This articles addresses the concept of (antenna) presence and the two related notions of hold time and proximity.
Further it provides some background on the process of reading tags and the relation between Smartspot devices and administrative locations on Brain.
How a Smartspot reads tags
A Smartspots continuously scans the environment for the existence of radio signals from RFID and BLE tags.
Usually tags will respond to the scanning cycle of a Smartspot given that they are within the detection range of the spot.
The raw response of a tag to the Smartspot is sometimes referred to as a raw tag read or raw hit.
Once a responding signal from an RFID or BLE tag is received for the first time, the Smartspot creates a so called Presence create event for that specific tag.
Associated with this presence detection is some extra data including:
- the time stamp of the first detection (presence create time)
- the estimated proximity of the tag to the reader (far, near, immediate)
- the time stamp the tag has been detected for the last time (last update)
In case the tag keeps respond again and again to the Smartspot scanning cycle, the tag will remain in the present state.
Once a tag has been registered as present but does not respond again to a scanning cycle of a Smartspot, the tag will become not present again. This may happen in case a tag passes by the Smartspot and goes out of range.
Due to various circumstances in practice a tag may not be able to respond to each and every scanning cycle of a Smartspot.
As indicated in the example above, in case a Smartspot wouldn't receive the response of a tag again it will register the tag as not present again.
Though in practice the same tag may respond again just a little later and become present again.
Reasons why tags that are in range of a Smartspot may not respond
There are various circumstances that will influence the response of a tag to a scanning cycle of a Smartspot. This includes:
- The number of tags that are in range.
- The more tags are in range, the longer it will take before the same tag will respond again.
- In case two tags respond at the same time, the information may get lost due to the collision
- Physical orientation and readability of the tag. Depending on the orientation of the tag the antennas of the tag and Smartpot may couple well or less. Moving tags may have periods of time that the coupling is poorer, resulting in no response.
- Tags may be far away from the reader. The tag signal may be just at the reader's sensitivity threshold, or in case of passive tags, the activation signal from the reader may be just at the tag's activation threshold.
On information level it will be helpful the keep the tag present for a particular time even in case a tag doesn't respond during that interval.
The period of time that a tag is logically kept present without physically responding to the Smartspot is called Hold time. This also means that the shortest time between a tag getting present and not present equals the hold time.
So there are 2 ways to look at the notion of Hold time:
- The shortest time between a tag getting present and not present again.
- The period of time that a tag is kept present without responding to the Smartspot
So, once present the tag will remain present for at least the hold time. This mechanism is to prevent tags hopping quickly between status of present and not present and feeding the system with unusable information
A Smartspot will have a couple of internal antennas. For RFID this is at least 2 and depending on the model, may go up to 25 internal antennas. There can also be a number of externally connected antennas.
A Smartspot can treat all connected antennas as just one big antenna, ignoring which antenna detected which tag. But it can also treat each and every antenna as a separate unit. The latter is the case if Antenna Presences are enabled, the presence of a tag is registered and sent to Brain for each individual Smartspot antenna.
Clearly this increases the amount of traffic substantially, since the presence status itself as well as all associated information including presence create time and proximity (changes) are included as well.
The hold time concept applies to both presence as well as antenna presence.
Next to the creation of antenna presences, a Smartspot may also create so called antenna reports. More details can be found in Antenna reports
How Brain uses (antenna) presences and proximities
Presences and proximities in relation to Brain locations and Smartspots
Both presence and proximity states of an item are created by the Smartspot devices. Though Brain will register presence on location level, rather than on device level.
Since Smartspot antennas are mapped to reporting locations, Brain assigns the presences and proximities to the associated reporting locations. By doing so, Brain keeps state of these presences and proximities for all individual items.
These presence and proximity states will be used by the Brain location engine to determine the actual location of an item.
Multiple Smartspot reporting to a single location
Every Smartspot device is mapped to a reporting location.
In case a reporting location requires multiple smartspots to cover the desired area, one could map multiple smartspot to a single reporting location.
So for example if Smartspots A and B are located in Hallroom A and one would like to keep Hallroom A as the single reporting location, both Smartspots A and B can be mapped to the same reporting location Hallroom A.
Further details on mapping Smartspots and antennas to locations can be found in Location Editor
A single Smartspot with external antenna reporting to two locations
Opposite to the above example, a Smartspot with multiple antennas can be configure to report to multiple locations as well.
So for example, a single Smartspot with internal Antennas A and with external antenna B can be configured to report to location Hallroom In (Smartspot antennas A) and location Hallroom_Out (external antennas B).
In order to do so, it's required to have 'antenna presences' enabled on that device. Otherwise the spot would collect all presences from all antennas and report them to one single location.
More details on mapping Smartspot antennas to multiple locations can be done found in Location Editor