.e. also during TWR.

In general the Anchors provide information about how occupied are the TWR slots. The Tags will choose the best fit Anchors and some free data slot advertised by of those Anchors to do the TWR. During the TWR it will request also data slots for the next update period. Each selected anchor will confirm or not the reservation for the next period.

If the reservation was successful the Tag will keep using that data slot. If the reservation was not successful the Tag will try to find another slot the next period. The successful or fail of the reservation depends on how many reservation confirmation is the minimum required for the Tag (depends on the amount of Anchors installed else minimum would be 3 because that’s the minimum amount of measurements needed for the trilateration).


1 Like

Thank you guys for the quick reply !

So what I understood is that Anchors transmit info about the free TWR slots in their respective beacon slots and the tag which do not have any assigned TWR slot will listen to this info and will use this info to reserve its slot by sending Group Poll message during a free/unassigned TWR slot.
But just imagine if there were multiple tags with unassigned TWR slot and they tried to transmit Group Poll message in the same unassigned TWR slot, then how will the collision resolution will happen.
Due to collision they will not receive Response message from Anchor so they will understand that collision has happened but i do not understand how Collision resolution will happen.
Because without any Collision resolution, I guess they will try the same thing in next superframe and similar collision of message may happen again if they are accessing same free TWR slot.
I did not found any info about Collision resolution in PANS library document nor any frame structure for collision resolution has been described in the document.
Can someone please explain the process flow about how such situations are handled in TDMA ?

Hi Ashish,

the receiver is coherent so multiple transmitter can transmit at the same time and the receiver can still receive the frame correctly. See the description of reservation I have described above. One of the worst case would be e.g. there would be 2 tags each would get 2 reservations from 4 required. So they will look for another slot from the next superframe. The selection is done in the way that it would reduce the probability of the next collision. That will allow the resolution of the collision.


Hi leapslabs,

the receiver is coherent so multiple transmitter can transmit at the same time and the receiver can still receive the frame correctly.

could you point out where this conclusion come from? As what I know is that Aloha/TDMA is required from multiple nodes to simultaneously send data.